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[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >
<chapter id='profile-manual-usage'>
<title>Basic Usage (with examples) for each of the Yocto Tracing Tools</title>
<para>
This chapter presents basic usage examples for each of the tracing
tools.
</para>
<section id='profile-manual-perf'>
<title>perf</title>
<para>
The 'perf' tool is the profiling and tracing tool that comes
bundled with the Linux kernel.
</para>
<para>
Don't let the fact that it's part of the kernel fool you into thinking
that it's only for tracing and profiling the kernel - you can indeed
use it to trace and profile just the kernel, but you can also use it
to profile specific applications separately (with or without kernel
context), and you can also use it to trace and profile the kernel
and all applications on the system simultaneously to gain a system-wide
view of what's going on.
</para>
<para>
In many ways, perf aims to be a superset of all the tracing and profiling
tools available in Linux today, including all the other tools covered
in this HOWTO. The past couple of years have seen perf subsume a lot
of the functionality of those other tools and, at the same time, those
other tools have removed large portions of their previous functionality
and replaced it with calls to the equivalent functionality now
implemented by the perf subsystem. Extrapolation suggests that at
some point those other tools will simply become completely redundant
and go away; until then, we'll cover those other tools in these pages
and in many cases show how the same things can be accomplished in
perf and the other tools when it seems useful to do so.
</para>
<para>
The coverage below details some of the most common ways you'll likely
want to apply the tool; full documentation can be found either within
the tool itself or in the man pages at
<ulink url='http://linux.die.net/man/1/perf'>perf(1)</ulink>.
</para>
<section id='perf-setup'>
<title>Setup</title>
<para>
For this section, we'll assume you've already performed the basic
setup outlined in the General Setup section.
</para>
<para>
In particular, you'll get the most mileage out of perf if you
profile an image built with the following in your
<filename>local.conf</filename> file:
<literallayout class='monospaced'>
<ulink url='&YOCTO_DOCS_REF_URL;#var-INHIBIT_PACKAGE_STRIP'>INHIBIT_PACKAGE_STRIP</ulink> = "1"
</literallayout>
</para>
<para>
perf runs on the target system for the most part. You can archive
profile data and copy it to the host for analysis, but for the
rest of this document we assume you've ssh'ed to the host and
will be running the perf commands on the target.
</para>
</section>
<section id='perf-basic-usage'>
<title>Basic Usage</title>
<para>
The perf tool is pretty much self-documenting. To remind yourself
of the available commands, simply type 'perf', which will show you
basic usage along with the available perf subcommands:
<literallayout class='monospaced'>
root@crownbay:~# perf
usage: perf [--version] [--help] COMMAND [ARGS]
The most commonly used perf commands are:
annotate Read perf.data (created by perf record) and display annotated code
archive Create archive with object files with build-ids found in perf.data file
bench General framework for benchmark suites
buildid-cache Manage build-id cache.
buildid-list List the buildids in a perf.data file
diff Read two perf.data files and display the differential profile
evlist List the event names in a perf.data file
inject Filter to augment the events stream with additional information
kmem Tool to trace/measure kernel memory(slab) properties
kvm Tool to trace/measure kvm guest os
list List all symbolic event types
lock Analyze lock events
probe Define new dynamic tracepoints
record Run a command and record its profile into perf.data
report Read perf.data (created by perf record) and display the profile
sched Tool to trace/measure scheduler properties (latencies)
script Read perf.data (created by perf record) and display trace output
stat Run a command and gather performance counter statistics
test Runs sanity tests.
timechart Tool to visualize total system behavior during a workload
top System profiling tool.
See 'perf help COMMAND' for more information on a specific command.
</literallayout>
</para>
<section id='using-perf-to-do-basic-profiling'>
<title>Using perf to do Basic Profiling</title>
<para>
As a simple test case, we'll profile the 'wget' of a fairly large
file, which is a minimally interesting case because it has both
file and network I/O aspects, and at least in the case of standard
Yocto images, it's implemented as part of busybox, so the methods
we use to analyze it can be used in a very similar way to the whole
host of supported busybox applets in Yocto.
<literallayout class='monospaced'>
root@crownbay:~# rm linux-2.6.19.2.tar.bz2; \
wget <ulink url='http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2'>http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2</ulink>
</literallayout>
The quickest and easiest way to get some basic overall data about
what's going on for a particular workload is to profile it using
'perf stat'. 'perf stat' basically profiles using a few default
counters and displays the summed counts at the end of the run:
<literallayout class='monospaced'>
root@crownbay:~# perf stat wget <ulink url='http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2'>http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2</ulink>
Connecting to downloads.yoctoproject.org (140.211.169.59:80)
linux-2.6.19.2.tar.b 100% |***************************************************| 41727k 0:00:00 ETA
Performance counter stats for 'wget <ulink url='http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2'>http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2</ulink>':
4597.223902 task-clock # 0.077 CPUs utilized
23568 context-switches # 0.005 M/sec
68 CPU-migrations # 0.015 K/sec
241 page-faults # 0.052 K/sec
3045817293 cycles # 0.663 GHz
<not supported> stalled-cycles-frontend
<not supported> stalled-cycles-backend
858909167 instructions # 0.28 insns per cycle
165441165 branches # 35.987 M/sec
19550329 branch-misses # 11.82% of all branches
59.836627620 seconds time elapsed
</literallayout>
Many times such a simple-minded test doesn't yield much of
interest, but sometimes it does (see Real-world Yocto bug
(slow loop-mounted write speed)).
</para>
<para>
Also, note that 'perf stat' isn't restricted to a fixed set of
counters - basically any event listed in the output of 'perf list'
can be tallied by 'perf stat'. For example, suppose we wanted to
see a summary of all the events related to kernel memory
allocation/freeing along with cache hits and misses:
<literallayout class='monospaced'>
root@crownbay:~# perf stat -e kmem:* -e cache-references -e cache-misses wget <ulink url='http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2'>http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2</ulink>
Connecting to downloads.yoctoproject.org (140.211.169.59:80)
linux-2.6.19.2.tar.b 100% |***************************************************| 41727k 0:00:00 ETA
Performance counter stats for 'wget <ulink url='http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2'>http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2</ulink>':
5566 kmem:kmalloc
125517 kmem:kmem_cache_alloc
0 kmem:kmalloc_node
0 kmem:kmem_cache_alloc_node
34401 kmem:kfree
69920 kmem:kmem_cache_free
133 kmem:mm_page_free
41 kmem:mm_page_free_batched
11502 kmem:mm_page_alloc
11375 kmem:mm_page_alloc_zone_locked
0 kmem:mm_page_pcpu_drain
0 kmem:mm_page_alloc_extfrag
66848602 cache-references
2917740 cache-misses # 4.365 % of all cache refs
44.831023415 seconds time elapsed
</literallayout>
So 'perf stat' gives us a nice easy way to get a quick overview of
what might be happening for a set of events, but normally we'd
need a little more detail in order to understand what's going on
in a way that we can act on in a useful way.
</para>
<para>
To dive down into a next level of detail, we can use 'perf
record'/'perf report' which will collect profiling data and
present it to use using an interactive text-based UI (or
simply as text if we specify --stdio to 'perf report').
</para>
<para>
As our first attempt at profiling this workload, we'll simply
run 'perf record', handing it the workload we want to profile
(everything after 'perf record' and any perf options we hand
it - here none - will be executed in a new shell). perf collects
samples until the process exits and records them in a file named
'perf.data' in the current working directory.
<literallayout class='monospaced'>
root@crownbay:~# perf record wget <ulink url='http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2'>http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2</ulink>
Connecting to downloads.yoctoproject.org (140.211.169.59:80)
linux-2.6.19.2.tar.b 100% |************************************************| 41727k 0:00:00 ETA
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.176 MB perf.data (~7700 samples) ]
</literallayout>
To see the results in a 'text-based UI' (tui), simply run
'perf report', which will read the perf.data file in the current
working directory and display the results in an interactive UI:
<literallayout class='monospaced'>
root@crownbay:~# perf report
</literallayout>
</para>
<para>
<imagedata fileref="figures/perf-wget-flat-stripped.png" width="6in" depth="7in" align="center" scalefit="1" />
</para>
<para>
The above screenshot displays a 'flat' profile, one entry for
each 'bucket' corresponding to the functions that were profiled
during the profiling run, ordered from the most popular to the
least (perf has options to sort in various orders and keys as
well as display entries only above a certain threshold and so
on - see the perf documentation for details). Note that this
includes both userspace functions (entries containing a [.]) and
kernel functions accounted to the process (entries containing
a [k]). (perf has command-line modifiers that can be used to
restrict the profiling to kernel or userspace, among others).
</para>
<para>
Notice also that the above report shows an entry for 'busybox',
which is the executable that implements 'wget' in Yocto, but that
instead of a useful function name in that entry, it displays
a not-so-friendly hex value instead. The steps below will show
how to fix that problem.
</para>
<para>
Before we do that, however, let's try running a different profile,
one which shows something a little more interesting. The only
difference between the new profile and the previous one is that
we'll add the -g option, which will record not just the address
of a sampled function, but the entire callchain to the sampled
function as well:
<literallayout class='monospaced'>
root@crownbay:~# perf record -g wget <ulink url='http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2'>http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2</ulink>
Connecting to downloads.yoctoproject.org (140.211.169.59:80)
linux-2.6.19.2.tar.b 100% |************************************************| 41727k 0:00:00 ETA
[ perf record: Woken up 3 times to write data ]
[ perf record: Captured and wrote 0.652 MB perf.data (~28476 samples) ]
root@crownbay:~# perf report
</literallayout>
</para>
<para>
<imagedata fileref="figures/perf-wget-g-copy-to-user-expanded-stripped.png" width="6in" depth="7in" align="center" scalefit="1" />
</para>
<para>
Using the callgraph view, we can actually see not only which
functions took the most time, but we can also see a summary of
how those functions were called and learn something about how the
program interacts with the kernel in the process.
</para>
<para>
Notice that each entry in the above screenshot now contains a '+'
on the left-hand side. This means that we can expand the entry and
drill down into the callchains that feed into that entry.
Pressing 'enter' on any one of them will expand the callchain
(you can also press 'E' to expand them all at the same time or 'C'
to collapse them all).
</para>
<para>
In the screenshot above, we've toggled the __copy_to_user_ll()
entry and several subnodes all the way down. This lets us see
which callchains contributed to the profiled __copy_to_user_ll()
function which contributed 1.77% to the total profile.
</para>
<para>
As a bit of background explanation for these callchains, think
about what happens at a high level when you run wget to get a file
out on the network. Basically what happens is that the data comes
into the kernel via the network connection (socket) and is passed
to the userspace program 'wget' (which is actually a part of
busybox, but that's not important for now), which takes the buffers
the kernel passes to it and writes it to a disk file to save it.
</para>
<para>
The part of this process that we're looking at in the above call
stacks is the part where the kernel passes the data it's read from
the socket down to wget i.e. a copy-to-user.
</para>
<para>
Notice also that here there's also a case where the hex value
is displayed in the callstack, here in the expanded
sys_clock_gettime() function. Later we'll see it resolve to a
userspace function call in busybox.
</para>
<para>
<imagedata fileref="figures/perf-wget-g-copy-from-user-expanded-stripped.png" width="6in" depth="7in" align="center" scalefit="1" />
</para>
<para>
The above screenshot shows the other half of the journey for the
data - from the wget program's userspace buffers to disk. To get
the buffers to disk, the wget program issues a write(2), which
does a copy-from-user to the kernel, which then takes care via
some circuitous path (probably also present somewhere in the
profile data), to get it safely to disk.
</para>
<para>
Now that we've seen the basic layout of the profile data and the
basics of how to extract useful information out of it, let's get
back to the task at hand and see if we can get some basic idea
about where the time is spent in the program we're profiling,
wget. Remember that wget is actually implemented as an applet
in busybox, so while the process name is 'wget', the executable
we're actually interested in is busybox. So let's expand the
first entry containing busybox:
</para>
<para>
<imagedata fileref="figures/perf-wget-busybox-expanded-stripped.png" width="6in" depth="7in" align="center" scalefit="1" />
</para>
<para>
Again, before we expanded we saw that the function was labeled
with a hex value instead of a symbol as with most of the kernel
entries. Expanding the busybox entry doesn't make it any better.
</para>
<para>
The problem is that perf can't find the symbol information for the
busybox binary, which is actually stripped out by the Yocto build
system.
</para>
<para>
One way around that is to put the following in your
<filename>local.conf</filename> file when you build the image:
<literallayout class='monospaced'>
<ulink url='&YOCTO_DOCS_REF_URL;#var-INHIBIT_PACKAGE_STRIP'>INHIBIT_PACKAGE_STRIP</ulink> = "1"
</literallayout>
However, we already have an image with the binaries stripped,
so what can we do to get perf to resolve the symbols? Basically
we need to install the debuginfo for the busybox package.
</para>
<para>
To generate the debug info for the packages in the image, we can
add dbg-pkgs to EXTRA_IMAGE_FEATURES in local.conf. For example:
<literallayout class='monospaced'>
EXTRA_IMAGE_FEATURES = "debug-tweaks tools-profile dbg-pkgs"
</literallayout>
Additionally, in order to generate the type of debuginfo that
perf understands, we also need to add the following to local.conf:
<literallayout class='monospaced'>
PACKAGE_DEBUG_SPLIT_STYLE = 'debug-file-directory'
</literallayout>
Once we've done that, we can install the debuginfo for busybox.
The debug packages once built can be found in
build/tmp/deploy/rpm/* on the host system. Find the
busybox-dbg-...rpm file and copy it to the target. For example:
<literallayout class='monospaced'>
[trz@empanada core2]$ scp /home/trz/yocto/crownbay-tracing-dbg/build/tmp/deploy/rpm/core2_32/busybox-dbg-1.20.2-r2.core2_32.rpm root@192.168.1.31:
root@192.168.1.31's password:
busybox-dbg-1.20.2-r2.core2_32.rpm 100% 1826KB 1.8MB/s 00:01
</literallayout>
Now install the debug rpm on the target:
<literallayout class='monospaced'>
root@crownbay:~# rpm -i busybox-dbg-1.20.2-r2.core2_32.rpm
</literallayout>
Now that the debuginfo is installed, we see that the busybox
entries now display their functions symbolically:
</para>
<para>
<imagedata fileref="figures/perf-wget-busybox-debuginfo.png" width="6in" depth="7in" align="center" scalefit="1" />
</para>
<para>
If we expand one of the entries and press 'enter' on a leaf node,
we're presented with a menu of actions we can take to get more
information related to that entry:
</para>
<para>
<imagedata fileref="figures/perf-wget-busybox-dso-zoom-menu.png" width="6in" depth="2in" align="center" scalefit="1" />
</para>
<para>
One of these actions allows us to show a view that displays a
busybox-centric view of the profiled functions (in this case we've
also expanded all the nodes using the 'E' key):
</para>
<para>
<imagedata fileref="figures/perf-wget-busybox-dso-zoom.png" width="6in" depth="7in" align="center" scalefit="1" />
</para>
<para>
Finally, we can see that now that the busybox debuginfo is
installed, the previously unresolved symbol in the
sys_clock_gettime() entry mentioned previously is now resolved,
and shows that the sys_clock_gettime system call that was the
source of 6.75% of the copy-to-user overhead was initiated by
the handle_input() busybox function:
</para>
<para>
<imagedata fileref="figures/perf-wget-g-copy-to-user-expanded-debuginfo.png" width="6in" depth="7in" align="center" scalefit="1" />
</para>
<para>
At the lowest level of detail, we can dive down to the assembly
level and see which instructions caused the most overhead in a
function. Pressing 'enter' on the 'udhcpc_main' function, we're
again presented with a menu:
</para>
<para>
<imagedata fileref="figures/perf-wget-busybox-annotate-menu.png" width="6in" depth="2in" align="center" scalefit="1" />
</para>
<para>
Selecting 'Annotate udhcpc_main', we get a detailed listing of
percentages by instruction for the udhcpc_main function. From the
display, we can see that over 50% of the time spent in this
function is taken up by a couple tests and the move of a
constant (1) to a register:
</para>
<para>
<imagedata fileref="figures/perf-wget-busybox-annotate-udhcpc.png" width="6in" depth="7in" align="center" scalefit="1" />
</para>
<para>
As a segue into tracing, let's try another profile using a
different counter, something other than the default 'cycles'.
</para>
<para>
The tracing and profiling infrastructure in Linux has become
unified in a way that allows us to use the same tool with a
completely different set of counters, not just the standard
hardware counters that traditional tools have had to restrict
themselves to (of course the traditional tools can also make use
of the expanded possibilities now available to them, and in some
cases have, as mentioned previously).
