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SubjectRe: [PATCH 5/6] tracing, documentation: Add a document describing how to do some performance analysis with tracepoints

* Mel Gorman <mel@csn.ul.ie> wrote:

> The documentation for ftrace, events and tracepoints is pretty
> extensive. Similarly, the perf PCL tools help files --help are
> there and the code simple enough to figure out what much of the
> switches mean. However, pulling the discrete bits and pieces
> together and translating that into "how do I solve a problem"
> requires a fair amount of imagination.
>
> This patch adds a simple document intended to get someone started
> on the different ways of using tracepoints to gather meaningful
> data.
>
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> ---
> Documentation/trace/tracepoint-analysis.txt | 327 +++++++++++++++++++++++++++
> 1 files changed, 327 insertions(+), 0 deletions(-)
> create mode 100644 Documentation/trace/tracepoint-analysis.txt
>
> diff --git a/Documentation/trace/tracepoint-analysis.txt b/Documentation/trace/tracepoint-analysis.txt
> new file mode 100644
> index 0000000..e7a7d3e
> --- /dev/null
> +++ b/Documentation/trace/tracepoint-analysis.txt
> @@ -0,0 +1,327 @@
> + Notes on Analysing Behaviour Using Events and Tracepoints
> +
> + Documentation written by Mel Gorman
> + PCL information heavily based on email from Ingo Molnar
> +
> +1. Introduction
> +===============
> +
> +Tracepoints (see Documentation/trace/tracepoints.txt) can be used without
> +creating custom kernel modules to register probe functions using the event
> +tracing infrastructure.
> +
> +Simplistically, tracepoints will represent an important event that when can
> +be taken in conjunction with other tracepoints to build a "Big Picture" of
> +what is going on within the system. There are a large number of methods for
> +gathering and interpreting these events. Lacking any current Best Practises,
> +this document describes some of the methods that can be used.
> +
> +This document assumes that debugfs is mounted on /sys/kernel/debug and that
> +the appropriate tracing options have been configured into the kernel. It is
> +assumed that the PCL tool tools/perf has been installed and is in your path.
> +
> +2. Listing Available Events
> +===========================
> +
> +2.1 Standard Utilities
> +----------------------
> +
> +All possible events are visible from /sys/kernel/debug/tracing/events. Simply
> +calling
> +
> + $ find /sys/kernel/debug/tracing/events -type d
> +
> +will give a fair indication of the number of events available.
> +
> +2.2 PCL
> +-------
> +
> +Discovery and enumeration of all counters and events, including tracepoints
> +are available with the perf tool. Getting a list of available events is a
> +simple case of
> +
> + $ perf list 2>&1 | grep Tracepoint
> + ext4:ext4_free_inode [Tracepoint event]
> + ext4:ext4_request_inode [Tracepoint event]
> + ext4:ext4_allocate_inode [Tracepoint event]
> + ext4:ext4_write_begin [Tracepoint event]
> + ext4:ext4_ordered_write_end [Tracepoint event]
> + [ .... remaining output snipped .... ]
> +
> +
> +2. Enabling Events
> +==================
> +
> +2.1 System-Wide Event Enabling
> +------------------------------
> +
> +See Documentation/trace/events.txt for a proper description on how events
> +can be enabled system-wide. A short example of enabling all events related
> +to page allocation would look something like
> +
> + $ for i in `find /sys/kernel/debug/tracing/events -name "enable" | grep mm_`; do echo 1 > $i; done
> +
> +2.2 System-Wide Event Enabling with SystemTap
> +---------------------------------------------
> +
> +In SystemTap, tracepoints are accessible using the kernel.trace() function
> +call. The following is an example that reports every 5 seconds what processes
> +were allocating the pages.
> +
> + global page_allocs
> +
> + probe kernel.trace("mm_page_alloc") {
> + page_allocs[execname()]++
> + }
> +
> + function print_count() {
> + printf ("%-25s %-s\n", "#Pages Allocated", "Process Name")
> + foreach (proc in page_allocs-)
> + printf("%-25d %s\n", page_allocs[proc], proc)
> + printf ("\n")
> + delete page_allocs
> + }
> +
> + probe timer.s(5) {
> + print_count()
> + }
> +
> +2.3 System-Wide Event Enabling with PCL
> +---------------------------------------
> +
> +By specifying the -a switch and analysing sleep, the system-wide events
> +for a duration of time can be examined.
> +
> + $ perf stat -a \
> + -e kmem:mm_page_alloc -e kmem:mm_page_free_direct \
> + -e kmem:mm_pagevec_free \
> + sleep 10
> + Performance counter stats for 'sleep 10':
> +
> + 9630 kmem:mm_page_alloc
