1 ==============================================
2 Notes on Analysing Behaviour Using Events and
3 ==============================================
4 :Author: Mel Gorman (PCL information heavily b
5
6 1. Introduction
7 ===============
8
9 Tracepoints (see Documentation/trace/tracepoin
10 creating custom kernel modules to register pro
11 tracing infrastructure.
12
13 Simplistically, tracepoints represent importan
14 taken in conjunction with other tracepoints to
15 what is going on within the system. There are
16 gathering and interpreting these events. Lacki
17 this document describes some of the methods th
18
19 This document assumes that debugfs is mounted
20 the appropriate tracing options have been conf
21 assumed that the PCL tool tools/perf has been
22
23 2. Listing Available Events
24 ===========================
25
26 2.1 Standard Utilities
27 ----------------------
28
29 All possible events are visible from /sys/kern
30 calling::
31
32 $ find /sys/kernel/tracing/events -type d
33
34 will give a fair indication of the number of e
35
36 2.2 PCL (Performance Counters for Linux)
37 ----------------------------------------
38
39 Discovery and enumeration of all counters and
40 are available with the perf tool. Getting a li
41 simple case of::
42
43 $ perf list 2>&1 | grep Tracepoint
44 ext4:ext4_free_inode [Tr
45 ext4:ext4_request_inode [Tr
46 ext4:ext4_allocate_inode [Tr
47 ext4:ext4_write_begin [Tr
48 ext4:ext4_ordered_write_end [Tr
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.rst for a prope
59 can be enabled system-wide. A short example of
60 to page allocation would look something like::
61
62 $ for i in `find /sys/kernel/tracing/events
63
64 3.2 System-Wide Event Enabling with SystemTap
65 ---------------------------------------------
66
67 In SystemTap, tracepoints are accessible using
68 call. The following is an example that reports
69 were 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 Allocat
80 foreach (proc in page_allocs-)
81 printf("%-25d %s\n", page_allo
82 printf ("\n")
83 delete page_allocs
84 }
85
86 probe timer.s(5) {
87 print_count()
88 }
89
90 3.3 System-Wide Event Enabling with PCL
91 ---------------------------------------
92
93 By specifying the -a switch and analysing slee
94 for a duration of time can be examined.
95 ::
96
97 $ perf stat -a \
98 -e kmem:mm_page_alloc -e kmem:mm_page_
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
109 Similarly, one could execute a shell and exit
110 at that point.
111
112 3.4 Local Event Enabling
113 ------------------------
114
115 Documentation/trace/ftrace.rst describes how t
116 basis using set_ftrace_pid.
117
118 3.5 Local Event Enablement with PCL
119 -----------------------------------
120
121 Events can be activated and tracked for the du
122 basis using PCL such as follows.
123 ::
124
125 $ perf stat -e kmem:mm_page_alloc -e kmem:mm
126 -e kmem:mm_page_free_batched
127 Time: 0.909
128
129 Performance counter stats for './hackbench
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
137 4. Event Filtering
138 ==================
139
140 Documentation/trace/ftrace.rst covers in-depth
141 ftrace. Obviously using grep and awk of trace
142 as any script reading trace_pipe.
143
144 5. Analysing Event Variances with PCL
145 =====================================
146
147 Any workload can exhibit variances between run
148 to know what the standard deviation is. By and
149 performance analyst to do it by hand. In the e
150 occurrences are useful to the performance anal
151 ::
152
153 $ perf stat --repeat 5 -e kmem:mm_page_alloc
154 -e kmem:mm_page_free_b
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
162
163 16630 kmem:mm_page_alloc (
164 11486 kmem:mm_page_free (
165 4730 kmem:mm_page_free_batched (
166
167 0.982653002 seconds time elapsed ( +-
168
169 In the event that some higher-level event is r
170 aggregation of discrete events, then a script
171
172 Using --repeat, it is also possible to view ho
173 time on a system-wide basis using -a and sleep
174 ::
175
176 $ perf stat -e kmem:mm_page_alloc -e kmem:mm
177 -e kmem:mm_page_free_batched \
178 -a --repeat 10 \
179 sleep 1
180 Performance counter stats for 'sleep 1' (10
181
182 1066 kmem:mm_page_alloc (
183 182 kmem:mm_page_free (
184 890 kmem:mm_page_free_batched (
185
186 1.002251757 seconds time elapsed ( +-
187
188 6. Higher-Level Analysis with Helper Scripts
189 ============================================
190
191 When events are enabled the events that are tr
192 /sys/kernel/tracing/trace_pipe in human-readab
193 options exist as well. By post-processing the
194 be gathered on-line as appropriate. Examples o
195
196 - Reading information from /proc for the PID
197 - Deriving a higher-level event from a serie
198 - Calculating latencies between two events
199
200 Documentation/trace/postprocess/trace-pageallo
201 script that can read trace_pipe from STDIN or
202 on-line, it can be interrupted once to generat
203 and twice to exit.