</para>
<para>
We can get a list of the available events that can be used to
profile a workload via 'perf list':
<literallayout class='monospaced'>
root@crownbay:~# perf list
List of pre-defined events (to be used in -e):
cpu-cycles OR cycles [Hardware event]
stalled-cycles-frontend OR idle-cycles-frontend [Hardware event]
stalled-cycles-backend OR idle-cycles-backend [Hardware event]
instructions [Hardware event]
cache-references [Hardware event]
cache-misses [Hardware event]
branch-instructions OR branches [Hardware event]
branch-misses [Hardware event]
bus-cycles [Hardware event]
ref-cycles [Hardware event]
cpu-clock [Software event]
task-clock [Software event]
page-faults OR faults [Software event]
minor-faults [Software event]
major-faults [Software event]
context-switches OR cs [Software event]
cpu-migrations OR migrations [Software event]
alignment-faults [Software event]
emulation-faults [Software event]
L1-dcache-loads [Hardware cache event]
L1-dcache-load-misses [Hardware cache event]
L1-dcache-prefetch-misses [Hardware cache event]
L1-icache-loads [Hardware cache event]
L1-icache-load-misses [Hardware cache event]
.
.
.
rNNN [Raw hardware event descriptor]
cpu/t1=v1[,t2=v2,t3 ...]/modifier [Raw hardware event descriptor]
(see 'perf list --help' on how to encode it)
mem:<addr>[:access] [Hardware breakpoint]
sunrpc:rpc_call_status [Tracepoint event]
sunrpc:rpc_bind_status [Tracepoint event]
sunrpc:rpc_connect_status [Tracepoint event]
sunrpc:rpc_task_begin [Tracepoint event]
skb:kfree_skb [Tracepoint event]
skb:consume_skb [Tracepoint event]
skb:skb_copy_datagram_iovec [Tracepoint event]
net:net_dev_xmit [Tracepoint event]
net:net_dev_queue [Tracepoint event]
net:netif_receive_skb [Tracepoint event]
net:netif_rx [Tracepoint event]
napi:napi_poll [Tracepoint event]
sock:sock_rcvqueue_full [Tracepoint event]
sock:sock_exceed_buf_limit [Tracepoint event]
udp:udp_fail_queue_rcv_skb [Tracepoint event]
hda:hda_send_cmd [Tracepoint event]
hda:hda_get_response [Tracepoint event]
hda:hda_bus_reset [Tracepoint event]
scsi:scsi_dispatch_cmd_start [Tracepoint event]
scsi:scsi_dispatch_cmd_error [Tracepoint event]
scsi:scsi_eh_wakeup [Tracepoint event]
drm:drm_vblank_event [Tracepoint event]
drm:drm_vblank_event_queued [Tracepoint event]
drm:drm_vblank_event_delivered [Tracepoint event]
random:mix_pool_bytes [Tracepoint event]
random:mix_pool_bytes_nolock [Tracepoint event]
random:credit_entropy_bits [Tracepoint event]
gpio:gpio_direction [Tracepoint event]
gpio:gpio_value [Tracepoint event]
block:block_rq_abort [Tracepoint event]
block:block_rq_requeue [Tracepoint event]
block:block_rq_issue [Tracepoint event]
block:block_bio_bounce [Tracepoint event]
block:block_bio_complete [Tracepoint event]
block:block_bio_backmerge [Tracepoint event]
.
.
writeback:writeback_wake_thread [Tracepoint event]
writeback:writeback_wake_forker_thread [Tracepoint event]
writeback:writeback_bdi_register [Tracepoint event]
.
.
writeback:writeback_single_inode_requeue [Tracepoint event]
writeback:writeback_single_inode [Tracepoint event]
kmem:kmalloc [Tracepoint event]
kmem:kmem_cache_alloc [Tracepoint event]
kmem:mm_page_alloc [Tracepoint event]
kmem:mm_page_alloc_zone_locked [Tracepoint event]
kmem:mm_page_pcpu_drain [Tracepoint event]
kmem:mm_page_alloc_extfrag [Tracepoint event]
vmscan:mm_vmscan_kswapd_sleep [Tracepoint event]
vmscan:mm_vmscan_kswapd_wake [Tracepoint event]
vmscan:mm_vmscan_wakeup_kswapd [Tracepoint event]
vmscan:mm_vmscan_direct_reclaim_begin [Tracepoint event]
.
.
module:module_get [Tracepoint event]
module:module_put [Tracepoint event]
module:module_request [Tracepoint event]
sched:sched_kthread_stop [Tracepoint event]
sched:sched_wakeup [Tracepoint event]
sched:sched_wakeup_new [Tracepoint event]
sched:sched_process_fork [Tracepoint event]
sched:sched_process_exec [Tracepoint event]
sched:sched_stat_runtime [Tracepoint event]
rcu:rcu_utilization [Tracepoint event]
workqueue:workqueue_queue_work [Tracepoint event]
workqueue:workqueue_execute_end [Tracepoint event]
signal:signal_generate [Tracepoint event]
signal:signal_deliver [Tracepoint event]
timer:timer_init [Tracepoint event]
timer:timer_start [Tracepoint event]
timer:hrtimer_cancel [Tracepoint event]
timer:itimer_state [Tracepoint event]
timer:itimer_expire [Tracepoint event]
irq:irq_handler_entry [Tracepoint event]
irq:irq_handler_exit [Tracepoint event]
irq:softirq_entry [Tracepoint event]
irq:softirq_exit [Tracepoint event]
irq:softirq_raise [Tracepoint event]
printk:console [Tracepoint event]
task:task_newtask [Tracepoint event]
task:task_rename [Tracepoint event]
syscalls:sys_enter_socketcall [Tracepoint event]
syscalls:sys_exit_socketcall [Tracepoint event]
.
.
.
syscalls:sys_enter_unshare [Tracepoint event]
syscalls:sys_exit_unshare [Tracepoint event]
raw_syscalls:sys_enter [Tracepoint event]
raw_syscalls:sys_exit [Tracepoint event]
</literallayout>
</para>
<informalexample>
<emphasis>Tying it Together:</emphasis> These are exactly the same set of events defined
by the trace event subsystem and exposed by
ftrace/tracecmd/kernelshark as files in
/sys/kernel/debug/tracing/events, by SystemTap as
kernel.trace("tracepoint_name") and (partially) accessed by LTTng.
</informalexample>
<para>
Only a subset of these would be of interest to us when looking at
this workload, so let's choose the most likely subsystems
(identified by the string before the colon in the Tracepoint events)
and do a 'perf stat' run using only those wildcarded subsystems:
<literallayout class='monospaced'>
root@crownbay:~# perf stat -e skb:* -e net:* -e napi:* -e sched:* -e workqueue:* -e irq:* -e syscalls:* wget <ulink url='http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2'>http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2</ulink>
Performance counter stats for 'wget <ulink url='http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2'>http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2</ulink>':
23323 skb:kfree_skb
0 skb:consume_skb
49897 skb:skb_copy_datagram_iovec
6217 net:net_dev_xmit
6217 net:net_dev_queue
7962 net:netif_receive_skb
2 net:netif_rx
8340 napi:napi_poll
0 sched:sched_kthread_stop
0 sched:sched_kthread_stop_ret
3749 sched:sched_wakeup
0 sched:sched_wakeup_new
0 sched:sched_switch
29 sched:sched_migrate_task
0 sched:sched_process_free
1 sched:sched_process_exit
0 sched:sched_wait_task
0 sched:sched_process_wait
0 sched:sched_process_fork
1 sched:sched_process_exec
0 sched:sched_stat_wait
2106519415641 sched:sched_stat_sleep
0 sched:sched_stat_iowait
147453613 sched:sched_stat_blocked
12903026955 sched:sched_stat_runtime
0 sched:sched_pi_setprio
3574 workqueue:workqueue_queue_work
3574 workqueue:workqueue_activate_work
0 workqueue:workqueue_execute_start
0 workqueue:workqueue_execute_end
16631 irq:irq_handler_entry
16631 irq:irq_handler_exit
28521 irq:softirq_entry
28521 irq:softirq_exit
28728 irq:softirq_raise
1 syscalls:sys_enter_sendmmsg
1 syscalls:sys_exit_sendmmsg
0 syscalls:sys_enter_recvmmsg
0 syscalls:sys_exit_recvmmsg
14 syscalls:sys_enter_socketcall
14 syscalls:sys_exit_socketcall
.
.
.
16965 syscalls:sys_enter_read
16965 syscalls:sys_exit_read
12854 syscalls:sys_enter_write
12854 syscalls:sys_exit_write
.
.
.
58.029710972 seconds time elapsed
</literallayout>
Let's pick one of these tracepoints and tell perf to do a profile
using it as the sampling event:
<literallayout class='monospaced'>
root@crownbay:~# perf record -g -e sched:sched_wakeup wget <ulink url='http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2'>http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2</ulink>
</literallayout>
</para>
<para>
<imagedata fileref="figures/sched-wakeup-profile.png" width="6in" depth="7in" align="center" scalefit="1" />
</para>
<para>
The screenshot above shows the results of running a profile using
sched:sched_switch tracepoint, which shows the relative costs of
various paths to sched_wakeup (note that sched_wakeup is the
name of the tracepoint - it's actually defined just inside
ttwu_do_wakeup(), which accounts for the function name actually
displayed in the profile:
<literallayout class='monospaced'>
/*
* Mark the task runnable and perform wakeup-preemption.
*/
static void
ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags)
{
trace_sched_wakeup(p, true);
.
.
.
}
</literallayout>
A couple of the more interesting callchains are expanded and
displayed above, basically some network receive paths that
presumably end up waking up wget (busybox) when network data is
ready.
</para>
<para>
Note that because tracepoints are normally used for tracing,
the default sampling period for tracepoints is 1 i.e. for
tracepoints perf will sample on every event occurrence (this
can be changed using the -c option). This is in contrast to
hardware counters such as for example the default 'cycles'
hardware counter used for normal profiling, where sampling
periods are much higher (in the thousands) because profiling should
have as low an overhead as possible and sampling on every cycle
would be prohibitively expensive.
</para>
</section>
<section id='using-perf-to-do-basic-tracing'>
<title>Using perf to do Basic Tracing</title>
<para>
Profiling is a great tool for solving many problems or for
getting a high-level view of what's going on with a workload or
across the system. It is however by definition an approximation,
as suggested by the most prominent word associated with it,
'sampling'. On the one hand, it allows a representative picture of
what's going on in the system to be cheaply taken, but on the other
hand, that cheapness limits its utility when that data suggests a
need to 'dive down' more deeply to discover what's really going
on. In such cases, the only way to see what's really going on is
to be able to look at (or summarize more intelligently) the
individual steps that go into the higher-level behavior exposed
by the coarse-grained profiling data.
</para>
<para>
As a concrete example, we can trace all the events we think might
be applicable to our workload:
<literallayout class='monospaced'>
root@crownbay:~# perf record -g -e skb:* -e net:* -e napi:* -e sched:sched_switch -e sched:sched_wakeup -e irq:*
-e syscalls:sys_enter_read -e syscalls:sys_exit_read -e syscalls:sys_enter_write -e syscalls:sys_exit_write
wget <ulink url='http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2'>http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2</ulink>
</literallayout>
We can look at the raw trace output using 'perf script' with no
arguments:
<literallayout class='monospaced'>
root@crownbay:~# perf script
perf 1262 [000] 11624.857082: sys_exit_read: 0x0
perf 1262 [000] 11624.857193: sched_wakeup: comm=migration/0 pid=6 prio=0 success=1 target_cpu=000
wget 1262 [001] 11624.858021: softirq_raise: vec=1 [action=TIMER]
wget 1262 [001] 11624.858074: softirq_entry: vec=1 [action=TIMER]
wget 1262 [001] 11624.858081: softirq_exit: vec=1 [action=TIMER]
wget 1262 [001] 11624.858166: sys_enter_read: fd: 0x0003, buf: 0xbf82c940, count: 0x0200
wget 1262 [001] 11624.858177: sys_exit_read: 0x200
wget 1262 [001] 11624.858878: kfree_skb: skbaddr=0xeb248d80 protocol=0 location=0xc15a5308
wget 1262 [001] 11624.858945: kfree_skb: skbaddr=0xeb248000 protocol=0 location=0xc15a5308
wget 1262 [001] 11624.859020: softirq_raise: vec=1 [action=TIMER]
wget 1262 [001] 11624.859076: softirq_entry: vec=1 [action=TIMER]
wget 1262 [001] 11624.859083: softirq_exit: vec=1 [action=TIMER]
wget 1262 [001] 11624.859167: sys_enter_read: fd: 0x0003, buf: 0xb7720000, count: 0x0400
wget 1262 [001] 11624.859192: sys_exit_read: 0x1d7
wget 1262 [001] 11624.859228: sys_enter_read: fd: 0x0003, buf: 0xb7720000, count: 0x0400
wget 1262 [001] 11624.859233: sys_exit_read: 0x0
wget 1262 [001] 11624.859573: sys_enter_read: fd: 0x0003, buf: 0xbf82c580, count: 0x0200
wget 1262 [001] 11624.859584: sys_exit_read: 0x200
wget 1262 [001] 11624.859864: sys_enter_read: fd: 0x0003, buf: 0xb7720000, count: 0x0400
wget 1262 [001] 11624.859888: sys_exit_read: 0x400
wget 1262 [001] 11624.859935: sys_enter_read: fd: 0x0003, buf: 0xb7720000, count: 0x0400
wget 1262 [001] 11624.859944: sys_exit_read: 0x400
</literallayout>
This gives us a detailed timestamped sequence of events that
occurred within the workload with respect to those events.
</para>
<para>
In many ways, profiling can be viewed as a subset of tracing -
theoretically, if you have a set of trace events that's sufficient
to capture all the important aspects of a workload, you can derive
any of the results or views that a profiling run can.
</para>
<para>
Another aspect of traditional profiling is that while powerful in
many ways, it's limited by the granularity of the underlying data.
Profiling tools offer various ways of sorting and presenting the
sample data, which make it much more useful and amenable to user
experimentation, but in the end it can't be used in an open-ended
way to extract data that just isn't present as a consequence of
the fact that conceptually, most of it has been thrown away.
</para>
<para>
Full-blown detailed tracing data does however offer the opportunity
to manipulate and present the information collected during a
tracing run in an infinite variety of ways.
</para>
<para>
Another way to look at it is that there are only so many ways that
the 'primitive' counters can be used on their own to generate
interesting output; to get anything more complicated than simple
counts requires some amount of additional logic, which is typically
very specific to the problem at hand. For example, if we wanted to
make use of a 'counter' that maps to the value of the time
difference between when a process was scheduled to run on a
processor and the time it actually ran, we wouldn't expect such
a counter to exist on its own, but we could derive one called say
'wakeup_latency' and use it to extract a useful view of that metric
from trace data. Likewise, we really can't figure out from standard
profiling tools how much data every process on the system reads and
writes, along with how many of those reads and writes fail
completely. If we have sufficient trace data, however, we could
with the right tools easily extract and present that information,
but we'd need something other than pre-canned profiling tools to
do that.
</para>
<para>
Luckily, there is a general-purpose way to handle such needs,
called 'programming languages'. Making programming languages
easily available to apply to such problems given the specific
format of data is called a 'programming language binding' for
that data and language. Perf supports two programming language
bindings, one for Python and one for Perl.
</para>
<informalexample>
<emphasis>Tying it Together:</emphasis> Language bindings for manipulating and
aggregating trace data are of course not a new
idea. One of the first projects to do this was IBM's DProbes
dpcc compiler, an ANSI C compiler which targeted a low-level
assembly language running on an in-kernel interpreter on the
target system. This is exactly analogous to what Sun's DTrace
did, except that DTrace invented its own language for the purpose.
Systemtap, heavily inspired by DTrace, also created its own
one-off language, but rather than running the product on an
in-kernel interpreter, created an elaborate compiler-based
machinery to translate its language into kernel modules written
in C.
