linux/Documentation/trace/tracepoint-analysis.txt
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   1                Notes on Analysing Behaviour Using Events and Tracepoints
   2
   3                        Documentation written by Mel Gorman
   4                PCL information heavily based on email from Ingo Molnar
   5
   61. Introduction
   7===============
   8
   9Tracepoints (see Documentation/trace/tracepoints.txt) can be used without
  10creating custom kernel modules to register probe functions using the event
  11tracing infrastructure.
  12
  13Simplistically, tracepoints represent important events that can be
  14taken in conjunction with other tracepoints to build a "Big Picture" of
  15what is going on within the system. There are a large number of methods for
  16gathering and interpreting these events. Lacking any current Best Practises,
  17this document describes some of the methods that can be used.
  18
  19This document assumes that debugfs is mounted on /sys/kernel/debug and that
  20the appropriate tracing options have been configured into the kernel. It is
  21assumed that the PCL tool tools/perf has been installed and is in your path.
  22
  232. Listing Available Events
  24===========================
  25
  262.1 Standard Utilities
  27----------------------
  28
  29All possible events are visible from /sys/kernel/debug/tracing/events. Simply
  30calling
  31
  32  $ find /sys/kernel/debug/tracing/events -type d
  33
  34will give a fair indication of the number of events available.
  35
  362.2 PCL (Performance Counters for Linux)
  37-------
  38
  39Discovery and enumeration of all counters and events, including tracepoints,
  40are available with the perf tool. Getting a list of available events is a
  41simple case of:
  42
  43  $ perf list 2>&1 | grep Tracepoint
  44  ext4:ext4_free_inode                     [Tracepoint event]
  45  ext4:ext4_request_inode                  [Tracepoint event]
  46  ext4:ext4_allocate_inode                 [Tracepoint event]
  47  ext4:ext4_write_begin                    [Tracepoint event]
  48  ext4:ext4_ordered_write_end              [Tracepoint event]
  49  [ .... remaining output snipped .... ]
  50
  51
  523. Enabling Events
  53==================
  54
  553.1 System-Wide Event Enabling
  56------------------------------
  57
  58See Documentation/trace/events.txt for a proper description on how events
  59can be enabled system-wide. A short example of enabling all events related
  60to page allocation would look something like:
  61
  62  $ for i in `find /sys/kernel/debug/tracing/events -name "enable" | grep mm_`; do echo 1 > $i; done
  63
  643.2 System-Wide Event Enabling with SystemTap
  65---------------------------------------------
  66
  67In SystemTap, tracepoints are accessible using the kernel.trace() function
  68call. The following is an example that reports every 5 seconds what processes
  69were allocating the pages.
  70
  71  global page_allocs
  72
  73  probe kernel.trace("mm_page_alloc") {
  74        page_allocs[execname()]++
  75  }
  76
  77  function print_count() {
  78        printf ("%-25s %-s\n", "#Pages Allocated", "Process Name")
  79        foreach (proc in page_allocs-)
  80                printf("%-25d %s\n", page_allocs[proc], proc)
  81        printf ("\n")
  82        delete page_allocs
  83  }
  84
  85  probe timer.s(5) {
  86          print_count()
  87  }
  88
  893.3 System-Wide Event Enabling with PCL
  90---------------------------------------
  91
  92By specifying the -a switch and analysing sleep, the system-wide events
  93for a duration of time can be examined.
  94
  95 $ perf stat -a \
  96        -e kmem:mm_page_alloc -e kmem:mm_page_free \
  97        -e kmem:mm_page_free_batched \
  98        sleep 10
  99 Performance counter stats for 'sleep 10':
 100
 101           9630  kmem:mm_page_alloc
 102           2143  kmem:mm_page_free
 103           7424  kmem:mm_page_free_batched
 104
 105   10.002577764  seconds time elapsed
 106
 107Similarly, one could execute a shell and exit it as desired to get a report
 108at that point.
 109
 1103.4 Local Event Enabling
 111------------------------
 112
 113Documentation/trace/ftrace.txt describes how to enable events on a per-thread
 114basis using set_ftrace_pid.
 115
 1163.5 Local Event Enablement with PCL
 117-----------------------------------
 118
 119Events can be activated and tracked for the duration of a process on a local
 120basis using PCL such as follows.
 121
 122  $ perf stat -e kmem:mm_page_alloc -e kmem:mm_page_free \
 123                 -e kmem:mm_page_free_batched ./hackbench 10
 124  Time: 0.909
 125
 126    Performance counter stats for './hackbench 10':
 127
 128          17803  kmem:mm_page_alloc
 129          12398  kmem:mm_page_free
 130           4827  kmem:mm_page_free_batched
 131
 132    0.973913387  seconds time elapsed
 133
 1344. Event Filtering
 135==================
 136
 137Documentation/trace/ftrace.txt covers in-depth how to filter events in
 138ftrace.  Obviously using grep and awk of trace_pipe is an option as well
 139as any script reading trace_pipe.
