1=========================================================
2Notes on Analysing Behaviour Using Events and Tracepoints
3=========================================================
4:Author: Mel Gorman (PCL information heavily based on email from Ingo Molnar)
5
61. Introduction
7===============
8
9Tracepoints (see Documentation/trace/tracepoints.rst) 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.rst 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
72  global page_allocs
73
74  probe kernel.trace("mm_page_alloc") {
75  	page_allocs[execname()]++
76  }
77
78  function print_count() {
79  	printf ("%-25s %-s\n", "#Pages Allocated", "Process Name")
80  	foreach (proc in page_allocs-)
81  		printf("%-25d %s\n", page_allocs[proc], proc)
82  	printf ("\n")
83  	delete page_allocs
84  }
85
86  probe timer.s(5) {
87          print_count()
88  }
89
903.3 System-Wide Event Enabling with PCL
91---------------------------------------
92
93By specifying the -a switch and analysing sleep, the system-wide events
94for a duration of time can be examined.
95::
96
97 $ perf stat -a \
98	-e kmem:mm_page_alloc -e kmem:mm_page_free \
99	-e kmem:mm_page_free_batched \
100	sleep 10
101 Performance counter stats for 'sleep 10':
102
103           9630  kmem:mm_page_alloc
104           2143  kmem:mm_page_free
105           7424  kmem:mm_page_free_batched
106
107   10.002577764  seconds time elapsed
108
109Similarly, one could execute a shell and exit it as desired to get a report
110at that point.
111
1123.4 Local Event Enabling
113------------------------
114
115Documentation/trace/ftrace.rst describes how to enable events on a per-thread
116basis using set_ftrace_pid.
117
1183.5 Local Event Enablement with PCL
119-----------------------------------
120
121Events can be activated and tracked for the duration of a process on a local
122basis using PCL such as follows.
123::
124
125  $ perf stat -e kmem:mm_page_alloc -e kmem:mm_page_free \
126		 -e kmem:mm_page_free_batched ./hackbench 10
127  Time: 0.909
128
129    Performance counter stats for './hackbench 10':
130
131          17803  kmem:mm_page_alloc
132          12398  kmem:mm_page_free
133           4827  kmem:mm_page_free_batched
134
135    0.973913387  seconds time elapsed
136
1374. Event Filtering
138==================
139
140Documentation/trace/ftrace.rst covers in-depth how to filter events in
141ftrace.  Obviously using grep and awk of trace_pipe is an option as well
142as any script reading trace_pipe.
143
1445. Analysing Event Variances with PCL
145=====================================
146
147Any workload can exhibit variances between runs and it can be important
148to know what the standard deviation is. By and large, this is left to the
149performance analyst to do it by hand. In the event that the discrete event
150occurrences are useful to the performance analyst, then perf can be used.
151::
152
153  $ perf stat --repeat 5 -e kmem:mm_page_alloc -e kmem:mm_page_free
154			-e kmem:mm_page_free_batched ./hackbench 10
155  Time: 0.890
156  Time: 0.895
157  Time: 0.915
158  Time: 1.001
159  Time: 0.899
160
161   Performance counter stats for './hackbench 10' (5 runs):
162
163          16630  kmem:mm_page_alloc         ( +-   3.542% )
164          11486  kmem:mm_page_free	    ( +-   4.771% )
165           4730  kmem:mm_page_free_batched  ( +-   2.325% )
166
167    0.982653002  seconds time elapsed   ( +-   1.448% )
168
169In the event that some higher-level event is required that depends on some
170aggregation of discrete events, then a script would need to be developed.
171
172Using --repeat, it is also possible to view how events are fluctuating over
173time on a system-wide basis using -a and sleep.
174::
175
176  $ perf stat -e kmem:mm_page_alloc -e kmem:mm_page_free \
177		-e kmem:mm_page_free_batched \
178		-a --repeat 10 \
179		sleep 1
180  Performance counter stats for 'sleep 1' (10 runs):
181
182           1066  kmem:mm_page_alloc         ( +-  26.148% )
183            182  kmem:mm_page_free          ( +-   5.464% )
184            890  kmem:mm_page_free_batched  ( +-  30.079% )
185
186    1.002251757  seconds time elapsed   ( +-   0.005% )
187
1886. Higher-Level Analysis with Helper Scripts
189============================================
190
191When events are enabled the events that are triggering can be read from
192/sys/kernel/debug/tracing/trace_pipe in human-readable format although binary
193options exist as well. By post-processing the output, further information can
194be gathered on-line as appropriate. Examples of post-processing might include
195
196  - Reading information from /proc for the PID that triggered the event
197  - Deriving a higher-level event from a series of lower-level events.
198  - Calculating latencies between two events
199
200Documentation/trace/postprocess/trace-pagealloc-postprocess.pl is an example
201script that can read trace_pipe from STDIN or a copy of a trace. When used
202on-line, it can be interrupted once to generate a report without exiting
203and twice to exit.
204
205Simplistically, the script just reads STDIN and counts up events but it
206also can do more such as
207
208  - Derive high-level events from many low-level events. If a number of pages
209    are freed to the main allocator from the per-CPU lists, it recognises
210    that as one per-CPU drain even though there is no specific tracepoint
211    for that event
212  - It can aggregate based on PID or individual process number
213  - In the event memory is getting externally fragmented, it reports
214    on whether the fragmentation event was severe or moderate.