</informalexample>
<para>
Now that we have the trace data in perf.data, we can use
'perf script -g' to generate a skeleton script with handlers
for the read/write entry/exit events we recorded:
<literallayout class='monospaced'>
root@crownbay:~# perf script -g python
generated Python script: perf-script.py
</literallayout>
The skeleton script simply creates a python function for each
event type in the perf.data file. The body of each function simply
prints the event name along with its parameters. For example:
<literallayout class='monospaced'>
def net__netif_rx(event_name, context, common_cpu,
common_secs, common_nsecs, common_pid, common_comm,
skbaddr, len, name):
print_header(event_name, common_cpu, common_secs, common_nsecs,
common_pid, common_comm)
print "skbaddr=%u, len=%u, name=%s\n" % (skbaddr, len, name),
</literallayout>
We can run that script directly to print all of the events
contained in the perf.data file:
<literallayout class='monospaced'>
root@crownbay:~# perf script -s perf-script.py
in trace_begin
syscalls__sys_exit_read 0 11624.857082795 1262 perf nr=3, ret=0
sched__sched_wakeup 0 11624.857193498 1262 perf comm=migration/0, pid=6, prio=0, success=1, target_cpu=0
irq__softirq_raise 1 11624.858021635 1262 wget vec=TIMER
irq__softirq_entry 1 11624.858074075 1262 wget vec=TIMER
irq__softirq_exit 1 11624.858081389 1262 wget vec=TIMER
syscalls__sys_enter_read 1 11624.858166434 1262 wget nr=3, fd=3, buf=3213019456, count=512
syscalls__sys_exit_read 1 11624.858177924 1262 wget nr=3, ret=512
skb__kfree_skb 1 11624.858878188 1262 wget skbaddr=3945041280, location=3243922184, protocol=0
skb__kfree_skb 1 11624.858945608 1262 wget skbaddr=3945037824, location=3243922184, protocol=0
irq__softirq_raise 1 11624.859020942 1262 wget vec=TIMER
irq__softirq_entry 1 11624.859076935 1262 wget vec=TIMER
irq__softirq_exit 1 11624.859083469 1262 wget vec=TIMER
syscalls__sys_enter_read 1 11624.859167565 1262 wget nr=3, fd=3, buf=3077701632, count=1024
syscalls__sys_exit_read 1 11624.859192533 1262 wget nr=3, ret=471
syscalls__sys_enter_read 1 11624.859228072 1262 wget nr=3, fd=3, buf=3077701632, count=1024
syscalls__sys_exit_read 1 11624.859233707 1262 wget nr=3, ret=0
syscalls__sys_enter_read 1 11624.859573008 1262 wget nr=3, fd=3, buf=3213018496, count=512
syscalls__sys_exit_read 1 11624.859584818 1262 wget nr=3, ret=512
syscalls__sys_enter_read 1 11624.859864562 1262 wget nr=3, fd=3, buf=3077701632, count=1024
syscalls__sys_exit_read 1 11624.859888770 1262 wget nr=3, ret=1024
syscalls__sys_enter_read 1 11624.859935140 1262 wget nr=3, fd=3, buf=3077701632, count=1024
syscalls__sys_exit_read 1 11624.859944032 1262 wget nr=3, ret=1024
</literallayout>
That in itself isn't very useful; after all, we can accomplish
pretty much the same thing by simply running 'perf script'
without arguments in the same directory as the perf.data file.
</para>
<para>
We can however replace the print statements in the generated
function bodies with whatever we want, and thereby make it
infinitely more useful.
</para>
<para>
As a simple example, let's just replace the print statements in
the function bodies with a simple function that does nothing but
increment a per-event count. When the program is run against a
perf.data file, each time a particular event is encountered,
a tally is incremented for that event. For example:
<literallayout class='monospaced'>
def net__netif_rx(event_name, context, common_cpu,
common_secs, common_nsecs, common_pid, common_comm,
skbaddr, len, name):
inc_counts(event_name)
</literallayout>
Each event handler function in the generated code is modified
to do this. For convenience, we define a common function called
inc_counts() that each handler calls; inc_counts() simply tallies
a count for each event using the 'counts' hash, which is a
specialized hash function that does Perl-like autovivification, a
capability that's extremely useful for kinds of multi-level
aggregation commonly used in processing traces (see perf's
documentation on the Python language binding for details):
<literallayout class='monospaced'>
counts = autodict()
def inc_counts(event_name):
try:
counts[event_name] += 1
except TypeError:
counts[event_name] = 1
</literallayout>
Finally, at the end of the trace processing run, we want to
print the result of all the per-event tallies. For that, we
use the special 'trace_end()' function:
<literallayout class='monospaced'>
def trace_end():
for event_name, count in counts.iteritems():
print "%-40s %10s\n" % (event_name, count)
</literallayout>
The end result is a summary of all the events recorded in the
trace:
<literallayout class='monospaced'>
skb__skb_copy_datagram_iovec 13148
irq__softirq_entry 4796
irq__irq_handler_exit 3805
irq__softirq_exit 4795
syscalls__sys_enter_write 8990
net__net_dev_xmit 652
skb__kfree_skb 4047
sched__sched_wakeup 1155
irq__irq_handler_entry 3804
irq__softirq_raise 4799
net__net_dev_queue 652
syscalls__sys_enter_read 17599
net__netif_receive_skb 1743
syscalls__sys_exit_read 17598
net__netif_rx 2
napi__napi_poll 1877
syscalls__sys_exit_write 8990
</literallayout>
Note that this is pretty much exactly the same information we get
from 'perf stat', which goes a little way to support the idea
mentioned previously that given the right kind of trace data,
higher-level profiling-type summaries can be derived from it.
</para>
<para>
Documentation on using the
<ulink url='http://linux.die.net/man/1/perf-script-python'>'perf script' python binding</ulink>.
</para>
</section>
<section id='system-wide-tracing-and-profiling'>
<title>System-Wide Tracing and Profiling</title>
<para>
The examples so far have focused on tracing a particular program or
workload - in other words, every profiling run has specified the
program to profile in the command-line e.g. 'perf record wget ...'.
</para>
<para>
It's also possible, and more interesting in many cases, to run a
system-wide profile or trace while running the workload in a
separate shell.
</para>
<para>
To do system-wide profiling or tracing, you typically use
the -a flag to 'perf record'.
</para>
<para>
To demonstrate this, open up one window and start the profile
using the -a flag (press Ctrl-C to stop tracing):
<literallayout class='monospaced'>
root@crownbay:~# perf record -g -a
^C[ perf record: Woken up 6 times to write data ]
[ perf record: Captured and wrote 1.400 MB perf.data (~61172 samples) ]
</literallayout>
In another window, run the wget test:
<literallayout class='monospaced'>
root@crownbay:~# wget <ulink url='http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2'>http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2</ulink>
Connecting to downloads.yoctoproject.org (140.211.169.59:80)
linux-2.6.19.2.tar.b 100% |*******************************| 41727k 0:00:00 ETA
</literallayout>
Here we see entries not only for our wget load, but for other
processes running on the system as well:
</para>
<para>
<imagedata fileref="figures/perf-systemwide.png" width="6in" depth="7in" align="center" scalefit="1" />
</para>
<para>
In the snapshot above, we can see callchains that originate in
libc, and a callchain from Xorg that demonstrates that we're
using a proprietary X driver in userspace (notice the presence
of 'PVR' and some other unresolvable symbols in the expanded
Xorg callchain).
</para>
<para>
Note also that we have both kernel and userspace entries in the
above snapshot. We can also tell perf to focus on userspace but
providing a modifier, in this case 'u', to the 'cycles' hardware
counter when we record a profile:
<literallayout class='monospaced'>
root@crownbay:~# perf record -g -a -e cycles:u
^C[ perf record: Woken up 2 times to write data ]
[ perf record: Captured and wrote 0.376 MB perf.data (~16443 samples) ]
</literallayout>
</para>
<para>
<imagedata fileref="figures/perf-report-cycles-u.png" width="6in" depth="7in" align="center" scalefit="1" />
</para>
<para>
Notice in the screenshot above, we see only userspace entries ([.])
</para>
<para>
Finally, we can press 'enter' on a leaf node and select the 'Zoom
into DSO' menu item to show only entries associated with a
specific DSO. In the screenshot below, we've zoomed into the
'libc' DSO which shows all the entries associated with the
libc-xxx.so DSO.
</para>
<para>
<imagedata fileref="figures/perf-systemwide-libc.png" width="6in" depth="7in" align="center" scalefit="1" />
</para>
<para>
We can also use the system-wide -a switch to do system-wide
tracing. Here we'll trace a couple of scheduler events:
<literallayout class='monospaced'>
root@crownbay:~# perf record -a -e sched:sched_switch -e sched:sched_wakeup
^C[ perf record: Woken up 38 times to write data ]
[ perf record: Captured and wrote 9.780 MB perf.data (~427299 samples) ]
</literallayout>
We can look at the raw output using 'perf script' with no
arguments:
<literallayout class='monospaced'>
root@crownbay:~# perf script
perf 1383 [001] 6171.460045: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
perf 1383 [001] 6171.460066: sched_switch: prev_comm=perf prev_pid=1383 prev_prio=120 prev_state=R+ ==> next_comm=kworker/1:1 next_pid=21 next_prio=120
kworker/1:1 21 [001] 6171.460093: sched_switch: prev_comm=kworker/1:1 prev_pid=21 prev_prio=120 prev_state=S ==> next_comm=perf next_pid=1383 next_prio=120
swapper 0 [000] 6171.468063: sched_wakeup: comm=kworker/0:3 pid=1209 prio=120 success=1 target_cpu=000
swapper 0 [000] 6171.468107: sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/0:3 next_pid=1209 next_prio=120
kworker/0:3 1209 [000] 6171.468143: sched_switch: prev_comm=kworker/0:3 prev_pid=1209 prev_prio=120 prev_state=S ==> next_comm=swapper/0 next_pid=0 next_prio=120
perf 1383 [001] 6171.470039: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
perf 1383 [001] 6171.470058: sched_switch: prev_comm=perf prev_pid=1383 prev_prio=120 prev_state=R+ ==> next_comm=kworker/1:1 next_pid=21 next_prio=120
kworker/1:1 21 [001] 6171.470082: sched_switch: prev_comm=kworker/1:1 prev_pid=21 prev_prio=120 prev_state=S ==> next_comm=perf next_pid=1383 next_prio=120
perf 1383 [001] 6171.480035: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
</literallayout>
</para>
<section id='perf-filtering'>
<title>Filtering</title>
<para>
Notice that there are a lot of events that don't really have
anything to do with what we're interested in, namely events
that schedule 'perf' itself in and out or that wake perf up.
We can get rid of those by using the '--filter' option -
for each event we specify using -e, we can add a --filter
after that to filter out trace events that contain fields
with specific values:
<literallayout class='monospaced'>
root@crownbay:~# perf record -a -e sched:sched_switch --filter 'next_comm != perf && prev_comm != perf' -e sched:sched_wakeup --filter 'comm != perf'
^C[ perf record: Woken up 38 times to write data ]
[ perf record: Captured and wrote 9.688 MB perf.data (~423279 samples) ]
root@crownbay:~# perf script
swapper 0 [000] 7932.162180: sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/0:3 next_pid=1209 next_prio=120
kworker/0:3 1209 [000] 7932.162236: sched_switch: prev_comm=kworker/0:3 prev_pid=1209 prev_prio=120 prev_state=S ==> next_comm=swapper/0 next_pid=0 next_prio=120
perf 1407 [001] 7932.170048: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
perf 1407 [001] 7932.180044: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
perf 1407 [001] 7932.190038: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
perf 1407 [001] 7932.200044: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
perf 1407 [001] 7932.210044: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
perf 1407 [001] 7932.220044: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
swapper 0 [001] 7932.230111: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
swapper 0 [001] 7932.230146: sched_switch: prev_comm=swapper/1 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/1:1 next_pid=21 next_prio=120
kworker/1:1 21 [001] 7932.230205: sched_switch: prev_comm=kworker/1:1 prev_pid=21 prev_prio=120 prev_state=S ==> next_comm=swapper/1 next_pid=0 next_prio=120
swapper 0 [000] 7932.326109: sched_wakeup: comm=kworker/0:3 pid=1209 prio=120 success=1 target_cpu=000
swapper 0 [000] 7932.326171: sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/0:3 next_pid=1209 next_prio=120
kworker/0:3 1209 [000] 7932.326214: sched_switch: prev_comm=kworker/0:3 prev_pid=1209 prev_prio=120 prev_state=S ==> next_comm=swapper/0 next_pid=0 next_prio=120
</literallayout>
In this case, we've filtered out all events that have 'perf'
in their 'comm' or 'comm_prev' or 'comm_next' fields. Notice
that there are still events recorded for perf, but notice
that those events don't have values of 'perf' for the filtered
fields. To completely filter out anything from perf will
require a bit more work, but for the purpose of demonstrating
how to use filters, it's close enough.
</para>
<informalexample>
<emphasis>Tying it Together:</emphasis> These are exactly the same set of event
filters defined by the trace event subsystem. See the
ftrace/tracecmd/kernelshark section for more discussion about
these event filters.
</informalexample>
<informalexample>
<emphasis>Tying it Together:</emphasis> These event filters are implemented by a
special-purpose pseudo-interpreter in the kernel and are an
integral and indispensable part of the perf design as it
relates to tracing. kernel-based event filters provide a
mechanism to precisely throttle the event stream that appears
in user space, where it makes sense to provide bindings to real
programming languages for postprocessing the event stream.
This architecture allows for the intelligent and flexible
partitioning of processing between the kernel and user space.
Contrast this with other tools such as SystemTap, which does
all of its processing in the kernel and as such requires a
special project-defined language in order to accommodate that
design, or LTTng, where everything is sent to userspace and
as such requires a super-efficient kernel-to-userspace
transport mechanism in order to function properly. While
perf certainly can benefit from for instance advances in
the design of the transport, it doesn't fundamentally depend
on them. Basically, if you find that your perf tracing
application is causing buffer I/O overruns, it probably
means that you aren't taking enough advantage of the
kernel filtering engine.
</informalexample>
</section>
</section>
<section id='using-dynamic-tracepoints'>
<title>Using Dynamic Tracepoints</title>
<para>
perf isn't restricted to the fixed set of static tracepoints
listed by 'perf list'. Users can also add their own 'dynamic'
tracepoints anywhere in the kernel. For instance, suppose we
want to define our own tracepoint on do_fork(). We can do that
using the 'perf probe' perf subcommand:
<literallayout class='monospaced'>
root@crownbay:~# perf probe do_fork
Added new event:
probe:do_fork (on do_fork)
You can now use it in all perf tools, such as:
perf record -e probe:do_fork -aR sleep 1
</literallayout>
Adding a new tracepoint via 'perf probe' results in an event
with all the expected files and format in
/sys/kernel/debug/tracing/events, just the same as for static
tracepoints (as discussed in more detail in the trace events
subsystem section:
<literallayout class='monospaced'>
root@crownbay:/sys/kernel/debug/tracing/events/probe/do_fork# ls -al
drwxr-xr-x 2 root root 0 Oct 28 11:42 .
drwxr-xr-x 3 root root 0 Oct 28 11:42 ..
-rw-r--r-- 1 root root 0 Oct 28 11:42 enable
-rw-r--r-- 1 root root 0 Oct 28 11:42 filter
-r--r--r-- 1 root root 0 Oct 28 11:42 format
-r--r--r-- 1 root root 0 Oct 28 11:42 id
root@crownbay:/sys/kernel/debug/tracing/events/probe/do_fork# cat format
name: do_fork
ID: 944
format:
field:unsigned short common_type; offset:0; size:2; signed:0;
field:unsigned char common_flags; offset:2; size:1; signed:0;
field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
field:int common_pid; offset:4; size:4; signed:1;
field:int common_padding; offset:8; size:4; signed:1;
field:unsigned long __probe_ip; offset:12; size:4; signed:0;
print fmt: "(%lx)", REC->__probe_ip
</literallayout>
We can list all dynamic tracepoints currently in existence:
<literallayout class='monospaced'>
root@crownbay:~# perf probe -l
probe:do_fork (on do_fork)
probe:schedule (on schedule)
</literallayout>
Let's record system-wide ('sleep 30' is a trick for recording
system-wide but basically do nothing and then wake up after
30 seconds):
<literallayout class='monospaced'>
root@crownbay:~# perf record -g -a -e probe:do_fork sleep 30
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.087 MB perf.data (~3812 samples) ]
</literallayout>
Using 'perf script' we can see each do_fork event that fired:
<literallayout class='monospaced'>
root@crownbay:~# perf script
# ========
# captured on: Sun Oct 28 11:55:18 2012
# hostname : crownbay
# os release : 3.4.11-yocto-standard
# perf version : 3.4.11
# arch : i686
# nrcpus online : 2
# nrcpus avail : 2
# cpudesc : Intel(R) Atom(TM) CPU E660 @ 1.30GHz
# cpuid : GenuineIntel,6,38,1
# total memory : 1017184 kB
# cmdline : /usr/bin/perf record -g -a -e probe:do_fork sleep 30
# event : name = probe:do_fork, type = 2, config = 0x3b0, config1 = 0x0, config2 = 0x0, excl_usr = 0, excl_kern
= 0, id = { 5, 6 }
# HEADER_CPU_TOPOLOGY info available, use -I to display
# ========
#
matchbox-deskto 1197 [001] 34211.378318: do_fork: (c1028460)
matchbox-deskto 1295 [001] 34211.380388: do_fork: (c1028460)
pcmanfm 1296 [000] 34211.632350: do_fork: (c1028460)
pcmanfm 1296 [000] 34211.639917: do_fork: (c1028460)
matchbox-deskto 1197 [001] 34217.541603: do_fork: (c1028460)
matchbox-deskto 1299 [001] 34217.543584: do_fork: (c1028460)
gthumb 1300 [001] 34217.697451: do_fork: (c1028460)
gthumb 1300 [001] 34219.085734: do_fork: (c1028460)
gthumb 1300 [000] 34219.121351: do_fork: (c1028460)
gthumb 1300 [001] 34219.264551: do_fork: (c1028460)
pcmanfm 1296 [000] 34219.590380: do_fork: (c1028460)
matchbox-deskto 1197 [001] 34224.955965: do_fork: (c1028460)
matchbox-deskto 1306 [001] 34224.957972: do_fork: (c1028460)
matchbox-termin 1307 [000] 34225.038214: do_fork: (c1028460)
matchbox-termin 1307 [001] 34225.044218: do_fork: (c1028460)
matchbox-termin 1307 [000] 34225.046442: do_fork: (c1028460)
matchbox-deskto 1197 [001] 34237.112138: do_fork: (c1028460)
matchbox-deskto 1311 [001] 34237.114106: do_fork: (c1028460)
gaku 1312 [000] 34237.202388: do_fork: (c1028460)
</literallayout>
And using 'perf report' on the same file, we can see the
callgraphs from starting a few programs during those 30 seconds:
</para>
<para>
<imagedata fileref="figures/perf-probe-do_fork-profile.png" width="6in" depth="7in" align="center" scalefit="1" />
</para>
<informalexample>
<emphasis>Tying it Together:</emphasis> The trace events subsystem accommodate static
and dynamic tracepoints in exactly the same way - there's no
difference as far as the infrastructure is concerned. See the
ftrace section for more details on the trace event subsystem.