204
205 Simplistically, the script just reads STDIN an
206 also can do more such as
207
208 - Derive high-level events from many low-lev
209 are freed to the main allocator from the p
210 that as one per-CPU drain even though ther
211 for that event
212 - It can aggregate based on PID or individua
213 - In the event memory is getting externally
214 on whether the fragmentation event was sev
215 - When receiving an event about a PID, it ca
216 that if large numbers of events are coming
217 processes, the parent process responsible
218 can be identified
219
220 7. Lower-Level Analysis with PCL
221 ================================
222
223 There may also be a requirement to identify wh
224 were generating events within the kernel. To b
225 data must be recorded. At the time of writing,
226 ::
227
228 $ perf record -c 1 \
229 -e kmem:mm_page_alloc -e kmem:mm_page_
230 -e kmem:mm_page_free_batched \
231 ./hackbench 10
232 Time: 0.894
233 [ perf record: Captured and wrote 0.733 MB p
234
235 Note the use of '-c 1' to set the event period
236 period is quite high to minimise overhead but
237 very coarse as a result.
238
239 This record outputted a file called perf.data
240 perf report.
241 ::
242
243 $ perf report
244 # Samples: 30922
245 #
246 # Overhead Command Sh
247 # ........ ......... .....................
248 #
249 87.27% hackbench [vdso]
250 6.85% hackbench /lib/i686/cmov/libc-2
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
256 0.01% perf /usr/bin/perf
257 0.01% perf /lib/ld-2.9.so
258 0.00% hackbench /lib/i686/cmov/libpth
259 #
260 # (For more details, try: perf report --sort
261 #
262
263 According to this, the vast majority of events
264 within the VDSO. With simple binaries, this wi
265 take a slightly different example. In the cour
266 noticed that X was generating an insane amount
267 at it:
268 ::
269
270 $ perf record -c 1 -f \
271 -e kmem:mm_page_alloc -e kmem:
272 -e kmem:mm_page_free_batched \
273 -p `pidof X`
274
275 This was interrupted after a few seconds and
276 ::
277
278 $ perf report
279 # Samples: 27666
280 #
281 # Overhead Command
282 # ........ ....... .......................
283 #
284 51.95% Xorg [vdso]
285 47.95% Xorg /opt/gfx-test/lib/libpi
286 0.09% Xorg /lib/i686/cmov/libc-2.9
287 0.01% Xorg [kernel]
288 #
289 # (For more details, try: perf report --sort
290 #
291
292 So, almost half of the events are occurring in
293 symbol:
294 ::
295
296 $ perf report --sort comm,dso,symbol
297 # Samples: 27666
298 #
299 # Overhead Command
300 # ........ ....... .......................
301 #
302 51.95% Xorg [vdso]
303 47.93% Xorg /opt/gfx-test/lib/libpi
304 0.09% Xorg /lib/i686/cmov/libc-2.9
305 0.01% Xorg /opt/gfx-test/lib/libpi
306 0.01% Xorg [kernel]
307 0.01% Xorg /opt/gfx-test/lib/libpi
308 0.00% Xorg [kernel]
309
310 To see where within the function pixmanFillsse
311 ::
312
313 $ perf annotate pixmanFillsse2
314 [ ... ]
315 0.00 : 34eeb: 0f 18 08
316 : }
317 :
318 : extern __inline void __attribu
319 : _mm_store_si128 (__m128i *__P,
320 : *__P = __B;
321 12.40 : 34eee: 66 0f 7f 80 40
322 0.00 : 34ef5: ff
323 12.40 : 34ef6: 66 0f 7f 80 50
324 0.00 : 34efd: ff
325 12.39 : 34efe: 66 0f 7f 80 60
326 0.00 : 34f05: ff
327 12.67 : 34f06: 66 0f 7f 80 70
328 0.00 : 34f0d: ff
329 12.58 : 34f0e: 66 0f 7f 40 80
330 12.31 : 34f13: 66 0f 7f 40 90
331 12.40 : 34f18: 66 0f 7f 40 a0
332 12.31 : 34f1d: 66 0f 7f 40 b0
333
334 At a glance, it looks like the time is being s
335 the card. Further investigation would be need
336 are being copied around so much but a starting
337 ancient build of libpixmap out of the library
338 forgotten about from months ago!
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