215  - When receiving an event about a PID, it can record who the parent was so
216    that if large numbers of events are coming from very short-lived
217    processes, the parent process responsible for creating all the helpers
218    can be identified
219
2207. Lower-Level Analysis with PCL
221================================
222
223There may also be a requirement to identify what functions within a program
224were generating events within the kernel. To begin this sort of analysis, the
225data must be recorded. At the time of writing, this required root:
226::
227
228  $ perf record -c 1 \
229	-e kmem:mm_page_alloc -e kmem:mm_page_free \
230	-e kmem:mm_page_free_batched \
231	./hackbench 10
232  Time: 0.894
233  [ perf record: Captured and wrote 0.733 MB perf.data (~32010 samples) ]
234
235Note the use of '-c 1' to set the event period to sample. The default sample
236period is quite high to minimise overhead but the information collected can be
237very coarse as a result.
238
239This record outputted a file called perf.data which can be analysed using
240perf report.
241::
242
243  $ perf report
244  # Samples: 30922
245  #
246  # Overhead    Command                     Shared Object
247  # ........  .........  ................................
248  #
249      87.27%  hackbench  [vdso]
250       6.85%  hackbench  /lib/i686/cmov/libc-2.9.so
251       2.62%  hackbench  /lib/ld-2.9.so
252       1.52%       perf  [vdso]
253       1.22%  hackbench  ./hackbench
254       0.48%  hackbench  [kernel]
255       0.02%       perf  /lib/i686/cmov/libc-2.9.so
256       0.01%       perf  /usr/bin/perf
257       0.01%       perf  /lib/ld-2.9.so
258       0.00%  hackbench  /lib/i686/cmov/libpthread-2.9.so
259  #
260  # (For more details, try: perf report --sort comm,dso,symbol)
261  #
262
263According to this, the vast majority of events triggered on events
264within the VDSO. With simple binaries, this will often be the case so let's
265take a slightly different example. In the course of writing this, it was
266noticed that X was generating an insane amount of page allocations so let's look
267at it:
268::
269
270  $ perf record -c 1 -f \
271		-e kmem:mm_page_alloc -e kmem:mm_page_free \
272		-e kmem:mm_page_free_batched \
273		-p `pidof X`
274
275This was interrupted after a few seconds and
276::
277
278  $ perf report
279  # Samples: 27666
280  #
281  # Overhead  Command                            Shared Object
282  # ........  .......  .......................................
283  #
284      51.95%     Xorg  [vdso]
285      47.95%     Xorg  /opt/gfx-test/lib/libpixman-1.so.0.13.1
286       0.09%     Xorg  /lib/i686/cmov/libc-2.9.so
287       0.01%     Xorg  [kernel]
288  #
289  # (For more details, try: perf report --sort comm,dso,symbol)
290  #
291
292So, almost half of the events are occurring in a library. To get an idea which
293symbol:
294::
295
296  $ perf report --sort comm,dso,symbol
297  # Samples: 27666
298  #
299  # Overhead  Command                            Shared Object  Symbol
300  # ........  .......  .......................................  ......
301  #
302      51.95%     Xorg  [vdso]                                   [.] 0x000000ffffe424
303      47.93%     Xorg  /opt/gfx-test/lib/libpixman-1.so.0.13.1  [.] pixmanFillsse2
304       0.09%     Xorg  /lib/i686/cmov/libc-2.9.so               [.] _int_malloc
305       0.01%     Xorg  /opt/gfx-test/lib/libpixman-1.so.0.13.1  [.] pixman_region32_copy_f
306       0.01%     Xorg  [kernel]                                 [k] read_hpet
307       0.01%     Xorg  /opt/gfx-test/lib/libpixman-1.so.0.13.1  [.] get_fast_path
308       0.00%     Xorg  [kernel]                                 [k] ftrace_trace_userstack
309
310To see where within the function pixmanFillsse2 things are going wrong:
311::
312
313  $ perf annotate pixmanFillsse2
314  [ ... ]
315    0.00 :         34eeb:       0f 18 08                prefetcht0 (%eax)
316         :      }
317         :
318         :      extern __inline void __attribute__((__gnu_inline__, __always_inline__, _
319         :      _mm_store_si128 (__m128i *__P, __m128i __B) :      {
320         :        *__P = __B;
321   12.40 :         34eee:       66 0f 7f 80 40 ff ff    movdqa %xmm0,-0xc0(%eax)
322    0.00 :         34ef5:       ff
323   12.40 :         34ef6:       66 0f 7f 80 50 ff ff    movdqa %xmm0,-0xb0(%eax)
324    0.00 :         34efd:       ff
325   12.39 :         34efe:       66 0f 7f 80 60 ff ff    movdqa %xmm0,-0xa0(%eax)
326    0.00 :         34f05:       ff
327   12.67 :         34f06:       66 0f 7f 80 70 ff ff    movdqa %xmm0,-0x90(%eax)
328    0.00 :         34f0d:       ff
329   12.58 :         34f0e:       66 0f 7f 40 80          movdqa %xmm0,-0x80(%eax)
330   12.31 :         34f13:       66 0f 7f 40 90          movdqa %xmm0,-0x70(%eax)
331   12.40 :         34f18:       66 0f 7f 40 a0          movdqa %xmm0,-0x60(%eax)
332   12.31 :         34f1d:       66 0f 7f 40 b0          movdqa %xmm0,-0x50(%eax)
333
334At a glance, it looks like the time is being spent copying pixmaps to
335the card.  Further investigation would be needed to determine why pixmaps
336are being copied around so much but a starting point would be to take an
337ancient build of libpixmap out of the library path where it was totally
338forgotten about from months ago!
339