</informalexample>
<informalexample>
<emphasis>Tying it Together:</emphasis> Dynamic tracepoints are implemented under the
covers by kprobes and uprobes. kprobes and uprobes are also used
by and in fact are the main focus of SystemTap.
</informalexample>
</section>
</section>
<section id='perf-documentation'>
<title>Documentation</title>
<para>
Online versions of the man pages for the commands discussed in this
section can be found here:
<itemizedlist>
<listitem><para>The <ulink url='http://linux.die.net/man/1/perf-stat'>'perf stat' manpage</ulink>.
</para></listitem>
<listitem><para>The <ulink url='http://linux.die.net/man/1/perf-record'>'perf record' manpage</ulink>.
</para></listitem>
<listitem><para>The <ulink url='http://linux.die.net/man/1/perf-report'>'perf report' manpage</ulink>.
</para></listitem>
<listitem><para>The <ulink url='http://linux.die.net/man/1/perf-probe'>'perf probe' manpage</ulink>.
</para></listitem>
<listitem><para>The <ulink url='http://linux.die.net/man/1/perf-script'>'perf script' manpage</ulink>.
</para></listitem>
<listitem><para>Documentation on using the
<ulink url='http://linux.die.net/man/1/perf-script-python'>'perf script' python binding</ulink>.
</para></listitem>
<listitem><para>The top-level
<ulink url='http://linux.die.net/man/1/perf'>perf(1) manpage</ulink>.
</para></listitem>
</itemizedlist>
</para>
<para>
Normally, you should be able to invoke the man pages via perf
itself e.g. 'perf help' or 'perf help record'.
</para>
<para>
However, by default Yocto doesn't install man pages, but perf
invokes the man pages for most help functionality. This is a bug
and is being addressed by a Yocto bug:
<ulink url='https://bugzilla.yoctoproject.org/show_bug.cgi?id=3388'>Bug 3388 - perf: enable man pages for basic 'help' functionality</ulink>.
</para>
<para>
The man pages in text form, along with some other files, such as
a set of examples, can be found in the 'perf' directory of the
kernel tree:
<literallayout class='monospaced'>
tools/perf/Documentation
</literallayout>
There's also a nice perf tutorial on the perf wiki that goes
into more detail than we do here in certain areas:
<ulink url='https://perf.wiki.kernel.org/index.php/Tutorial'>Perf Tutorial</ulink>
</para>
</section>
</section>
<section id='profile-manual-ftrace'>
<title>ftrace</title>
<para>
'ftrace' literally refers to the 'ftrace function tracer' but in
reality this encompasses a number of related tracers along with
the infrastructure that they all make use of.
</para>
<section id='ftrace-setup'>
<title>Setup</title>
<para>
For this section, we'll assume you've already performed the basic
setup outlined in the General Setup section.
</para>
<para>
ftrace, trace-cmd, and kernelshark run on the target system,
and are ready to go out-of-the-box - no additional setup is
necessary. For the rest of this section we assume you've ssh'ed
to the host and will be running ftrace on the target. kernelshark
is a GUI application and if you use the '-X' option to ssh you
can have the kernelshark GUI run on the target but display
remotely on the host if you want.
</para>
</section>
<section id='basic-ftrace-usage'>
<title>Basic ftrace usage</title>
<para>
'ftrace' essentially refers to everything included in
the /tracing directory of the mounted debugfs filesystem
(Yocto follows the standard convention and mounts it
at /sys/kernel/debug). Here's a listing of all the files
found in /sys/kernel/debug/tracing on a Yocto system:
<literallayout class='monospaced'>
root@sugarbay:/sys/kernel/debug/tracing# ls
README kprobe_events trace
available_events kprobe_profile trace_clock
available_filter_functions options trace_marker
available_tracers per_cpu trace_options
buffer_size_kb printk_formats trace_pipe
buffer_total_size_kb saved_cmdlines tracing_cpumask
current_tracer set_event tracing_enabled
dyn_ftrace_total_info set_ftrace_filter tracing_on
enabled_functions set_ftrace_notrace tracing_thresh
events set_ftrace_pid
free_buffer set_graph_function
</literallayout>
The files listed above are used for various purposes -
some relate directly to the tracers themselves, others are
used to set tracing options, and yet others actually contain
the tracing output when a tracer is in effect. Some of the
functions can be guessed from their names, others need
explanation; in any case, we'll cover some of the files we
see here below but for an explanation of the others, please
see the ftrace documentation.
</para>
<para>
We'll start by looking at some of the available built-in
tracers.
</para>
<para>
cat'ing the 'available_tracers' file lists the set of
available tracers:
<literallayout class='monospaced'>
root@sugarbay:/sys/kernel/debug/tracing# cat available_tracers
blk function_graph function nop
</literallayout>
The 'current_tracer' file contains the tracer currently in
effect:
<literallayout class='monospaced'>
root@sugarbay:/sys/kernel/debug/tracing# cat current_tracer
nop
</literallayout>
The above listing of current_tracer shows that
the 'nop' tracer is in effect, which is just another
way of saying that there's actually no tracer
currently in effect.
</para>
<para>
echo'ing one of the available_tracers into current_tracer
makes the specified tracer the current tracer:
<literallayout class='monospaced'>
root@sugarbay:/sys/kernel/debug/tracing# echo function > current_tracer
root@sugarbay:/sys/kernel/debug/tracing# cat current_tracer
function
</literallayout>
The above sets the current tracer to be the
'function tracer'. This tracer traces every function
call in the kernel and makes it available as the
contents of the 'trace' file. Reading the 'trace' file
lists the currently buffered function calls that have been
traced by the function tracer:
<literallayout class='monospaced'>
root@sugarbay:/sys/kernel/debug/tracing# cat trace | less
# tracer: function
#
# entries-in-buffer/entries-written: 310629/766471 #P:8
#
# _-----=> irqs-off
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth
# ||| / delay
# TASK-PID CPU# |||| TIMESTAMP FUNCTION
# | | | |||| | |
<idle>-0 [004] d..1 470.867169: ktime_get_real <-intel_idle
<idle>-0 [004] d..1 470.867170: getnstimeofday <-ktime_get_real
<idle>-0 [004] d..1 470.867171: ns_to_timeval <-intel_idle
<idle>-0 [004] d..1 470.867171: ns_to_timespec <-ns_to_timeval
<idle>-0 [004] d..1 470.867172: smp_apic_timer_interrupt <-apic_timer_interrupt
<idle>-0 [004] d..1 470.867172: native_apic_mem_write <-smp_apic_timer_interrupt
<idle>-0 [004] d..1 470.867172: irq_enter <-smp_apic_timer_interrupt
<idle>-0 [004] d..1 470.867172: rcu_irq_enter <-irq_enter
<idle>-0 [004] d..1 470.867173: rcu_idle_exit_common.isra.33 <-rcu_irq_enter
<idle>-0 [004] d..1 470.867173: local_bh_disable <-irq_enter
<idle>-0 [004] d..1 470.867173: add_preempt_count <-local_bh_disable
<idle>-0 [004] d.s1 470.867174: tick_check_idle <-irq_enter
<idle>-0 [004] d.s1 470.867174: tick_check_oneshot_broadcast <-tick_check_idle
<idle>-0 [004] d.s1 470.867174: ktime_get <-tick_check_idle
<idle>-0 [004] d.s1 470.867174: tick_nohz_stop_idle <-tick_check_idle
<idle>-0 [004] d.s1 470.867175: update_ts_time_stats <-tick_nohz_stop_idle
<idle>-0 [004] d.s1 470.867175: nr_iowait_cpu <-update_ts_time_stats
<idle>-0 [004] d.s1 470.867175: tick_do_update_jiffies64 <-tick_check_idle
<idle>-0 [004] d.s1 470.867175: _raw_spin_lock <-tick_do_update_jiffies64
<idle>-0 [004] d.s1 470.867176: add_preempt_count <-_raw_spin_lock
<idle>-0 [004] d.s2 470.867176: do_timer <-tick_do_update_jiffies64
<idle>-0 [004] d.s2 470.867176: _raw_spin_lock <-do_timer
<idle>-0 [004] d.s2 470.867176: add_preempt_count <-_raw_spin_lock
<idle>-0 [004] d.s3 470.867177: ntp_tick_length <-do_timer
<idle>-0 [004] d.s3 470.867177: _raw_spin_lock_irqsave <-ntp_tick_length
.
.
.
</literallayout>
Each line in the trace above shows what was happening in
the kernel on a given cpu, to the level of detail of
function calls. Each entry shows the function called,
followed by its caller (after the arrow).
</para>
<para>
The function tracer gives you an extremely detailed idea
of what the kernel was doing at the point in time the trace
was taken, and is a great way to learn about how the kernel
code works in a dynamic sense.
</para>
<informalexample>
<emphasis>Tying it Together:</emphasis> The ftrace function tracer is also
available from within perf, as the ftrace:function tracepoint.
</informalexample>
<para>
It is a little more difficult to follow the call chains than
it needs to be - luckily there's a variant of the function
tracer that displays the callchains explicitly, called the
'function_graph' tracer:
<literallayout class='monospaced'>
root@sugarbay:/sys/kernel/debug/tracing# echo function_graph > current_tracer
root@sugarbay:/sys/kernel/debug/tracing# cat trace | less
tracer: function_graph
CPU DURATION FUNCTION CALLS
| | | | | | |
7) 0.046 us | pick_next_task_fair();
7) 0.043 us | pick_next_task_stop();
7) 0.042 us | pick_next_task_rt();
7) 0.032 us | pick_next_task_fair();
7) 0.030 us | pick_next_task_idle();
7) | _raw_spin_unlock_irq() {
7) 0.033 us | sub_preempt_count();
7) 0.258 us | }
7) 0.032 us | sub_preempt_count();
7) + 13.341 us | } /* __schedule */
7) 0.095 us | } /* sub_preempt_count */
7) | schedule() {
7) | __schedule() {
7) 0.060 us | add_preempt_count();
7) 0.044 us | rcu_note_context_switch();
7) | _raw_spin_lock_irq() {
7) 0.033 us | add_preempt_count();
7) 0.247 us | }
7) | idle_balance() {
7) | _raw_spin_unlock() {
7) 0.031 us | sub_preempt_count();
7) 0.246 us | }
7) | update_shares() {
7) 0.030 us | __rcu_read_lock();
7) 0.029 us | __rcu_read_unlock();
7) 0.484 us | }
7) 0.030 us | __rcu_read_lock();
7) | load_balance() {
7) | find_busiest_group() {
7) 0.031 us | idle_cpu();
7) 0.029 us | idle_cpu();
7) 0.035 us | idle_cpu();
7) 0.906 us | }
7) 1.141 us | }
7) 0.022 us | msecs_to_jiffies();
7) | load_balance() {
7) | find_busiest_group() {
7) 0.031 us | idle_cpu();
.
.
.
4) 0.062 us | msecs_to_jiffies();
4) 0.062 us | __rcu_read_unlock();
4) | _raw_spin_lock() {
4) 0.073 us | add_preempt_count();
4) 0.562 us | }
4) + 17.452 us | }
4) 0.108 us | put_prev_task_fair();
4) 0.102 us | pick_next_task_fair();
4) 0.084 us | pick_next_task_stop();
4) 0.075 us | pick_next_task_rt();
4) 0.062 us | pick_next_task_fair();
4) 0.066 us | pick_next_task_idle();
------------------------------------------
4) kworker-74 => <idle>-0
------------------------------------------
4) | finish_task_switch() {
4) | _raw_spin_unlock_irq() {
4) 0.100 us | sub_preempt_count();
4) 0.582 us | }
4) 1.105 us | }
4) 0.088 us | sub_preempt_count();
4) ! 100.066 us | }
.
.
.
3) | sys_ioctl() {
3) 0.083 us | fget_light();
3) | security_file_ioctl() {
3) 0.066 us | cap_file_ioctl();
3) 0.562 us | }
3) | do_vfs_ioctl() {
3) | drm_ioctl() {
3) 0.075 us | drm_ut_debug_printk();
3) | i915_gem_pwrite_ioctl() {
3) | i915_mutex_lock_interruptible() {
3) 0.070 us | mutex_lock_interruptible();
3) 0.570 us | }
3) | drm_gem_object_lookup() {
3) | _raw_spin_lock() {
3) 0.080 us | add_preempt_count();
3) 0.620 us | }
3) | _raw_spin_unlock() {
3) 0.085 us | sub_preempt_count();
3) 0.562 us | }
3) 2.149 us | }
3) 0.133 us | i915_gem_object_pin();
3) | i915_gem_object_set_to_gtt_domain() {
3) 0.065 us | i915_gem_object_flush_gpu_write_domain();
3) 0.065 us | i915_gem_object_wait_rendering();
3) 0.062 us | i915_gem_object_flush_cpu_write_domain();
3) 1.612 us | }
3) | i915_gem_object_put_fence() {
3) 0.097 us | i915_gem_object_flush_fence.constprop.36();
3) 0.645 us | }
3) 0.070 us | add_preempt_count();
3) 0.070 us | sub_preempt_count();
3) 0.073 us | i915_gem_object_unpin();
3) 0.068 us | mutex_unlock();
3) 9.924 us | }
3) + 11.236 us | }
3) + 11.770 us | }
3) + 13.784 us | }
3) | sys_ioctl() {
</literallayout>
As you can see, the function_graph display is much easier to
follow. Also note that in addition to the function calls and
associated braces, other events such as scheduler events
are displayed in context. In fact, you can freely include
any tracepoint available in the trace events subsystem described
in the next section by simply enabling those events, and they'll
appear in context in the function graph display. Quite a
powerful tool for understanding kernel dynamics.
</para>
<para>
Also notice that there are various annotations on the left
hand side of the display. For example if the total time it
took for a given function to execute is above a certain
threshold, an exclamation point or plus sign appears on the
left hand side. Please see the ftrace documentation for
details on all these fields.
</para>
</section>
<section id='the-trace-events-subsystem'>
<title>The 'trace events' Subsystem</title>
<para>
One especially important directory contained within
the /sys/kernel/debug/tracing directory is the 'events'
subdirectory, which contains representations of every
tracepoint in the system. Listing out the contents of
the 'events' subdirectory, we see mainly another set of
subdirectories:
<literallayout class='monospaced'>
root@sugarbay:/sys/kernel/debug/tracing# cd events
root@sugarbay:/sys/kernel/debug/tracing/events# ls -al
drwxr-xr-x 38 root root 0 Nov 14 23:19 .
drwxr-xr-x 5 root root 0 Nov 14 23:19 ..
drwxr-xr-x 19 root root 0 Nov 14 23:19 block
drwxr-xr-x 32 root root 0 Nov 14 23:19 btrfs
drwxr-xr-x 5 root root 0 Nov 14 23:19 drm
-rw-r--r-- 1 root root 0 Nov 14 23:19 enable
drwxr-xr-x 40 root root 0 Nov 14 23:19 ext3
drwxr-xr-x 79 root root 0 Nov 14 23:19 ext4
drwxr-xr-x 14 root root 0 Nov 14 23:19 ftrace
drwxr-xr-x 8 root root 0 Nov 14 23:19 hda
-r--r--r-- 1 root root 0 Nov 14 23:19 header_event
-r--r--r-- 1 root root 0 Nov 14 23:19 header_page
drwxr-xr-x 25 root root 0 Nov 14 23:19 i915
drwxr-xr-x 7 root root 0 Nov 14 23:19 irq
drwxr-xr-x 12 root root 0 Nov 14 23:19 jbd
drwxr-xr-x 14 root root 0 Nov 14 23:19 jbd2
drwxr-xr-x 14 root root 0 Nov 14 23:19 kmem
drwxr-xr-x 7 root root 0 Nov 14 23:19 module
drwxr-xr-x 3 root root 0 Nov 14 23:19 napi
drwxr-xr-x 6 root root 0 Nov 14 23:19 net
drwxr-xr-x 3 root root 0 Nov 14 23:19 oom
drwxr-xr-x 12 root root 0 Nov 14 23:19 power
drwxr-xr-x 3 root root 0 Nov 14 23:19 printk
drwxr-xr-x 8 root root 0 Nov 14 23:19 random
drwxr-xr-x 4 root root 0 Nov 14 23:19 raw_syscalls
drwxr-xr-x 3 root root 0 Nov 14 23:19 rcu
drwxr-xr-x 6 root root 0 Nov 14 23:19 rpm
drwxr-xr-x 20 root root 0 Nov 14 23:19 sched
drwxr-xr-x 7 root root 0 Nov 14 23:19 scsi
drwxr-xr-x 4 root root 0 Nov 14 23:19 signal
drwxr-xr-x 5 root root 0 Nov 14 23:19 skb
drwxr-xr-x 4 root root 0 Nov 14 23:19 sock
drwxr-xr-x 10 root root 0 Nov 14 23:19 sunrpc
drwxr-xr-x 538 root root 0 Nov 14 23:19 syscalls
drwxr-xr-x 4 root root 0 Nov 14 23:19 task
drwxr-xr-x 14 root root 0 Nov 14 23:19 timer
drwxr-xr-x 3 root root 0 Nov 14 23:19 udp
drwxr-xr-x 21 root root 0 Nov 14 23:19 vmscan
drwxr-xr-x 3 root root 0 Nov 14 23:19 vsyscall
drwxr-xr-x 6 root root 0 Nov 14 23:19 workqueue
drwxr-xr-x 26 root root 0 Nov 14 23:19 writeback
</literallayout>
Each one of these subdirectories corresponds to a
'subsystem' and contains yet again more subdirectories,
each one of those finally corresponding to a tracepoint.