 140
 1415. Analysing Event Variances with PCL
 142=====================================
 143
 144Any workload can exhibit variances between runs and it can be important
 145to know what the standard deviation is. By and large, this is left to the
 146performance analyst to do it by hand. In the event that the discrete event
 147occurrences are useful to the performance analyst, then perf can be used.
 148
 149  $ perf stat --repeat 5 -e kmem:mm_page_alloc -e kmem:mm_page_free
 150                        -e kmem:mm_page_free_batched ./hackbench 10
 151  Time: 0.890
 152  Time: 0.895
 153  Time: 0.915
 154  Time: 1.001
 155  Time: 0.899
 156
 157   Performance counter stats for './hackbench 10' (5 runs):
 158
 159          16630  kmem:mm_page_alloc         ( +-   3.542% )
 160          11486  kmem:mm_page_free          ( +-   4.771% )
 161           4730  kmem:mm_page_free_batched  ( +-   2.325% )
 162
 163    0.982653002  seconds time elapsed   ( +-   1.448% )
 164
 165In the event that some higher-level event is required that depends on some
 166aggregation of discrete events, then a script would need to be developed.
 167
 168Using --repeat, it is also possible to view how events are fluctuating over
 169time on a system-wide basis using -a and sleep.
 170
 171  $ perf stat -e kmem:mm_page_alloc -e kmem:mm_page_free \
 172                -e kmem:mm_page_free_batched \
 173                -a --repeat 10 \
 174                sleep 1
 175  Performance counter stats for 'sleep 1' (10 runs):
 176
 177           1066  kmem:mm_page_alloc         ( +-  26.148% )
 178            182  kmem:mm_page_free          ( +-   5.464% )
 179            890  kmem:mm_page_free_batched  ( +-  30.079% )
 180
 181    1.002251757  seconds time elapsed   ( +-   0.005% )
 182
 1836. Higher-Level Analysis with Helper Scripts
 184============================================
 185
 186When events are enabled the events that are triggering can be read from
 187/sys/kernel/debug/tracing/trace_pipe in human-readable format although binary
 188options exist as well. By post-processing the output, further information can
 189be gathered on-line as appropriate. Examples of post-processing might include
 190
 191  o Reading information from /proc for the PID that triggered the event
 192  o Deriving a higher-level event from a series of lower-level events.
 193  o Calculating latencies between two events
 194
 195Documentation/trace/postprocess/trace-pagealloc-postprocess.pl is an example
 196script that can read trace_pipe from STDIN or a copy of a trace. When used
 197on-line, it can be interrupted once to generate a report without exiting
 198and twice to exit.
 199
 200Simplistically, the script just reads STDIN and counts up events but it
 201also can do more such as
 202
 203  o Derive high-level events from many low-level events. If a number of pages
 204    are freed to the main allocator from the per-CPU lists, it recognises
 205    that as one per-CPU drain even though there is no specific tracepoint
 206    for that event
 207  o It can aggregate based on PID or individual process number
 208  o In the event memory is getting externally fragmented, it reports
 209    on whether the fragmentation event was severe or moderate.
 210  o When receiving an event about a PID, it can record who the parent was so
 211    that if large numbers of events are coming from very short-lived
 212    processes, the parent process responsible for creating all the helpers
 213    can be identified
 214
 2157. Lower-Level Analysis with PCL
 216================================
 217
 218There may also be a requirement to identify what functions within a program
 219were generating events within the kernel. To begin this sort of analysis, the
 220data must be recorded. At the time of writing, this required root:
 221
 222  $ perf record -c 1 \
 223        -e kmem:mm_page_alloc -e kmem:mm_page_free \
 224        -e kmem:mm_page_free_batched \
 225        ./hackbench 10
 226  Time: 0.894
 227  [ perf record: Captured and wrote 0.733 MB perf.data (~32010 samples) ]
 228
 229Note the use of '-c 1' to set the event period to sample. The default sample
 230period is quite high to minimise overhead but the information collected can be
 231very coarse as a result.
 232
 233This record outputted a file called perf.data which can be analysed using
 234perf report.
 235
 236  $ perf report
 237  # Samples: 30922
 238  #
 239  # Overhead    Command                     Shared Object
 240  # ........  .........  ................................
