tracing, documentation: add a document describing how to do some performance...

		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!