For example, here are the contents of the 'kmem' subsystem:
<literallayout class='monospaced'>
root@sugarbay:/sys/kernel/debug/tracing/events# cd kmem
root@sugarbay:/sys/kernel/debug/tracing/events/kmem# ls -al
drwxr-xr-x 14 root root 0 Nov 14 23:19 .
drwxr-xr-x 38 root root 0 Nov 14 23:19 ..
-rw-r--r-- 1 root root 0 Nov 14 23:19 enable
-rw-r--r-- 1 root root 0 Nov 14 23:19 filter
drwxr-xr-x 2 root root 0 Nov 14 23:19 kfree
drwxr-xr-x 2 root root 0 Nov 14 23:19 kmalloc
drwxr-xr-x 2 root root 0 Nov 14 23:19 kmalloc_node
drwxr-xr-x 2 root root 0 Nov 14 23:19 kmem_cache_alloc
drwxr-xr-x 2 root root 0 Nov 14 23:19 kmem_cache_alloc_node
drwxr-xr-x 2 root root 0 Nov 14 23:19 kmem_cache_free
drwxr-xr-x 2 root root 0 Nov 14 23:19 mm_page_alloc
drwxr-xr-x 2 root root 0 Nov 14 23:19 mm_page_alloc_extfrag
drwxr-xr-x 2 root root 0 Nov 14 23:19 mm_page_alloc_zone_locked
drwxr-xr-x 2 root root 0 Nov 14 23:19 mm_page_free
drwxr-xr-x 2 root root 0 Nov 14 23:19 mm_page_free_batched
drwxr-xr-x 2 root root 0 Nov 14 23:19 mm_page_pcpu_drain
</literallayout>
Let's see what's inside the subdirectory for a specific
tracepoint, in this case the one for kmalloc:
<literallayout class='monospaced'>
root@sugarbay:/sys/kernel/debug/tracing/events/kmem# cd kmalloc
root@sugarbay:/sys/kernel/debug/tracing/events/kmem/kmalloc# ls -al
drwxr-xr-x 2 root root 0 Nov 14 23:19 .
drwxr-xr-x 14 root root 0 Nov 14 23:19 ..
-rw-r--r-- 1 root root 0 Nov 14 23:19 enable
-rw-r--r-- 1 root root 0 Nov 14 23:19 filter
-r--r--r-- 1 root root 0 Nov 14 23:19 format
-r--r--r-- 1 root root 0 Nov 14 23:19 id
</literallayout>
The 'format' file for the tracepoint describes the event
in memory, which is used by the various tracing tools
that now make use of these tracepoint to parse the event
and make sense of it, along with a 'print fmt' field that
allows tools like ftrace to display the event as text.
Here's what the format of the kmalloc event looks like:
<literallayout class='monospaced'>
root@sugarbay:/sys/kernel/debug/tracing/events/kmem/kmalloc# cat format
name: kmalloc
ID: 313
format:
field:unsigned short common_type; offset:0; size:2; signed:0;
field:unsigned char common_flags; offset:2; size:1; signed:0;
field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
field:int common_pid; offset:4; size:4; signed:1;
field:int common_padding; offset:8; size:4; signed:1;
field:unsigned long call_site; offset:16; size:8; signed:0;
field:const void * ptr; offset:24; size:8; signed:0;
field:size_t bytes_req; offset:32; size:8; signed:0;
field:size_t bytes_alloc; offset:40; size:8; signed:0;
field:gfp_t gfp_flags; offset:48; size:4; signed:0;
print fmt: "call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s", REC->call_site, REC->ptr, REC->bytes_req, REC->bytes_alloc,
(REC->gfp_flags) ? __print_flags(REC->gfp_flags, "|", {(unsigned long)(((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | ((
gfp_t)0x20000u) | (( gfp_t)0x02u) | (( gfp_t)0x08u)) | (( gfp_t)0x4000u) | (( gfp_t)0x10000u) | (( gfp_t)0x1000u) | (( gfp_t)0x200u) | ((
gfp_t)0x400000u)), "GFP_TRANSHUGE"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | (( gfp_t)0x20000u) | ((
gfp_t)0x02u) | (( gfp_t)0x08u)), "GFP_HIGHUSER_MOVABLE"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | ((
gfp_t)0x20000u) | (( gfp_t)0x02u)), "GFP_HIGHUSER"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | ((
gfp_t)0x20000u)), "GFP_USER"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | (( gfp_t)0x80000u)), GFP_TEMPORARY"},
{(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u)), "GFP_KERNEL"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u)),
"GFP_NOFS"}, {(unsigned long)((( gfp_t)0x20u)), "GFP_ATOMIC"}, {(unsigned long)((( gfp_t)0x10u)), "GFP_NOIO"}, {(unsigned long)((
gfp_t)0x20u), "GFP_HIGH"}, {(unsigned long)(( gfp_t)0x10u), "GFP_WAIT"}, {(unsigned long)(( gfp_t)0x40u), "GFP_IO"}, {(unsigned long)((
gfp_t)0x100u), "GFP_COLD"}, {(unsigned long)(( gfp_t)0x200u), "GFP_NOWARN"}, {(unsigned long)(( gfp_t)0x400u), "GFP_REPEAT"}, {(unsigned
long)(( gfp_t)0x800u), "GFP_NOFAIL"}, {(unsigned long)(( gfp_t)0x1000u), "GFP_NORETRY"}, {(unsigned long)(( gfp_t)0x4000u), "GFP_COMP"},
{(unsigned long)(( gfp_t)0x8000u), "GFP_ZERO"}, {(unsigned long)(( gfp_t)0x10000u), "GFP_NOMEMALLOC"}, {(unsigned long)(( gfp_t)0x20000u),
"GFP_HARDWALL"}, {(unsigned long)(( gfp_t)0x40000u), "GFP_THISNODE"}, {(unsigned long)(( gfp_t)0x80000u), "GFP_RECLAIMABLE"}, {(unsigned
long)(( gfp_t)0x08u), "GFP_MOVABLE"}, {(unsigned long)(( gfp_t)0), "GFP_NOTRACK"}, {(unsigned long)(( gfp_t)0x400000u), "GFP_NO_KSWAPD"},
{(unsigned long)(( gfp_t)0x800000u), "GFP_OTHER_NODE"} ) : "GFP_NOWAIT"
</literallayout>
The 'enable' file in the tracepoint directory is what allows
the user (or tools such as trace-cmd) to actually turn the
tracepoint on and off. When enabled, the corresponding
tracepoint will start appearing in the ftrace 'trace'
file described previously. For example, this turns on the
kmalloc tracepoint:
<literallayout class='monospaced'>
root@sugarbay:/sys/kernel/debug/tracing/events/kmem/kmalloc# echo 1 > enable
</literallayout>
At the moment, we're not interested in the function tracer or
some other tracer that might be in effect, so we first turn
it off, but if we do that, we still need to turn tracing on in
order to see the events in the output buffer:
<literallayout class='monospaced'>
root@sugarbay:/sys/kernel/debug/tracing# echo nop > current_tracer
root@sugarbay:/sys/kernel/debug/tracing# echo 1 > tracing_on
</literallayout>
Now, if we look at the the 'trace' file, we see nothing
but the kmalloc events we just turned on:
<literallayout class='monospaced'>
root@sugarbay:/sys/kernel/debug/tracing# cat trace | less
# tracer: nop
#
# entries-in-buffer/entries-written: 1897/1897 #P:8
#
# _-----=> irqs-off
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth
# ||| / delay
# TASK-PID CPU# |||| TIMESTAMP FUNCTION
# | | | |||| | |
dropbear-1465 [000] ...1 18154.620753: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
<idle>-0 [000] ..s3 18154.621640: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
<idle>-0 [000] ..s3 18154.621656: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
matchbox-termin-1361 [001] ...1 18154.755472: kmalloc: call_site=ffffffff81614050 ptr=ffff88006d5f0e00 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_KERNEL|GFP_REPEAT
Xorg-1264 [002] ...1 18154.755581: kmalloc: call_site=ffffffff8141abe8 ptr=ffff8800734f4cc0 bytes_req=168 bytes_alloc=192 gfp_flags=GFP_KERNEL|GFP_NOWARN|GFP_NORETRY
Xorg-1264 [002] ...1 18154.755583: kmalloc: call_site=ffffffff814192a3 ptr=ffff88001f822520 bytes_req=24 bytes_alloc=32 gfp_flags=GFP_KERNEL|GFP_ZERO
Xorg-1264 [002] ...1 18154.755589: kmalloc: call_site=ffffffff81419edb ptr=ffff8800721a2f00 bytes_req=64 bytes_alloc=64 gfp_flags=GFP_KERNEL|GFP_ZERO
matchbox-termin-1361 [001] ...1 18155.354594: kmalloc: call_site=ffffffff81614050 ptr=ffff88006db35400 bytes_req=576 bytes_alloc=1024 gfp_flags=GFP_KERNEL|GFP_REPEAT
Xorg-1264 [002] ...1 18155.354703: kmalloc: call_site=ffffffff8141abe8 ptr=ffff8800734f4cc0 bytes_req=168 bytes_alloc=192 gfp_flags=GFP_KERNEL|GFP_NOWARN|GFP_NORETRY
Xorg-1264 [002] ...1 18155.354705: kmalloc: call_site=ffffffff814192a3 ptr=ffff88001f822520 bytes_req=24 bytes_alloc=32 gfp_flags=GFP_KERNEL|GFP_ZERO
Xorg-1264 [002] ...1 18155.354711: kmalloc: call_site=ffffffff81419edb ptr=ffff8800721a2f00 bytes_req=64 bytes_alloc=64 gfp_flags=GFP_KERNEL|GFP_ZERO
<idle>-0 [000] ..s3 18155.673319: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
dropbear-1465 [000] ...1 18155.673525: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
<idle>-0 [000] ..s3 18155.674821: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
<idle>-0 [000] ..s3 18155.793014: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
dropbear-1465 [000] ...1 18155.793219: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
<idle>-0 [000] ..s3 18155.794147: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
<idle>-0 [000] ..s3 18155.936705: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
dropbear-1465 [000] ...1 18155.936910: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
<idle>-0 [000] ..s3 18155.937869: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
matchbox-termin-1361 [001] ...1 18155.953667: kmalloc: call_site=ffffffff81614050 ptr=ffff88006d5f2000 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_KERNEL|GFP_REPEAT
Xorg-1264 [002] ...1 18155.953775: kmalloc: call_site=ffffffff8141abe8 ptr=ffff8800734f4cc0 bytes_req=168 bytes_alloc=192 gfp_flags=GFP_KERNEL|GFP_NOWARN|GFP_NORETRY
Xorg-1264 [002] ...1 18155.953777: kmalloc: call_site=ffffffff814192a3 ptr=ffff88001f822520 bytes_req=24 bytes_alloc=32 gfp_flags=GFP_KERNEL|GFP_ZERO
Xorg-1264 [002] ...1 18155.953783: kmalloc: call_site=ffffffff81419edb ptr=ffff8800721a2f00 bytes_req=64 bytes_alloc=64 gfp_flags=GFP_KERNEL|GFP_ZERO
<idle>-0 [000] ..s3 18156.176053: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
dropbear-1465 [000] ...1 18156.176257: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
<idle>-0 [000] ..s3 18156.177717: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
<idle>-0 [000] ..s3 18156.399229: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
dropbear-1465 [000] ...1 18156.399434: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_http://rostedt.homelinux.com/kernelshark/req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
<idle>-0 [000] ..s3 18156.400660: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
matchbox-termin-1361 [001] ...1 18156.552800: kmalloc: call_site=ffffffff81614050 ptr=ffff88006db34800 bytes_req=576 bytes_alloc=1024 gfp_flags=GFP_KERNEL|GFP_REPEAT
</literallayout>
To again disable the kmalloc event, we need to send 0 to the
enable file:
<literallayout class='monospaced'>
root@sugarbay:/sys/kernel/debug/tracing/events/kmem/kmalloc# echo 0 > enable
</literallayout>
You can enable any number of events or complete subsystems
(by using the 'enable' file in the subsystem directory) and
get an arbitrarily fine-grained idea of what's going on in the
system by enabling as many of the appropriate tracepoints
as applicable.
</para>
<para>
A number of the tools described in this HOWTO do just that,
including trace-cmd and kernelshark in the next section.
</para>
<informalexample>
<emphasis>Tying it Together:</emphasis> These tracepoints and their representation
are used not only by ftrace, but by many of the other tools
covered in this document and they form a central point of
integration for the various tracers available in Linux.
They form a central part of the instrumentation for the
following tools: perf, lttng, ftrace, blktrace and SystemTap
</informalexample>
<informalexample>
<emphasis>Tying it Together:</emphasis> Eventually all the special-purpose tracers
currently available in /sys/kernel/debug/tracing will be
removed and replaced with equivalent tracers based on the
'trace events' subsystem.
</informalexample>
</section>
<section id='trace-cmd-kernelshark'>
<title>trace-cmd/kernelshark</title>
<para>
trace-cmd is essentially an extensive command-line 'wrapper'
interface that hides the details of all the individual files
in /sys/kernel/debug/tracing, allowing users to specify
specific particular events within the
/sys/kernel/debug/tracing/events/ subdirectory and to collect
traces and avoid having to deal with those details directly.
</para>
<para>
As yet another layer on top of that, kernelshark provides a GUI
that allows users to start and stop traces and specify sets
of events using an intuitive interface, and view the
output as both trace events and as a per-CPU graphical
display. It directly uses 'trace-cmd' as the plumbing
that accomplishes all that underneath the covers (and
actually displays the trace-cmd command it uses, as we'll see).
</para>
<para>
To start a trace using kernelshark, first start kernelshark:
<literallayout class='monospaced'>
root@sugarbay:~# kernelshark
</literallayout>
Then bring up the 'Capture' dialog by choosing from the
kernelshark menu:
<literallayout class='monospaced'>
Capture | Record
</literallayout>
That will display the following dialog, which allows you to
choose one or more events (or even one or more complete
subsystems) to trace:
</para>
<para>
<imagedata fileref="figures/kernelshark-choose-events.png" width="6in" depth="6in" align="center" scalefit="1" />
</para>
<para>
Note that these are exactly the same sets of events described
in the previous trace events subsystem section, and in fact
is where trace-cmd gets them for kernelshark.
</para>
<para>
In the above screenshot, we've decided to explore the
graphics subsystem a bit and so have chosen to trace all
the tracepoints contained within the 'i915' and 'drm'
subsystems.
</para>
<para>
After doing that, we can start and stop the trace using
the 'Run' and 'Stop' button on the lower right corner of
the dialog (the same button will turn into the 'Stop'
button after the trace has started):
</para>
<para>
<imagedata fileref="figures/kernelshark-output-display.png" width="6in" depth="6in" align="center" scalefit="1" />
</para>
<para>
Notice that the right-hand pane shows the exact trace-cmd
command-line that's used to run the trace, along with the
results of the trace-cmd run.
</para>
<para>
Once the 'Stop' button is pressed, the graphical view magically
fills up with a colorful per-cpu display of the trace data,
along with the detailed event listing below that:
</para>
<para>
<imagedata fileref="figures/kernelshark-i915-display.png" width="6in" depth="7in" align="center" scalefit="1" />
</para>
<para>
Here's another example, this time a display resulting
from tracing 'all events':
</para>
<para>
<imagedata fileref="figures/kernelshark-all.png" width="6in" depth="7in" align="center" scalefit="1" />
</para>
<para>
The tool is pretty self-explanatory, but for more detailed
information on navigating through the data, see the
<ulink url='http://rostedt.homelinux.com/kernelshark/'>kernelshark website</ulink>.
