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 represent important events that 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 (Performance Counters for Linux)
  -------
  
  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 .... ]
  
  
  3. Enabling Events
  ==================
  
  3.1 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
  
  3.2 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
  ", "#Pages Allocated", "Process Name")
    	foreach (proc in page_allocs-)
    		printf("%-25d %s
  ", page_allocs[proc], proc)
    	printf ("
  ")
    	delete page_allocs
    }
  
    probe timer.s(5) {
            print_count()
    }
  
  3.3 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 \
  	-e kmem:mm_page_free_batched \
  	sleep 10
   Performance counter stats for 'sleep 10':
  
             9630  kmem:mm_page_alloc
             2143  kmem:mm_page_free
             7424  kmem:mm_page_free_batched
  
     10.002577764  seconds time elapsed
  
  Similarly, one could execute a shell and exit it as desired to get a report
  at that point.
  
  3.4 Local Event Enabling
  ------------------------
  
  Documentation/trace/ftrace.txt describes how to enable events on a per-thread
  basis using set_ftrace_pid.
  
  3.5 Local Event Enablement with PCL
  -----------------------------------
  
  Events can be activated 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 \
  		 -e kmem:mm_page_free_batched ./hackbench 10
    Time: 0.909
  
      Performance counter stats for './hackbench 10':
  
            17803  kmem:mm_page_alloc
            12398  kmem:mm_page_free
             4827  kmem:mm_page_free_batched
  
      0.973913387  seconds time elapsed
  
  4. 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.
  
  5. Analysing Event Variances with PCL
  =====================================
  
  Any workload can exhibit variances between runs and it can be important
  to know what the standard deviation is. 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
  			-e kmem:mm_page_free_batched ./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	    ( +-   4.771% )
             4730  kmem:mm_page_free_batched  ( +-   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 \
  		-e kmem:mm_page_free_batched \
  		-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          ( +-   5.464% )
              890  kmem:mm_page_free_batched  ( +-  30.079% )
  
      1.002251757  seconds time elapsed   ( +-   0.005% )
  
  6. 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 Calculating 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 exiting
  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
  
  7. Lower-Level Analysis with PCL
  ================================
  
  There may also be a requirement to identify what functions within 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 \
  	-e kmem:mm_page_free_batched \
  	./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 triggered on events
  within the VDSO. With simple binaries, this will often be the case so let's
  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 let's look
  at it:
  
    $ perf record -c 1 -f \
  		-e kmem:mm_page_alloc -e kmem:mm_page_free \
  		-e kmem:mm_page_free_batched \
  		-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 occurring 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!