> + 2143 kmem:mm_page_free_direct
> + 7424 kmem:mm_pagevec_free
> +
> + 10.002577764 seconds time elapsed
> +
> +Similarly, one could execute a shell and exit it as desired to get a report
> +at that point.
> +
> +2.4 Local Event Enabling
> +------------------------
> +
> +Documentation/trace/ftrace.txt describes how to enable events on a per-thread
> +basis using set_ftrace_pid.
> +
> +2.5 Local Event Enablement with PCL
> +-----------------------------------
> +
> +Events can be activate and tracked for the duration of a process on a local
> +basis using PCL such as follows.
> +
> + $ perf stat -e kmem:mm_page_alloc -e kmem:mm_page_free_direct \
> + -e kmem:mm_pagevec_free ./hackbench 10
> + Time: 0.909
> +
> + Performance counter stats for './hackbench 10':
> +
> + 17803 kmem:mm_page_alloc
> + 12398 kmem:mm_page_free_direct
> + 4827 kmem:mm_pagevec_free
> +
> + 0.973913387 seconds time elapsed
> +
> +3. Event Filtering
> +==================
> +
> +Documentation/trace/ftrace.txt covers in-depth how to filter events in
> +ftrace. Obviously using grep and awk of trace_pipe is an option as well
> +as any script reading trace_pipe.
> +
> +4. Analysing Event Variances with PCL
> +=====================================
> +
> +Any workload can exhibit variances between runs and it can be important
> +to know what the standard deviation in. By and large, this is left to the
> +performance analyst to do it by hand. In the event that the discrete event
> +occurrences are useful to the performance analyst, then perf can be used.
> +
> + $ perf stat --repeat 5 -e kmem:mm_page_alloc -e kmem:mm_page_free_direct
> + -e kmem:mm_pagevec_free ./hackbench 10
> + Time: 0.890
> + Time: 0.895
> + Time: 0.915
> + Time: 1.001
> + Time: 0.899
> +
> + Performance counter stats for './hackbench 10' (5 runs):
> +
> + 16630 kmem:mm_page_alloc ( +- 3.542% )
> + 11486 kmem:mm_page_free_direct ( +- 4.771% )
> + 4730 kmem:mm_pagevec_free ( +- 2.325% )
> +
> + 0.982653002 seconds time elapsed ( +- 1.448% )
> +
> +In the event that some higher-level event is required that depends on some
> +aggregation of discrete events, then a script would need to be developed.
> +
> +Using --repeat, it is also possible to view how events are fluctuating over
> +time on a system wide basis using -a and sleep.
> +
> + $ perf stat -e kmem:mm_page_alloc -e kmem:mm_page_free_direct \
> + -e kmem:mm_pagevec_free \
> + -a --repeat 10 \
> + sleep 1
> + Performance counter stats for 'sleep 1' (10 runs):
> +
> + 1066 kmem:mm_page_alloc ( +- 26.148% )
> + 182 kmem:mm_page_free_direct ( +- 5.464% )
> + 890 kmem:mm_pagevec_free ( +- 30.079% )
> +
> + 1.002251757 seconds time elapsed ( +- 0.005% )
> +
> +5. Higher-Level Analysis with Helper Scripts
> +============================================
> +
> +When events are enabled the events that are triggering can be read from
> +/sys/kernel/debug/tracing/trace_pipe in human-readable format although binary
> +options exist as well. By post-processing the output, further information can
> +be gathered on-line as appropriate. Examples of post-processing might include
> +
> + o Reading information from /proc for the PID that triggered the event
> + o Deriving a higher-level event from a series of lower-level events.
> + o Calculate latencies between two events
> +
> +Documentation/trace/postprocess/trace-pagealloc-postprocess.pl is an example
> +script that can read trace_pipe from STDIN or a copy of a trace. When used
> +on-line, it can be interrupted once to generate a report without existing
> +and twice to exit.
> +
> +Simplistically, the script just reads STDIN and counts up events but it
> +also can do more such as
> +
> + o Derive high-level events from many low-level events. If a number of pages
> + are freed to the main allocator from the per-CPU lists, it recognises
> + that as one per-CPU drain even though there is no specific tracepoint
> + for that event
> + o It can aggregate based on PID or individual process number
> + o In the event memory is getting externally fragmented, it reports
> + on whether the fragmentation event was severe or moderate.
> + o When receiving an event about a PID, it can record who the parent was so
> + that if large numbers of events are coming from very short-lived
> + processes, the parent process responsible for creating all the helpers
> + can be identified
> +
> +6. Lower-Level Analysis with PCL
> +================================
> +
> +There may also be a requirement to identify what functions with a program
> +were generating events within the kernel. To begin this sort of analysis, the
> +data must be recorded. At the time of writing, this required root
