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Linux/Documentation/trace/tracepoint-analysis.txt

  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 
  6 1. Introduction
  7 ===============
  8 
  9 Tracepoints (see Documentation/trace/tracepoints.txt) can be used without
 10 creating custom kernel modules to register probe functions using the event
 11 tracing infrastructure.
 12 
 13 Simplistically, tracepoints represent important events that can be
 14 taken in conjunction with other tracepoints to build a "Big Picture" of
 15 what is going on within the system. There are a large number of methods for
 16 gathering and interpreting these events. Lacking any current Best Practises,
 17 this document describes some of the methods that can be used.
 18 
 19 This document assumes that debugfs is mounted on /sys/kernel/debug and that
 20 the appropriate tracing options have been configured into the kernel. It is
 21 assumed that the PCL tool tools/perf has been installed and is in your path.
 22 
 23 2. Listing Available Events
 24 ===========================
 25 
 26 2.1 Standard Utilities
 27 ----------------------
 28 
 29 All possible events are visible from /sys/kernel/debug/tracing/events. Simply
 30 calling
 31 
 32   $ find /sys/kernel/debug/tracing/events -type d
 33 
 34 will give a fair indication of the number of events available.
 35 
 36 2.2 PCL (Performance Counters for Linux)
 37 -------
 38 
 39 Discovery and enumeration of all counters and events, including tracepoints,
 40 are available with the perf tool. Getting a list of available events is a
 41 simple 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 
 52 3. Enabling Events
 53 ==================
 54 
 55 3.1 System-Wide Event Enabling
 56 ------------------------------
 57 
 58 See Documentation/trace/events.txt for a proper description on how events
 59 can be enabled system-wide. A short example of enabling all events related
 60 to 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 
 64 3.2 System-Wide Event Enabling with SystemTap
 65 ---------------------------------------------
 66 
 67 In SystemTap, tracepoints are accessible using the kernel.trace() function
 68 call. The following is an example that reports every 5 seconds what processes
 69 were 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 
 89 3.3 System-Wide Event Enabling with PCL
 90 ---------------------------------------
 91 
 92 By specifying the -a switch and analysing sleep, the system-wide events
 93 for 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 
107 Similarly, one could execute a shell and exit it as desired to get a report
108 at that point.
109 
110 3.4 Local Event Enabling
111 ------------------------
112 
113 Documentation/trace/ftrace.txt describes how to enable events on a per-thread
114 basis using set_ftrace_pid.
115 
116 3.5 Local Event Enablement with PCL
117 -----------------------------------
118 
119 Events can be activated and tracked for the duration of a process on a local
120 basis 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 
134 4. Event Filtering
135 ==================
136 
137 Documentation/trace/ftrace.txt covers in-depth how to filter events in
138 ftrace.  Obviously using grep and awk of trace_pipe is an option as well
139 as any script reading trace_pipe.
140 
141 5. Analysing Event Variances with PCL
142 =====================================
143 
144 Any workload can exhibit variances between runs and it can be important
145 to know what the standard deviation is. By and large, this is left to the
146 performance analyst to do it by hand. In the event that the discrete event
147 occurrences 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 
165 In the event that some higher-level event is required that depends on some
166 aggregation of discrete events, then a script would need to be developed.
167 
168 Using --repeat, it is also possible to view how events are fluctuating over
169 time 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 
183 6. Higher-Level Analysis with Helper Scripts
184 ============================================
185 
186 When 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
188 options exist as well. By post-processing the output, further information can
189 be 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 
195 Documentation/trace/postprocess/trace-pagealloc-postprocess.pl is an example
196 script that can read trace_pipe from STDIN or a copy of a trace. When used
197 on-line, it can be interrupted once to generate a report without exiting
198 and twice to exit.
199 
200 Simplistically, the script just reads STDIN and counts up events but it
201 also 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 
215 7. Lower-Level Analysis with PCL
216 ================================
217 
218 There may also be a requirement to identify what functions within a program
219 were generating events within the kernel. To begin this sort of analysis, the
220 data 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 
229 Note the use of '-c 1' to set the event period to sample. The default sample
230 period is quite high to minimise overhead but the information collected can be
231 very coarse as a result.
232 
233 This record outputted a file called perf.data which can be analysed using
234 perf 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 
256 According to this, the vast majority of events triggered on events
257 within the VDSO. With simple binaries, this will often be the case so let's
258 take a slightly different example. In the course of writing this, it was
259 noticed that X was generating an insane amount of page allocations so let's look
260 at 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 
267 This 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 
283 So, almost half of the events are occurring in a library. To get an idea which
284 symbol:
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 
300 To 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 
323 At a glance, it looks like the time is being spent copying pixmaps to
324 the card.  Further investigation would be needed to determine why pixmaps
325 are being copied around so much but a starting point would be to take an
326 ancient build of libpixmap out of the library path where it was totally
327 forgotten about from months ago!

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