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Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.

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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