</para>
</section>
<section id='ftrace-documentation'>
<title>Documentation</title>
<para>
The documentation for ftrace can be found in the kernel
Documentation directory:
<literallayout class='monospaced'>
Documentation/trace/ftrace.txt
</literallayout>
The documentation for the trace event subsystem can also
be found in the kernel Documentation directory:
<literallayout class='monospaced'>
Documentation/trace/events.txt
</literallayout>
There is a nice series of articles on using
ftrace and trace-cmd at LWN:
<itemizedlist>
<listitem><para><ulink url='http://lwn.net/Articles/365835/'>Debugging the kernel using Ftrace - part 1</ulink>
</para></listitem>
<listitem><para><ulink url='http://lwn.net/Articles/366796/'>Debugging the kernel using Ftrace - part 2</ulink>
</para></listitem>
<listitem><para><ulink url='http://lwn.net/Articles/370423/'>Secrets of the Ftrace function tracer</ulink>
</para></listitem>
<listitem><para><ulink url='https://lwn.net/Articles/410200/'>trace-cmd: A front-end for Ftrace</ulink>
</para></listitem>
</itemizedlist>
</para>
<para>
There's more detailed documentation kernelshark usage here:
<ulink url='http://rostedt.homelinux.com/kernelshark/'>KernelShark</ulink>
</para>
<para>
An amusing yet useful README (a tracing mini-HOWTO) can be
found in /sys/kernel/debug/tracing/README.
</para>
</section>
</section>
<section id='profile-manual-systemtap'>
<title>systemtap</title>
<para>
SystemTap is a system-wide script-based tracing and profiling tool.
</para>
<para>
SystemTap scripts are C-like programs that are executed in the
kernel to gather/print/aggregate data extracted from the context
they end up being invoked under.
</para>
<para>
For example, this probe from the
<ulink url='http://sourceware.org/systemtap/tutorial/'>SystemTap tutorial</ulink>
simply prints a line every time any process on the system open()s
a file. For each line, it prints the executable name of the
program that opened the file, along with its PID, and the name
of the file it opened (or tried to open), which it extracts
from the open syscall's argstr.
<literallayout class='monospaced'>
probe syscall.open
{
printf ("%s(%d) open (%s)\n", execname(), pid(), argstr)
}
probe timer.ms(4000) # after 4 seconds
{
exit ()
}
</literallayout>
Normally, to execute this probe, you'd simply install
systemtap on the system you want to probe, and directly run
the probe on that system e.g. assuming the name of the file
containing the above text is trace_open.stp:
<literallayout class='monospaced'>
# stap trace_open.stp
</literallayout>
What systemtap does under the covers to run this probe is 1)
parse and convert the probe to an equivalent 'C' form, 2)
compile the 'C' form into a kernel module, 3) insert the
module into the kernel, which arms it, and 4) collect the data
generated by the probe and display it to the user.
</para>
<para>
In order to accomplish steps 1 and 2, the 'stap' program needs
access to the kernel build system that produced the kernel
that the probed system is running. In the case of a typical
embedded system (the 'target'), the kernel build system
unfortunately isn't typically part of the image running on
the target. It is normally available on the 'host' system
that produced the target image however; in such cases,
steps 1 and 2 are executed on the host system, and steps
3 and 4 are executed on the target system, using only the
systemtap 'runtime'.
</para>
<para>
The systemtap support in Yocto assumes that only steps
3 and 4 are run on the target; it is possible to do
everything on the target, but this section assumes only
the typical embedded use-case.
</para>
<para>
So basically what you need to do in order to run a systemtap
script on the target is to 1) on the host system, compile the
probe into a kernel module that makes sense to the target, 2)
copy the module onto the target system and 3) insert the
module into the target kernel, which arms it, and 4) collect
the data generated by the probe and display it to the user.
</para>
<section id='systemtap-setup'>
<title>Setup</title>
<para>
Those are a lot of steps and a lot of details, but
fortunately Yocto includes a script called 'crosstap'
that will take care of those details, allowing you to
simply execute a systemtap script on the remote target,
with arguments if necessary.
</para>
<para>
In order to do this from a remote host, however, you
need to have access to the build for the image you
booted. The 'crosstap' script provides details on how
to do this if you run the script on the host without having
done a build:
<note>
SystemTap, which uses 'crosstap', assumes you can establish an
ssh connection to the remote target.
Please refer to the crosstap wiki page for details on verifying
ssh connections at
<ulink url='https://wiki.yoctoproject.org/wiki/Tracing_and_Profiling#systemtap'></ulink>.
Also, the ability to ssh into the target system is not enabled
by default in *-minimal images.
</note>
<literallayout class='monospaced'>
$ crosstap root@192.168.1.88 trace_open.stp
Error: No target kernel build found.
Did you forget to create a local build of your image?
'crosstap' requires a local sdk build of the target system
(or a build that includes 'tools-profile') in order to build
kernel modules that can probe the target system.
Practically speaking, that means you need to do the following:
- If you're running a pre-built image, download the release
and/or BSP tarballs used to build the image.
- If you're working from git sources, just clone the metadata
and BSP layers needed to build the image you'll be booting.
- Make sure you're properly set up to build a new image (see
the BSP README and/or the widely available basic documentation
that discusses how to build images).
- Build an -sdk version of the image e.g.:
$ bitbake core-image-sato-sdk
OR
- Build a non-sdk image but include the profiling tools:
[ edit local.conf and add 'tools-profile' to the end of
the EXTRA_IMAGE_FEATURES variable ]
$ bitbake core-image-sato
Once you've build the image on the host system, you're ready to
boot it (or the equivalent pre-built image) and use 'crosstap'
to probe it (you need to source the environment as usual first):
$ source oe-init-build-env
$ cd ~/my/systemtap/scripts
$ crosstap root@192.168.1.xxx myscript.stp
</literallayout>
So essentially what you need to do is build an SDK image or
image with 'tools-profile' as detailed in the
"<link linkend='profile-manual-general-setup'>General Setup</link>"
section of this manual, and boot the resulting target image.
</para>
<note>
If you have a build directory containing multiple machines,
you need to have the MACHINE you're connecting to selected
in local.conf, and the kernel in that machine's build
directory must match the kernel on the booted system exactly,
or you'll get the above 'crosstap' message when you try to
invoke a script.
</note>
</section>
<section id='running-a-script-on-a-target'>
<title>Running a Script on a Target</title>
<para>
Once you've done that, you should be able to run a systemtap
script on the target:
<literallayout class='monospaced'>
$ cd /path/to/yocto
$ source oe-init-build-env
### Shell environment set up for builds. ###
You can now run 'bitbake <target>'
Common targets are:
core-image-minimal
core-image-sato
meta-toolchain
meta-ide-support
You can also run generated qemu images with a command like 'runqemu qemux86'
</literallayout>
Once you've done that, you can cd to whatever directory
contains your scripts and use 'crosstap' to run the script:
<literallayout class='monospaced'>
$ cd /path/to/my/systemap/script
$ crosstap root@192.168.7.2 trace_open.stp
</literallayout>
If you get an error connecting to the target e.g.:
<literallayout class='monospaced'>
$ crosstap root@192.168.7.2 trace_open.stp
error establishing ssh connection on remote 'root@192.168.7.2'
</literallayout>
Try ssh'ing to the target and see what happens:
<literallayout class='monospaced'>
$ ssh root@192.168.7.2
</literallayout>
A lot of the time, connection problems are due specifying a
wrong IP address or having a 'host key verification error'.
</para>
<para>
If everything worked as planned, you should see something
like this (enter the password when prompted, or press enter
if it's set up to use no password):
<literallayout class='monospaced'>
$ crosstap root@192.168.7.2 trace_open.stp
root@192.168.7.2's password:
matchbox-termin(1036) open ("/tmp/vte3FS2LW", O_RDWR|O_CREAT|O_EXCL|O_LARGEFILE, 0600)
matchbox-termin(1036) open ("/tmp/vteJMC7LW", O_RDWR|O_CREAT|O_EXCL|O_LARGEFILE, 0600)
</literallayout>
</para>
</section>
<section id='systemtap-documentation'>
<title>Documentation</title>
<para>
The SystemTap language reference can be found here:
<ulink url='http://sourceware.org/systemtap/langref/'>SystemTap Language Reference</ulink>
</para>
<para>
Links to other SystemTap documents, tutorials, and examples can be
found here:
<ulink url='http://sourceware.org/systemtap/documentation.html'>SystemTap documentation page</ulink>
</para>
</section>
</section>
<section id='profile-manual-sysprof'>
<title>Sysprof</title>
<para>
Sysprof is a very easy to use system-wide profiler that consists
of a single window with three panes and a few buttons which allow
you to start, stop, and view the profile from one place.
</para>
<section id='sysprof-setup'>
<title>Setup</title>
<para>
For this section, we'll assume you've already performed the
basic setup outlined in the General Setup section.
</para>
<para>
Sysprof is a GUI-based application that runs on the target
system. For the rest of this document we assume you've
ssh'ed to the host and will be running Sysprof on the
target (you can use the '-X' option to ssh and have the
Sysprof GUI run on the target but display remotely on the
host if you want).
</para>
</section>
<section id='sysprof-basic-usage'>
<title>Basic Usage</title>
<para>
To start profiling the system, you simply press the 'Start'
button. To stop profiling and to start viewing the profile data
in one easy step, press the 'Profile' button.
</para>
<para>
Once you've pressed the profile button, the three panes will
fill up with profiling data:
</para>
<para>
<imagedata fileref="figures/sysprof-copy-to-user.png" width="6in" depth="4in" align="center" scalefit="1" />
</para>
<para>
The left pane shows a list of functions and processes.
Selecting one of those expands that function in the right
pane, showing all its callees. Note that this caller-oriented
display is essentially the inverse of perf's default
callee-oriented callchain display.
</para>
<para>
In the screenshot above, we're focusing on __copy_to_user_ll()
and looking up the callchain we can see that one of the callers
of __copy_to_user_ll is sys_read() and the complete callpath
between them. Notice that this is essentially a portion of the
same information we saw in the perf display shown in the perf
section of this page.
</para>
<para>
<imagedata fileref="figures/sysprof-copy-from-user.png" width="6in" depth="4in" align="center" scalefit="1" />
</para>
<para>
Similarly, the above is a snapshot of the Sysprof display of a
copy-from-user callchain.
</para>
<para>
Finally, looking at the third Sysprof pane in the lower left,
we can see a list of all the callers of a particular function
selected in the top left pane. In this case, the lower pane is
showing all the callers of __mark_inode_dirty:
</para>
<para>
<imagedata fileref="figures/sysprof-callers.png" width="6in" depth="4in" align="center" scalefit="1" />
</para>
<para>
Double-clicking on one of those functions will in turn change the
focus to the selected function, and so on.
</para>
<informalexample>
<emphasis>Tying it Together:</emphasis> If you like sysprof's 'caller-oriented'
display, you may be able to approximate it in other tools as
well. For example, 'perf report' has the -g (--call-graph)
option that you can experiment with; one of the options is
'caller' for an inverted caller-based callgraph display.
</informalexample>
</section>
<section id='sysprof-documentation'>
<title>Documentation</title>
<para>
There doesn't seem to be any documentation for Sysprof, but
maybe that's because it's pretty self-explanatory.
The Sysprof website, however, is here:
<ulink url='http://sysprof.com/'>Sysprof, System-wide Performance Profiler for Linux</ulink>
</para>
</section>
</section>
<section id='lttng-linux-trace-toolkit-next-generation'>
<title>LTTng (Linux Trace Toolkit, next generation)</title>
<section id='lttng-setup'>
<title>Setup</title>
<para>
For this section, we'll assume you've already performed the
basic setup outlined in the General Setup section.
</para>
<para>
LTTng is run on the target system by ssh'ing to it.
However, if you want to see the traces graphically,
install Eclipse as described in section
"<link linkend='manually-copying-a-trace-to-the-host-and-viewing-it-in-eclipse'>Manually copying a trace to the host and viewing it in Eclipse (i.e. using Eclipse without network support)</link>"
and follow the directions to manually copy traces to the host and
view them in Eclipse (i.e. using Eclipse without network support).
</para>
<note>
Be sure to download and install/run the 'SR1' or later Juno release
of eclipse e.g.:
<ulink url='http://www.eclipse.org/downloads/download.php?file=/technology/epp/downloads/release/juno/SR1/eclipse-cpp-juno-SR1-linux-gtk-x86_64.tar.gz'>http://www.eclipse.org/downloads/download.php?file=/technology/epp/downloads/release/juno/SR1/eclipse-cpp-juno-SR1-linux-gtk-x86_64.tar.gz</ulink>
</note>
</section>
<section id='collecting-and-viewing-traces'>
<title>Collecting and Viewing Traces</title>
<para>
Once you've applied the above commits and built and booted your
image (you need to build the core-image-sato-sdk image or use one of the
other methods described in the General Setup section), you're
ready to start tracing.
</para>
<section id='collecting-and-viewing-a-trace-on-the-target-inside-a-shell'>
<title>Collecting and viewing a trace on the target (inside a shell)</title>
<para>
First, from the host, ssh to the target:
<literallayout class='monospaced'>
$ ssh -l root 192.168.1.47
The authenticity of host '192.168.1.47 (192.168.1.47)' can't be established.
RSA key fingerprint is 23:bd:c8:b1:a8:71:52:00:ee:00:4f:64:9e:10:b9:7e.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.1.47' (RSA) to the list of known hosts.
root@192.168.1.47's password:
</literallayout>
Once on the target, use these steps to create a trace:
<literallayout class='monospaced'>
root@crownbay:~# lttng create
Spawning a session daemon
Session auto-20121015-232120 created.
Traces will be written in /home/root/lttng-traces/auto-20121015-232120
</literallayout>
Enable the events you want to trace (in this case all
kernel events):
<literallayout class='monospaced'>
root@crownbay:~# lttng enable-event --kernel --all
All kernel events are enabled in channel channel0
</literallayout>
Start the trace:
<literallayout class='monospaced'>
root@crownbay:~# lttng start
Tracing started for session auto-20121015-232120
</literallayout>
And then stop the trace after awhile or after running
a particular workload that you want to trace:
<literallayout class='monospaced'>
root@crownbay:~# lttng stop
Tracing stopped for session auto-20121015-232120
</literallayout>
You can now view the trace in text form on the target:
<literallayout class='monospaced'>
root@crownbay:~# lttng view
[23:21:56.989270399] (+?.?????????) sys_geteuid: { 1 }, { }
[23:21:56.989278081] (+0.000007682) exit_syscall: { 1 }, { ret = 0 }
[23:21:56.989286043] (+0.000007962) sys_pipe: { 1 }, { fildes = 0xB77B9E8C }
[23:21:56.989321802] (+0.000035759) exit_syscall: { 1 }, { ret = 0 }
[23:21:56.989329345] (+0.000007543) sys_mmap_pgoff: { 1 }, { addr = 0x0, len = 10485760, prot = 3, flags = 131362, fd = 4294967295, pgoff = 0 }
[23:21:56.989351694] (+0.000022349) exit_syscall: { 1 }, { ret = -1247805440 }
[23:21:56.989432989] (+0.000081295) sys_clone: { 1 }, { clone_flags = 0x411, newsp = 0xB5EFFFE4, parent_tid = 0xFFFFFFFF, child_tid = 0x0 }
[23:21:56.989477129] (+0.000044140) sched_stat_runtime: { 1 }, { comm = "lttng-consumerd", tid = 1193, runtime = 681660, vruntime = 43367983388 }
[23:21:56.989486697] (+0.000009568) sched_migrate_task: { 1 }, { comm = "lttng-consumerd", tid = 1193, prio = 20, orig_cpu = 1, dest_cpu = 1 }
[23:21:56.989508418] (+0.000021721) hrtimer_init: { 1 }, { hrtimer = 3970832076, clockid = 1, mode = 1 }
[23:21:56.989770462] (+0.000262044) hrtimer_cancel: { 1 }, { hrtimer = 3993865440 }
[23:21:56.989771580] (+0.000001118) hrtimer_cancel: { 0 }, { hrtimer = 3993812192 }
[23:21:56.989776957] (+0.000005377) hrtimer_expire_entry: { 1 }, { hrtimer = 3993865440, now = 79815980007057, function = 3238465232 }
[23:21:56.989778145] (+0.000001188) hrtimer_expire_entry: { 0 }, { hrtimer = 3993812192, now = 79815980008174, function = 3238465232 }
[23:21:56.989791695] (+0.000013550) softirq_raise: { 1 }, { vec = 1 }
[23:21:56.989795396] (+0.000003701) softirq_raise: { 0 }, { vec = 1 }
[23:21:56.989800635] (+0.000005239) softirq_raise: { 0 }, { vec = 9 }
[23:21:56.989807130] (+0.000006495) sched_stat_runtime: { 1 }, { comm = "lttng-consumerd", tid = 1193, runtime = 330710, vruntime = 43368314098 }
[23:21:56.989809993] (+0.000002863) sched_stat_runtime: { 0 }, { comm = "lttng-sessiond", tid = 1181, runtime = 1015313, vruntime = 36976733240 }
[23:21:56.989818514] (+0.000008521) hrtimer_expire_exit: { 0 }, { hrtimer = 3993812192 }
[23:21:56.989819631] (+0.000001117) hrtimer_expire_exit: { 1 }, { hrtimer = 3993865440 }
[23:21:56.989821866] (+0.000002235) hrtimer_start: { 0 }, { hrtimer = 3993812192, function = 3238465232, expires = 79815981000000, softexpires = 79815981000000 }
[23:21:56.989822984] (+0.000001118) hrtimer_start: { 1 }, { hrtimer = 3993865440, function = 3238465232, expires = 79815981000000, softexpires = 79815981000000 }
[23:21:56.989832762] (+0.000009778) softirq_entry: { 1 }, { vec = 1 }
[23:21:56.989833879] (+0.000001117) softirq_entry: { 0 }, { vec = 1 }
[23:21:56.989838069] (+0.000004190) timer_cancel: { 1 }, { timer = 3993871956 }
[23:21:56.989839187] (+0.000001118) timer_cancel: { 0 }, { timer = 3993818708 }
[23:21:56.989841492] (+0.000002305) timer_expire_entry: { 1 }, { timer = 3993871956, now = 79515980, function = 3238277552 }
[23:21:56.989842819] (+0.000001327) timer_expire_entry: { 0 }, { timer = 3993818708, now = 79515980, function = 3238277552 }
[23:21:56.989854831] (+0.000012012) sched_stat_runtime: { 1 }, { comm = "lttng-consumerd", tid = 1193, runtime = 49237, vruntime = 43368363335 }
[23:21:56.989855949] (+0.000001118) sched_stat_runtime: { 0 }, { comm = "lttng-sessiond", tid = 1181, runtime = 45121, vruntime = 36976778361 }
[23:21:56.989861257] (+0.000005308) sched_stat_sleep: { 1 }, { comm = "kworker/1:1", tid = 21, delay = 9451318 }
[23:21:56.989862374] (+0.000001117) sched_stat_sleep: { 0 }, { comm = "kworker/0:0", tid = 4, delay = 9958820 }
[23:21:56.989868241] (+0.000005867) sched_wakeup: { 0 }, { comm = "kworker/0:0", tid = 4, prio = 120, success = 1, target_cpu = 0 }
[23:21:56.989869358] (+0.000001117) sched_wakeup: { 1 }, { comm = "kworker/1:1", tid = 21, prio = 120, success = 1, target_cpu = 1 }
[23:21:56.989877460] (+0.000008102) timer_expire_exit: { 1 }, { timer = 3993871956 }
[23:21:56.989878577] (+0.000001117) timer_expire_exit: { 0 }, { timer = 3993818708 }
.