> +
> + $ perf record -c 1 \
> + -e kmem:mm_page_alloc -e kmem:mm_page_free_direct \
> + -e kmem:mm_pagevec_free \
> + ./hackbench 10
> + Time: 0.894
> + [ perf record: Captured and wrote 0.733 MB perf.data (~32010 samples) ]
> +
> +Note the use of '-c 1' to set the event period to sample. The default sample
> +period is quite high to minimise overhead but the information collected can be
> +very coarse as a result.
> +
> +This record outputted a file called perf.data which can be analysed using
> +perf report.
> +
> + $ perf report
> + # Samples: 30922
> + #
> + # Overhead Command Shared Object
> + # ........ ......... ................................
> + #
> + 87.27% hackbench [vdso]
> + 6.85% hackbench /lib/i686/cmov/libc-2.9.so
> + 2.62% hackbench /lib/ld-2.9.so
> + 1.52% perf [vdso]
> + 1.22% hackbench ./hackbench
> + 0.48% hackbench [kernel]
> + 0.02% perf /lib/i686/cmov/libc-2.9.so
> + 0.01% perf /usr/bin/perf
> + 0.01% perf /lib/ld-2.9.so
> + 0.00% hackbench /lib/i686/cmov/libpthread-2.9.so
> + #
> + # (For more details, try: perf report --sort comm,dso,symbol)
> + #
> +
> +According to this, the vast majority of events occured triggered on events
> +within the VDSO. With simple binaries, this will often be the case so lets
> +take a slightly different example. In the course of writing this, it was
> +noticed that X was generating an insane amount of page allocations so lets look
> +at it
> +
> + $ perf record -c 1 -f \
> + -e kmem:mm_page_alloc -e kmem:mm_page_free_direct \
> + -e kmem:mm_pagevec_free \
> + -p `pidof X`
> +
> +This was interrupted after a few seconds and
> +
> + $ perf report
> + # Samples: 27666
> + #
> + # Overhead Command Shared Object
> + # ........ ....... .......................................
> + #
> + 51.95% Xorg [vdso]
> + 47.95% Xorg /opt/gfx-test/lib/libpixman-1.so.0.13.1
> + 0.09% Xorg /lib/i686/cmov/libc-2.9.so
> + 0.01% Xorg [kernel]
> + #
> + # (For more details, try: perf report --sort comm,dso,symbol)
> + #
> +
> +So, almost half of the events are occuring in a library. To get an idea which
> +symbol.
> +
> + $ perf report --sort comm,dso,symbol
> + # Samples: 27666
> + #
> + # Overhead Command Shared Object Symbol
> + # ........ ....... ....................................... ......
> + #
> + 51.95% Xorg [vdso] [.] 0x000000ffffe424
> + 47.93% Xorg /opt/gfx-test/lib/libpixman-1.so.0.13.1 [.] pixmanFillsse2
> + 0.09% Xorg /lib/i686/cmov/libc-2.9.so [.] _int_malloc
> + 0.01% Xorg /opt/gfx-test/lib/libpixman-1.so.0.13.1 [.] pixman_region32_copy_f
> + 0.01% Xorg [kernel] [k] read_hpet
> + 0.01% Xorg /opt/gfx-test/lib/libpixman-1.so.0.13.1 [.] get_fast_path
> + 0.00% Xorg [kernel] [k] ftrace_trace_userstack
> +
> +To see where within the function pixmanFillsse2 things are going wrong
> +
> + $ perf annotate pixmanFillsse2
> + [ ... ]
> + 0.00 : 34eeb: 0f 18 08 prefetcht0 (%eax)
> + : }
> + :
> + : extern __inline void __attribute__((__gnu_inline__, __always_inline__, _
> + : _mm_store_si128 (__m128i *__P, __m128i __B) : {
> + : *__P = __B;
> + 12.40 : 34eee: 66 0f 7f 80 40 ff ff movdqa %xmm0,-0xc0(%eax)
> + 0.00 : 34ef5: ff
> + 12.40 : 34ef6: 66 0f 7f 80 50 ff ff movdqa %xmm0,-0xb0(%eax)
> + 0.00 : 34efd: ff
> + 12.39 : 34efe: 66 0f 7f 80 60 ff ff movdqa %xmm0,-0xa0(%eax)
> + 0.00 : 34f05: ff
> + 12.67 : 34f06: 66 0f 7f 80 70 ff ff movdqa %xmm0,-0x90(%eax)
> + 0.00 : 34f0d: ff
> + 12.58 : 34f0e: 66 0f 7f 40 80 movdqa %xmm0,-0x80(%eax)
> + 12.31 : 34f13: 66 0f 7f 40 90 movdqa %xmm0,-0x70(%eax)
> + 12.40 : 34f18: 66 0f 7f 40 a0 movdqa %xmm0,-0x60(%eax)
> + 12.31 : 34f1d: 66 0f 7f 40 b0 movdqa %xmm0,-0x50(%eax)
> +
> +At a glance, it looks like the time is being spent copying pixmaps to
> +the card. Further investigation would be needed to determine why pixmaps
> +are being copied around so much but a starting point would be to take an
> +ancient build of libpixmap out of the library path where it was totally
> +forgotten about from months ago!

This is a very nice and comprehensive description!

I'm wondering: would you mind if we integrated the analysis ideas
from your perl script into 'perf trace'? Those kinds of high-level
counts and summaries are useful not just for MM events.

Another thing that was raise dbefore is a 'perf mem' special-purpose
tool to help the analysis of all things memory related: leak
detection, high level stats, etc. That could have some turn-key
modes of analysis for the page allocator too.

perf will do a proper format-string evaluation of
/debug/tracing/events/*/format as well, thus any tweaks to the
tracepoints get automatically adopted to.
Ingo


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