 241  #
 242      87.27%  hackbench  [vdso]
 243       6.85%  hackbench  /lib/i686/cmov/libc-2.9.so
 244       2.62%  hackbench  /lib/ld-2.9.so
 245       1.52%       perf  [vdso]
 246       1.22%  hackbench  ./hackbench
 247       0.48%  hackbench  [kernel]
 248       0.02%       perf  /lib/i686/cmov/libc-2.9.so
 249       0.01%       perf  /usr/bin/perf
 250       0.01%       perf  /lib/ld-2.9.so
 251       0.00%  hackbench  /lib/i686/cmov/libpthread-2.9.so
 252  #
 253  # (For more details, try: perf report --sort comm,dso,symbol)
 254  #
 255
 256According to this, the vast majority of events triggered on events
 257within the VDSO. With simple binaries, this will often be the case so let's
 258take a slightly different example. In the course of writing this, it was
 259noticed that X was generating an insane amount of page allocations so let's look
 260at it:
 261
 262  $ perf record -c 1 -f \
 263                -e kmem:mm_page_alloc -e kmem:mm_page_free \
 264                -e kmem:mm_page_free_batched \
 265                -p `pidof X`
 266
 267This was interrupted after a few seconds and
 268
 269  $ perf report
 270  # Samples: 27666
 271  #
 272  # Overhead  Command                            Shared Object
 273  # ........  .......  .......................................
 274  #
 275      51.95%     Xorg  [vdso]
 276      47.95%     Xorg  /opt/gfx-test/lib/libpixman-1.so.0.13.1
 277       0.09%     Xorg  /lib/i686/cmov/libc-2.9.so
 278       0.01%     Xorg  [kernel]
 279  #
 280  # (For more details, try: perf report --sort comm,dso,symbol)
 281  #
 282
 283So, almost half of the events are occurring in a library. To get an idea which
 284symbol:
 285
 286  $ perf report --sort comm,dso,symbol
 287  # Samples: 27666
 288  #
 289  # Overhead  Command                            Shared Object  Symbol
 290  # ........  .......  .......................................  ......
 291  #
 292      51.95%     Xorg  [vdso]                                   [.] 0x000000ffffe424
 293      47.93%     Xorg  /opt/gfx-test/lib/libpixman-1.so.0.13.1  [.] pixmanFillsse2
 294       0.09%     Xorg  /lib/i686/cmov/libc-2.9.so               [.] _int_malloc
 295       0.01%     Xorg  /opt/gfx-test/lib/libpixman-1.so.0.13.1  [.] pixman_region32_copy_f
 296       0.01%     Xorg  [kernel]                                 [k] read_hpet
 297       0.01%     Xorg  /opt/gfx-test/lib/libpixman-1.so.0.13.1  [.] get_fast_path
 298       0.00%     Xorg  [kernel]                                 [k] ftrace_trace_userstack
 299
 300To see where within the function pixmanFillsse2 things are going wrong:
 301
 302  $ perf annotate pixmanFillsse2
 303  [ ... ]
 304    0.00 :         34eeb:       0f 18 08                prefetcht0 (%eax)
 305         :      }
 306         :
 307         :      extern __inline void __attribute__((__gnu_inline__, __always_inline__, _
 308         :      _mm_store_si128 (__m128i *__P, __m128i __B) :      {
 309         :        *__P = __B;
 310   12.40 :         34eee:       66 0f 7f 80 40 ff ff    movdqa %xmm0,-0xc0(%eax)
 311    0.00 :         34ef5:       ff
 312   12.40 :         34ef6:       66 0f 7f 80 50 ff ff    movdqa %xmm0,-0xb0(%eax)
 313    0.00 :         34efd:       ff
 314   12.39 :         34efe:       66 0f 7f 80 60 ff ff    movdqa %xmm0,-0xa0(%eax)
 315    0.00 :         34f05:       ff
 316   12.67 :         34f06:       66 0f 7f 80 70 ff ff    movdqa %xmm0,-0x90(%eax)
 317    0.00 :         34f0d:       ff
 318   12.58 :         34f0e:       66 0f 7f 40 80          movdqa %xmm0,-0x80(%eax)
 319   12.31 :         34f13:       66 0f 7f 40 90          movdqa %xmm0,-0x70(%eax)
 320   12.40 :         34f18:       66 0f 7f 40 a0          movdqa %xmm0,-0x60(%eax)
 321   12.31 :         34f1d:       66 0f 7f 40 b0          movdqa %xmm0,-0x50(%eax)
 322
 323At a glance, it looks like the time is being spent copying pixmaps to
 324the card.  Further investigation would be needed to determine why pixmaps
 325are being copied around so much but a starting point would be to take an
 326ancient build of libpixmap out of the library path where it was totally
 327forgotten about from months ago!
 328
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