.
.
</literallayout>
You can now safely destroy the trace session (note that
this doesn't delete the trace - it's still there
in ~/lttng-traces):
<literallayout class='monospaced'>
root@crownbay:~# lttng destroy
Session auto-20121015-232120 destroyed at /home/root
</literallayout>
Note that the trace is saved in a directory of the same
name as returned by 'lttng create', under the ~/lttng-traces
directory (note that you can change this by supplying your
own name to 'lttng create'):
<literallayout class='monospaced'>
root@crownbay:~# ls -al ~/lttng-traces
drwxrwx--- 3 root root 1024 Oct 15 23:21 .
drwxr-xr-x 5 root root 1024 Oct 15 23:57 ..
drwxrwx--- 3 root root 1024 Oct 15 23:21 auto-20121015-232120
</literallayout>
</para>
</section>
<section id='collecting-and-viewing-a-userspace-trace-on-the-target-inside-a-shell'>
<title>Collecting and viewing a userspace trace on the target (inside a shell)</title>
<para>
For LTTng userspace tracing, you need to have a properly
instrumented userspace program. For this example, we'll use
the 'hello' test program generated by the lttng-ust build.
</para>
<para>
The 'hello' test program isn't installed on the rootfs by
the lttng-ust build, so we need to copy it over manually.
First cd into the build directory that contains the hello
executable:
<literallayout class='monospaced'>
$ cd build/tmp/work/core2_32-poky-linux/lttng-ust/2.0.5-r0/git/tests/hello/.libs
</literallayout>
Copy that over to the target machine:
<literallayout class='monospaced'>
$ scp hello root@192.168.1.20:
</literallayout>
You now have the instrumented lttng 'hello world' test
program on the target, ready to test.
</para>
<para>
First, from the host, ssh to the target:
<literallayout class='monospaced'>
$ ssh -l root 192.168.1.47
The authenticity of host '192.168.1.47 (192.168.1.47)' can't be established.
RSA key fingerprint is 23:bd:c8:b1:a8:71:52:00:ee:00:4f:64:9e:10:b9:7e.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.1.47' (RSA) to the list of known hosts.
root@192.168.1.47's password:
</literallayout>
Once on the target, use these steps to create a trace:
<literallayout class='monospaced'>
root@crownbay:~# lttng create
Session auto-20190303-021943 created.
Traces will be written in /home/root/lttng-traces/auto-20190303-021943
</literallayout>
Enable the events you want to trace (in this case all
userspace events):
<literallayout class='monospaced'>
root@crownbay:~# lttng enable-event --userspace --all
All UST events are enabled in channel channel0
</literallayout>
Start the trace:
<literallayout class='monospaced'>
root@crownbay:~# lttng start
Tracing started for session auto-20190303-021943
</literallayout>
Run the instrumented hello world program:
<literallayout class='monospaced'>
root@crownbay:~# ./hello
Hello, World!
Tracing... done.
</literallayout>
And then stop the trace after awhile or after running a
particular workload that you want to trace:
<literallayout class='monospaced'>
root@crownbay:~# lttng stop
Tracing stopped for session auto-20190303-021943
</literallayout>
You can now view the trace in text form on the target:
<literallayout class='monospaced'>
root@crownbay:~# lttng view
[02:31:14.906146544] (+?.?????????) hello:1424 ust_tests_hello:tptest: { cpu_id = 1 }, { intfield = 0, intfield2 = 0x0, longfield = 0, netintfield = 0, netintfieldhex = 0x0, arrfield1 = [ [0] = 1, [1] = 2, [2] = 3 ], arrfield2 = "test", _seqfield1_length = 4, seqfield1 = [ [0] = 116, [1] = 101, [2] = 115, [3] = 116 ], _seqfield2_length = 4, seqfield2 = "test", stringfield = "test", floatfield = 2222, doublefield = 2, boolfield = 1 }
[02:31:14.906170360] (+0.000023816) hello:1424 ust_tests_hello:tptest: { cpu_id = 1 }, { intfield = 1, intfield2 = 0x1, longfield = 1, netintfield = 1, netintfieldhex = 0x1, arrfield1 = [ [0] = 1, [1] = 2, [2] = 3 ], arrfield2 = "test", _seqfield1_length = 4, seqfield1 = [ [0] = 116, [1] = 101, [2] = 115, [3] = 116 ], _seqfield2_length = 4, seqfield2 = "test", stringfield = "test", floatfield = 2222, doublefield = 2, boolfield = 1 }
[02:31:14.906183140] (+0.000012780) hello:1424 ust_tests_hello:tptest: { cpu_id = 1 }, { intfield = 2, intfield2 = 0x2, longfield = 2, netintfield = 2, netintfieldhex = 0x2, arrfield1 = [ [0] = 1, [1] = 2, [2] = 3 ], arrfield2 = "test", _seqfield1_length = 4, seqfield1 = [ [0] = 116, [1] = 101, [2] = 115, [3] = 116 ], _seqfield2_length = 4, seqfield2 = "test", stringfield = "test", floatfield = 2222, doublefield = 2, boolfield = 1 }
[02:31:14.906194385] (+0.000011245) hello:1424 ust_tests_hello:tptest: { cpu_id = 1 }, { intfield = 3, intfield2 = 0x3, longfield = 3, netintfield = 3, netintfieldhex = 0x3, arrfield1 = [ [0] = 1, [1] = 2, [2] = 3 ], arrfield2 = "test", _seqfield1_length = 4, seqfield1 = [ [0] = 116, [1] = 101, [2] = 115, [3] = 116 ], _seqfield2_length = 4, seqfield2 = "test", stringfield = "test", floatfield = 2222, doublefield = 2, boolfield = 1 }
.
.
.
</literallayout>
You can now safely destroy the trace session (note that
this doesn't delete the trace - it's still
there in ~/lttng-traces):
<literallayout class='monospaced'>
root@crownbay:~# lttng destroy
Session auto-20190303-021943 destroyed at /home/root
</literallayout>
</para>
</section>
<section id='manually-copying-a-trace-to-the-host-and-viewing-it-in-eclipse'>
<title>Manually copying a trace to the host and viewing it in Eclipse (i.e. using Eclipse without network support)</title>
<para>
If you already have an LTTng trace on a remote target and
would like to view it in Eclipse on the host, you can easily
copy it from the target to the host and import it into
Eclipse to view it using the LTTng Eclipse plug-in already
bundled in the Eclipse (Juno SR1 or greater).
</para>
<para>
Using the trace we created in the previous section, archive
it and copy it to your host system:
<literallayout class='monospaced'>
root@crownbay:~/lttng-traces# tar zcvf auto-20121015-232120.tar.gz auto-20121015-232120
auto-20121015-232120/
auto-20121015-232120/kernel/
auto-20121015-232120/kernel/metadata
auto-20121015-232120/kernel/channel0_1
auto-20121015-232120/kernel/channel0_0
$ scp root@192.168.1.47:lttng-traces/auto-20121015-232120.tar.gz .
root@192.168.1.47's password:
auto-20121015-232120.tar.gz 100% 1566KB 1.5MB/s 00:01
</literallayout>
Unarchive it on the host:
<literallayout class='monospaced'>
$ gunzip -c auto-20121015-232120.tar.gz | tar xvf -
auto-20121015-232120/
auto-20121015-232120/kernel/
auto-20121015-232120/kernel/metadata
auto-20121015-232120/kernel/channel0_1
auto-20121015-232120/kernel/channel0_0
</literallayout>
We can now import the trace into Eclipse and view it:
<orderedlist>
<listitem><para>First, start eclipse and open the
'LTTng Kernel' perspective by selecting the following
menu item:
<literallayout class='monospaced'>
Window | Open Perspective | Other...
</literallayout></para></listitem>
<listitem><para>In the dialog box that opens, select
'LTTng Kernel' from the list.</para></listitem>
<listitem><para>Back at the main menu, select the
following menu item:
<literallayout class='monospaced'>
File | New | Project...
</literallayout></para></listitem>
<listitem><para>In the dialog box that opens, select
the 'Tracing | Tracing Project' wizard and press
'Next>'.</para></listitem>
<listitem><para>Give the project a name and press
'Finish'.</para></listitem>
<listitem><para>In the 'Project Explorer' pane under
the project you created, right click on the
'Traces' item.</para></listitem>
<listitem><para>Select 'Import..." and in the dialog
that's displayed:</para></listitem>
<listitem><para>Browse the filesystem and find the
select the 'kernel' directory containing the trace
you copied from the target
e.g. auto-20121015-232120/kernel</para></listitem>
<listitem><para>'Checkmark' the directory in the tree
that's displayed for the trace</para></listitem>
<listitem><para>Below that, select 'Common Trace Format:
Kernel Trace' for the 'Trace Type'</para></listitem>
<listitem><para>Press 'Finish' to close the dialog
</para></listitem>
<listitem><para>Back in the 'Project Explorer' pane,
double-click on the 'kernel' item for the
trace you just imported under 'Traces'
</para></listitem>
</orderedlist>
You should now see your trace data displayed graphically
in several different views in Eclipse:
</para>
<para>
<imagedata fileref="figures/lttngmain0.png" width="6in" depth="6in" align="center" scalefit="1" />
</para>
<para>
You can access extensive help information on how to use
the LTTng plug-in to search and analyze captured traces via
the Eclipse help system:
<literallayout class='monospaced'>
Help | Help Contents | LTTng Plug-in User Guide
</literallayout>
</para>
</section>
<section id='collecting-and-viewing-a-trace-in-eclipse'>
<title>Collecting and viewing a trace in Eclipse</title>
<note>
This section on collecting traces remotely doesn't currently
work because of Eclipse 'RSE' connectivity problems. Manually
tracing on the target, copying the trace files to the host,
and viewing the trace in Eclipse on the host as outlined in
previous steps does work however - please use the manual
steps outlined above to view traces in Eclipse.
</note>
<para>
In order to trace a remote target, you also need to add
a 'tracing' group on the target and connect as a user
who's part of that group e.g:
<literallayout class='monospaced'>
# adduser tomz
# groupadd -r tracing
# usermod -a -G tracing tomz
</literallayout>
<orderedlist>
<listitem><para>First, start eclipse and open the
'LTTng Kernel' perspective by selecting the following
menu item:
<literallayout class='monospaced'>
Window | Open Perspective | Other...
</literallayout></para></listitem>
<listitem><para>In the dialog box that opens, select
'LTTng Kernel' from the list.</para></listitem>
<listitem><para>Back at the main menu, select the
following menu item:
<literallayout class='monospaced'>
File | New | Project...
</literallayout></para></listitem>
<listitem><para>In the dialog box that opens, select
the 'Tracing | Tracing Project' wizard and
press 'Next>'.</para></listitem>
<listitem><para>Give the project a name and press
'Finish'. That should result in an entry in the
'Project' subwindow.</para></listitem>
<listitem><para>In the 'Control' subwindow just below
it, press 'New Connection'.</para></listitem>
<listitem><para>Add a new connection, giving it the
hostname or IP address of the target system.
</para></listitem>
<listitem><para>Provide the username and password
of a qualified user (a member of the 'tracing' group)
or root account on the target system.
</para></listitem>
<listitem><para>Provide appropriate answers to whatever
else is asked for e.g. 'secure storage password'
can be anything you want.
If you get an 'RSE Error' it may be due to proxies.
It may be possible to get around the problem by
changing the following setting:
<literallayout class='monospaced'>
Window | Preferences | Network Connections
</literallayout>
Switch 'Active Provider' to 'Direct'
</para></listitem>
</orderedlist>
</para>
</section>
</section>
<section id='lltng-documentation'>
<title>Documentation</title>
<para>
You can find the primary LTTng Documentation on the
<ulink url='https://lttng.org/docs/'>LTTng Documentation</ulink>
site.
The documentation on this site is appropriate for intermediate to
advanced software developers who are working in a Linux environment
and are interested in efficient software tracing.
</para>
<para>
For information on LTTng in general, visit the
<ulink url='http://lttng.org/lttng2.0'>LTTng Project</ulink>
site.
You can find a "Getting Started" link on this site that takes
you to an LTTng Quick Start.
</para>
<para>
Finally, you can access extensive help information on how to use
the LTTng plug-in to search and analyze captured traces via the
Eclipse help system:
<literallayout class='monospaced'>
Help | Help Contents | LTTng Plug-in User Guide
</literallayout>
</para>
</section>
</section>
<section id='profile-manual-blktrace'>
<title>blktrace</title>
<para>
blktrace is a tool for tracing and reporting low-level disk I/O.
blktrace provides the tracing half of the equation; its output can
be piped into the blkparse program, which renders the data in a
human-readable form and does some basic analysis:
</para>
<section id='blktrace-setup'>
<title>Setup</title>
<para>
For this section, we'll assume you've already performed the
basic setup outlined in the
"<link linkend='profile-manual-general-setup'>General Setup</link>"
section.
</para>
<para>
blktrace is an application that runs on the target system.
You can run the entire blktrace and blkparse pipeline on the
target, or you can run blktrace in 'listen' mode on the target
and have blktrace and blkparse collect and analyze the data on
the host (see the
"<link linkend='using-blktrace-remotely'>Using blktrace Remotely</link>"
section below).
For the rest of this section we assume you've ssh'ed to the
host and will be running blkrace on the target.
</para>
</section>
<section id='blktrace-basic-usage'>
<title>Basic Usage</title>
<para>
To record a trace, simply run the 'blktrace' command, giving it
the name of the block device you want to trace activity on:
<literallayout class='monospaced'>
root@crownbay:~# blktrace /dev/sdc
</literallayout>
In another shell, execute a workload you want to trace.
<literallayout class='monospaced'>
root@crownbay:/media/sdc# rm linux-2.6.19.2.tar.bz2; wget <ulink url='http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2'>http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2</ulink>; sync
Connecting to downloads.yoctoproject.org (140.211.169.59:80)
linux-2.6.19.2.tar.b 100% |*******************************| 41727k 0:00:00 ETA
</literallayout>
Press Ctrl-C in the blktrace shell to stop the trace. It will
display how many events were logged, along with the per-cpu file
sizes (blktrace records traces in per-cpu kernel buffers and
simply dumps them to userspace for blkparse to merge and sort
later).
<literallayout class='monospaced'>
^C=== sdc ===
CPU 0: 7082 events, 332 KiB data
CPU 1: 1578 events, 74 KiB data
Total: 8660 events (dropped 0), 406 KiB data
</literallayout>
If you examine the files saved to disk, you see multiple files,
one per CPU and with the device name as the first part of the
filename:
<literallayout class='monospaced'>
root@crownbay:~# ls -al
drwxr-xr-x 6 root root 1024 Oct 27 22:39 .
drwxr-sr-x 4 root root 1024 Oct 26 18:24 ..
-rw-r--r-- 1 root root 339938 Oct 27 22:40 sdc.blktrace.0
-rw-r--r-- 1 root root 75753 Oct 27 22:40 sdc.blktrace.1
</literallayout>
To view the trace events, simply invoke 'blkparse' in the
directory containing the trace files, giving it the device name
that forms the first part of the filenames:
<literallayout class='monospaced'>
root@crownbay:~# blkparse sdc
8,32 1 1 0.000000000 1225 Q WS 3417048 + 8 [jbd2/sdc-8]
8,32 1 2 0.000025213 1225 G WS 3417048 + 8 [jbd2/sdc-8]
8,32 1 3 0.000033384 1225 P N [jbd2/sdc-8]
8,32 1 4 0.000043301 1225 I WS 3417048 + 8 [jbd2/sdc-8]
8,32 1 0 0.000057270 0 m N cfq1225 insert_request
8,32 1 0 0.000064813 0 m N cfq1225 add_to_rr
8,32 1 5 0.000076336 1225 U N [jbd2/sdc-8] 1
8,32 1 0 0.000088559 0 m N cfq workload slice:150
8,32 1 0 0.000097359 0 m N cfq1225 set_active wl_prio:0 wl_type:1
8,32 1 0 0.000104063 0 m N cfq1225 Not idling. st->count:1
8,32 1 0 0.000112584 0 m N cfq1225 fifo= (null)
8,32 1 0 0.000118730 0 m N cfq1225 dispatch_insert
8,32 1 0 0.000127390 0 m N cfq1225 dispatched a request
8,32 1 0 0.000133536 0 m N cfq1225 activate rq, drv=1
8,32 1 6 0.000136889 1225 D WS 3417048 + 8 [jbd2/sdc-8]
8,32 1 7 0.000360381 1225 Q WS 3417056 + 8 [jbd2/sdc-8]
8,32 1 8 0.000377422 1225 G WS 3417056 + 8 [jbd2/sdc-8]
8,32 1 9 0.000388876 1225 P N [jbd2/sdc-8]
8,32 1 10 0.000397886 1225 Q WS 3417064 + 8 [jbd2/sdc-8]
8,32 1 11 0.000404800 1225 M WS 3417064 + 8 [jbd2/sdc-8]
8,32 1 12 0.000412343 1225 Q WS 3417072 + 8 [jbd2/sdc-8]
8,32 1 13 0.000416533 1225 M WS 3417072 + 8 [jbd2/sdc-8]
8,32 1 14 0.000422121 1225 Q WS 3417080 + 8 [jbd2/sdc-8]
8,32 1 15 0.000425194 1225 M WS 3417080 + 8 [jbd2/sdc-8]
8,32 1 16 0.000431968 1225 Q WS 3417088 + 8 [jbd2/sdc-8]
8,32 1 17 0.000435251 1225 M WS 3417088 + 8 [jbd2/sdc-8]
8,32 1 18 0.000440279 1225 Q WS 3417096 + 8 [jbd2/sdc-8]
8,32 1 19 0.000443911 1225 M WS 3417096 + 8 [jbd2/sdc-8]
8,32 1 20 0.000450336 1225 Q WS 3417104 + 8 [jbd2/sdc-8]
8,32 1 21 0.000454038 1225 M WS 3417104 + 8 [jbd2/sdc-8]
8,32 1 22 0.000462070 1225 Q WS 3417112 + 8 [jbd2/sdc-8]
8,32 1 23 0.000465422 1225 M WS 3417112 + 8 [jbd2/sdc-8]
8,32 1 24 0.000474222 1225 I WS 3417056 + 64 [jbd2/sdc-8]
8,32 1 0 0.000483022 0 m N cfq1225 insert_request
8,32 1 25 0.000489727 1225 U N [jbd2/sdc-8] 1
8,32 1 0 0.000498457 0 m N cfq1225 Not idling. st->count:1
8,32 1 0 0.000503765 0 m N cfq1225 dispatch_insert
8,32 1 0 0.000512914 0 m N cfq1225 dispatched a request
8,32 1 0 0.000518851 0 m N cfq1225 activate rq, drv=2
.
.
.
8,32 0 0 58.515006138 0 m N cfq3551 complete rqnoidle 1
8,32 0 2024 58.516603269 3 C WS 3156992 + 16 [0]
8,32 0 0 58.516626736 0 m N cfq3551 complete rqnoidle 1
8,32 0 0 58.516634558 0 m N cfq3551 arm_idle: 8 group_idle: 0
8,32 0 0 58.516636933 0 m N cfq schedule dispatch
8,32 1 0 58.516971613 0 m N cfq3551 slice expired t=0
8,32 1 0 58.516982089 0 m N cfq3551 sl_used=13 disp=6 charge=13 iops=0 sect=80
8,32 1 0 58.516985511 0 m N cfq3551 del_from_rr
8,32 1 0 58.516990819 0 m N cfq3551 put_queue
CPU0 (sdc):
Reads Queued: 0, 0KiB Writes Queued: 331, 26,284KiB
Read Dispatches: 0, 0KiB Write Dispatches: 485, 40,484KiB
Reads Requeued: 0 Writes Requeued: 0
Reads Completed: 0, 0KiB Writes Completed: 511, 41,000KiB
Read Merges: 0, 0KiB Write Merges: 13, 160KiB
Read depth: 0 Write depth: 2
IO unplugs: 23 Timer unplugs: 0
CPU1 (sdc):
Reads Queued: 0, 0KiB Writes Queued: 249, 15,800KiB
Read Dispatches: 0, 0KiB Write Dispatches: 42, 1,600KiB
Reads Requeued: 0 Writes Requeued: 0
Reads Completed: 0, 0KiB Writes Completed: 16, 1,084KiB
Read Merges: 0, 0KiB Write Merges: 40, 276KiB
Read depth: 0 Write depth: 2
IO unplugs: 30 Timer unplugs: 1
Total (sdc):
Reads Queued: 0, 0KiB Writes Queued: 580, 42,084KiB
Read Dispatches: 0, 0KiB Write Dispatches: 527, 42,084KiB
Reads Requeued: 0 Writes Requeued: 0
Reads Completed: 0, 0KiB Writes Completed: 527, 42,084KiB
Read Merges: 0, 0KiB Write Merges: 53, 436KiB
IO unplugs: 53 Timer unplugs: 1
Throughput (R/W): 0KiB/s / 719KiB/s
Events (sdc): 6,592 entries
Skips: 0 forward (0 - 0.0%)
Input file sdc.blktrace.0 added
Input file sdc.blktrace.1 added
</literallayout>
The report shows each event that was found in the blktrace data,
along with a summary of the overall block I/O traffic during
the run. You can look at the
<ulink url='http://linux.die.net/man/1/blkparse'>blkparse</ulink>
manpage to learn the
meaning of each field displayed in the trace listing.
</para>
<section id='blktrace-live-mode'>
<title>Live Mode</title>
<para>
blktrace and blkparse are designed from the ground up to
be able to operate together in a 'pipe mode' where the
stdout of blktrace can be fed directly into the stdin of
blkparse:
<literallayout class='monospaced'>
root@crownbay:~# blktrace /dev/sdc -o - | blkparse -i -
</literallayout>
This enables long-lived tracing sessions to run without
writing anything to disk, and allows the user to look for
certain conditions in the trace data in 'real-time' by
viewing the trace output as it scrolls by on the screen or
by passing it along to yet another program in the pipeline
such as grep which can be used to identify and capture
conditions of interest.
</para>
<para>
There's actually another blktrace command that implements
the above pipeline as a single command, so the user doesn't
have to bother typing in the above command sequence:
<literallayout class='monospaced'>
root@crownbay:~# btrace /dev/sdc
</literallayout>
</para>
</section>
<section id='using-blktrace-remotely'>
<title>Using blktrace Remotely</title>
<para>
Because blktrace traces block I/O and at the same time
normally writes its trace data to a block device, and
in general because it's not really a great idea to make
the device being traced the same as the device the tracer
writes to, blktrace provides a way to trace without
perturbing the traced device at all by providing native
support for sending all trace data over the network.
</para>
<para>
To have blktrace operate in this mode, start blktrace on
the target system being traced with the -l option, along with
the device to trace:
<literallayout class='monospaced'>
root@crownbay:~# blktrace -l /dev/sdc
server: waiting for connections...
</literallayout>
On the host system, use the -h option to connect to the
target system, also passing it the device to trace:
<literallayout class='monospaced'>
$ blktrace -d /dev/sdc -h 192.168.1.43
blktrace: connecting to 192.168.1.43
blktrace: connected!
</literallayout>
On the target system, you should see this:
<literallayout class='monospaced'>
server: connection from 192.168.1.43
</literallayout>
In another shell, execute a workload you want to trace.
<literallayout class='monospaced'>
root@crownbay:/media/sdc# rm linux-2.6.19.2.tar.bz2; wget <ulink url='http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2'>http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2</ulink>; sync
Connecting to downloads.yoctoproject.org (140.211.169.59:80)
linux-2.6.19.2.tar.b 100% |*******************************| 41727k 0:00:00 ETA
</literallayout>
When it's done, do a Ctrl-C on the host system to
stop the trace:
<literallayout class='monospaced'>
^C=== sdc ===
CPU 0: 7691 events, 361 KiB data
CPU 1: 4109 events, 193 KiB data
Total: 11800 events (dropped 0), 554 KiB data
</literallayout>
On the target system, you should also see a trace
summary for the trace just ended:
<literallayout class='monospaced'>
server: end of run for 192.168.1.43:sdc
=== sdc ===
CPU 0: 7691 events, 361 KiB data
CPU 1: 4109 events, 193 KiB data
Total: 11800 events (dropped 0), 554 KiB data
</literallayout>
The blktrace instance on the host will save the target
output inside a hostname-timestamp directory:
<literallayout class='monospaced'>
$ ls -al
drwxr-xr-x 10 root root 1024 Oct 28 02:40 .
drwxr-sr-x 4 root root 1024 Oct 26 18:24 ..
drwxr-xr-x 2 root root 1024 Oct 28 02:40 192.168.1.43-2012-10-28-02:40:56
</literallayout>
cd into that directory to see the output files:
<literallayout class='monospaced'>
$ ls -l
-rw-r--r-- 1 root root 369193 Oct 28 02:44 sdc.blktrace.0
-rw-r--r-- 1 root root 197278 Oct 28 02:44 sdc.blktrace.1
</literallayout>
And run blkparse on the host system using the device name:
<literallayout class='monospaced'>
$ blkparse sdc
8,32 1 1 0.000000000 1263 Q RM 6016 + 8 [ls]
8,32 1 0 0.000036038 0 m N cfq1263 alloced
8,32 1 2 0.000039390 1263 G RM 6016 + 8 [ls]
8,32 1 3 0.000049168 1263 I RM 6016 + 8 [ls]
8,32 1 0 0.000056152 0 m N cfq1263 insert_request
8,32 1 0 0.000061600 0 m N cfq1263 add_to_rr
8,32 1 0 0.000075498 0 m N cfq workload slice:300
.
.
.
8,32 0 0 177.266385696 0 m N cfq1267 arm_idle: 8 group_idle: 0
8,32 0 0 177.266388140 0 m N cfq schedule dispatch
8,32 1 0 177.266679239 0 m N cfq1267 slice expired t=0
8,32 1 0 177.266689297 0 m N cfq1267 sl_used=9 disp=6 charge=9 iops=0 sect=56
8,32 1 0 177.266692649 0 m N cfq1267 del_from_rr
8,32 1 0 177.266696560 0 m N cfq1267 put_queue
CPU0 (sdc):
Reads Queued: 0, 0KiB Writes Queued: 270, 21,708KiB
Read Dispatches: 59, 2,628KiB Write Dispatches: 495, 39,964KiB
Reads Requeued: 0 Writes Requeued: 0
Reads Completed: 90, 2,752KiB Writes Completed: 543, 41,596KiB
Read Merges: 0, 0KiB Write Merges: 9, 344KiB
Read depth: 2 Write depth: 2
IO unplugs: 20 Timer unplugs: 1
CPU1 (sdc):
Reads Queued: 688, 2,752KiB Writes Queued: 381, 20,652KiB
Read Dispatches: 31, 124KiB Write Dispatches: 59, 2,396KiB
Reads Requeued: 0 Writes Requeued: 0
Reads Completed: 0, 0KiB Writes Completed: 11, 764KiB
Read Merges: 598, 2,392KiB Write Merges: 88, 448KiB
Read depth: 2 Write depth: 2
IO unplugs: 52 Timer unplugs: 0
Total (sdc):
Reads Queued: 688, 2,752KiB Writes Queued: 651, 42,360KiB
Read Dispatches: 90, 2,752KiB Write Dispatches: 554, 42,360KiB
Reads Requeued: 0 Writes Requeued: 0
Reads Completed: 90, 2,752KiB Writes Completed: 554, 42,360KiB
Read Merges: 598, 2,392KiB Write Merges: 97, 792KiB
IO unplugs: 72 Timer unplugs: 1
Throughput (R/W): 15KiB/s / 238KiB/s
Events (sdc): 9,301 entries
Skips: 0 forward (0 - 0.0%)
</literallayout>
You should see the trace events and summary just as
you would have if you'd run the same command on the target.
</para>
</section>
<section id='tracing-block-io-via-ftrace'>
<title>Tracing Block I/O via 'ftrace'</title>
<para>
It's also possible to trace block I/O using only
<link linkend='the-trace-events-subsystem'>trace events subsystem</link>,
which can be useful for casual tracing
if you don't want to bother dealing with the userspace tools.
</para>
<para>
To enable tracing for a given device, use
/sys/block/xxx/trace/enable, where xxx is the device name.
This for example enables tracing for /dev/sdc:
<literallayout class='monospaced'>
root@crownbay:/sys/kernel/debug/tracing# echo 1 > /sys/block/sdc/trace/enable
</literallayout>
Once you've selected the device(s) you want to trace,
selecting the 'blk' tracer will turn the blk tracer on:
<literallayout class='monospaced'>
root@crownbay:/sys/kernel/debug/tracing# cat available_tracers
blk function_graph function nop
root@crownbay:/sys/kernel/debug/tracing# echo blk > current_tracer
</literallayout>
Execute the workload you're interested in:
<literallayout class='monospaced'>
root@crownbay:/sys/kernel/debug/tracing# cat /media/sdc/testfile.txt
</literallayout>
And look at the output (note here that we're using
'trace_pipe' instead of trace to capture this trace -
this allows us to wait around on the pipe for data to
appear):
<literallayout class='monospaced'>
root@crownbay:/sys/kernel/debug/tracing# cat trace_pipe
cat-3587 [001] d..1 3023.276361: 8,32 Q R 1699848 + 8 [cat]
cat-3587 [001] d..1 3023.276410: 8,32 m N cfq3587 alloced
cat-3587 [001] d..1 3023.276415: 8,32 G R 1699848 + 8 [cat]
cat-3587 [001] d..1 3023.276424: 8,32 P N [cat]
cat-3587 [001] d..2 3023.276432: 8,32 I R 1699848 + 8 [cat]
cat-3587 [001] d..1 3023.276439: 8,32 m N cfq3587 insert_request
cat-3587 [001] d..1 3023.276445: 8,32 m N cfq3587 add_to_rr
cat-3587 [001] d..2 3023.276454: 8,32 U N [cat] 1
cat-3587 [001] d..1 3023.276464: 8,32 m N cfq workload slice:150
cat-3587 [001] d..1 3023.276471: 8,32 m N cfq3587 set_active wl_prio:0 wl_type:2
cat-3587 [001] d..1 3023.276478: 8,32 m N cfq3587 fifo= (null)
cat-3587 [001] d..1 3023.276483: 8,32 m N cfq3587 dispatch_insert
cat-3587 [001] d..1 3023.276490: 8,32 m N cfq3587 dispatched a request
cat-3587 [001] d..1 3023.276497: 8,32 m N cfq3587 activate rq, drv=1
cat-3587 [001] d..2 3023.276500: 8,32 D R 1699848 + 8 [cat]
</literallayout>
And this turns off tracing for the specified device:
<literallayout class='monospaced'>
root@crownbay:/sys/kernel/debug/tracing# echo 0 > /sys/block/sdc/trace/enable
</literallayout>
</para>
</section>
</section>
<section id='blktrace-documentation'>
<title>Documentation</title>
<para>
Online versions of the man pages for the commands discussed
in this section can be found here:
<itemizedlist>
<listitem><para><ulink url='http://linux.die.net/man/8/blktrace'>http://linux.die.net/man/8/blktrace</ulink>
</para></listitem>
<listitem><para><ulink url='http://linux.die.net/man/1/blkparse'>http://linux.die.net/man/1/blkparse</ulink>
</para></listitem>
<listitem><para><ulink url='http://linux.die.net/man/8/btrace'>http://linux.die.net/man/8/btrace</ulink>
</para></listitem>
</itemizedlist>
</para>
<para>
The above manpages, along with manpages for the other
blktrace utilities (btt, blkiomon, etc) can be found in the
/doc directory of the blktrace tools git repo:
<literallayout class='monospaced'>
$ git clone git://git.kernel.dk/blktrace.git
</literallayout>
</para>
</section>
</section>
</chapter>
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