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eb6d42ea SR |
1 | ftrace - Function Tracer |
2 | ======================== | |
3 | ||
4 | Copyright 2008 Red Hat Inc. | |
a41eebab SR |
5 | Author: Steven Rostedt <srostedt@redhat.com> |
6 | License: The GNU Free Documentation License, Version 1.2 | |
a97762a7 | 7 | (dual licensed under the GPL v2) |
f2d9c740 SR |
8 | Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton, |
9 | John Kacur, and David Teigland. | |
42ec632e | 10 | Written for: 2.6.28-rc2 |
eb6d42ea SR |
11 | |
12 | Introduction | |
13 | ------------ | |
14 | ||
15 | Ftrace is an internal tracer designed to help out developers and | |
16 | designers of systems to find what is going on inside the kernel. | |
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17 | It can be used for debugging or analyzing latencies and |
18 | performance issues that take place outside of user-space. | |
eb6d42ea SR |
19 | |
20 | Although ftrace is the function tracer, it also includes an | |
5752674e IM |
21 | infrastructure that allows for other types of tracing. Some of |
22 | the tracers that are currently in ftrace include a tracer to | |
23 | trace context switches, the time it takes for a high priority | |
24 | task to run after it was woken up, the time interrupts are | |
25 | disabled, and more (ftrace allows for tracer plugins, which | |
26 | means that the list of tracers can always grow). | |
eb6d42ea SR |
27 | |
28 | ||
555f386c MF |
29 | Implementation Details |
30 | ---------------------- | |
31 | ||
32 | See ftrace-design.txt for details for arch porters and such. | |
33 | ||
34 | ||
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35 | The File System |
36 | --------------- | |
37 | ||
5752674e IM |
38 | Ftrace uses the debugfs file system to hold the control files as |
39 | well as the files to display output. | |
eb6d42ea | 40 | |
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41 | When debugfs is configured into the kernel (which selecting any ftrace |
42 | option will do) the directory /sys/kernel/debug will be created. To mount | |
43 | this directory, you can add to your /etc/fstab file: | |
44 | ||
45 | debugfs /sys/kernel/debug debugfs defaults 0 0 | |
46 | ||
47 | Or you can mount it at run time with: | |
48 | ||
49 | mount -t debugfs nodev /sys/kernel/debug | |
eb6d42ea | 50 | |
156f5a78 GL |
51 | For quicker access to that directory you may want to make a soft link to |
52 | it: | |
eb6d42ea | 53 | |
156f5a78 GL |
54 | ln -s /sys/kernel/debug /debug |
55 | ||
56 | Any selected ftrace option will also create a directory called tracing | |
57 | within the debugfs. The rest of the document will assume that you are in | |
58 | the ftrace directory (cd /sys/kernel/debug/tracing) and will only concentrate | |
59 | on the files within that directory and not distract from the content with | |
60 | the extended "/sys/kernel/debug/tracing" path name. | |
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61 | |
62 | That's it! (assuming that you have ftrace configured into your kernel) | |
63 | ||
64 | After mounting the debugfs, you can see a directory called | |
65 | "tracing". This directory contains the control and output files | |
66 | of ftrace. Here is a list of some of the key files: | |
67 | ||
68 | ||
69 | Note: all time values are in microseconds. | |
70 | ||
5752674e IM |
71 | current_tracer: |
72 | ||
73 | This is used to set or display the current tracer | |
74 | that is configured. | |
75 | ||
76 | available_tracers: | |
77 | ||
78 | This holds the different types of tracers that | |
79 | have been compiled into the kernel. The | |
80 | tracers listed here can be configured by | |
81 | echoing their name into current_tracer. | |
82 | ||
6752ab4a | 83 | tracing_on: |
5752674e | 84 | |
6752ab4a SR |
85 | This sets or displays whether writing to the trace |
86 | ring buffer is enabled. Echo 0 into this file to disable | |
87 | the tracer or 1 to enable it. | |
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88 | |
89 | trace: | |
90 | ||
91 | This file holds the output of the trace in a human | |
92 | readable format (described below). | |
93 | ||
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94 | trace_pipe: |
95 | ||
96 | The output is the same as the "trace" file but this | |
97 | file is meant to be streamed with live tracing. | |
4a88d44a AT |
98 | Reads from this file will block until new data is |
99 | retrieved. Unlike the "trace" file, this file is a | |
100 | consumer. This means reading from this file causes | |
101 | sequential reads to display more current data. Once | |
102 | data is read from this file, it is consumed, and | |
103 | will not be read again with a sequential read. The | |
104 | "trace" file is static, and if the tracer is not | |
105 | adding more data,they will display the same | |
106 | information every time they are read. | |
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107 | |
108 | trace_options: | |
109 | ||
110 | This file lets the user control the amount of data | |
111 | that is displayed in one of the above output | |
112 | files. | |
113 | ||
42b40b3d | 114 | tracing_max_latency: |
5752674e IM |
115 | |
116 | Some of the tracers record the max latency. | |
117 | For example, the time interrupts are disabled. | |
118 | This time is saved in this file. The max trace | |
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119 | will also be stored, and displayed by "trace". |
120 | A new max trace will only be recorded if the | |
121 | latency is greater than the value in this | |
122 | file. (in microseconds) | |
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123 | |
124 | buffer_size_kb: | |
125 | ||
126 | This sets or displays the number of kilobytes each CPU | |
127 | buffer can hold. The tracer buffers are the same size | |
128 | for each CPU. The displayed number is the size of the | |
129 | CPU buffer and not total size of all buffers. The | |
130 | trace buffers are allocated in pages (blocks of memory | |
131 | that the kernel uses for allocation, usually 4 KB in size). | |
132 | If the last page allocated has room for more bytes | |
133 | than requested, the rest of the page will be used, | |
134 | making the actual allocation bigger than requested. | |
135 | ( Note, the size may not be a multiple of the page size | |
3dbda77e | 136 | due to buffer management overhead. ) |
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137 | |
138 | This can only be updated when the current_tracer | |
139 | is set to "nop". | |
140 | ||
141 | tracing_cpumask: | |
142 | ||
143 | This is a mask that lets the user only trace | |
144 | on specified CPUS. The format is a hex string | |
145 | representing the CPUS. | |
146 | ||
147 | set_ftrace_filter: | |
148 | ||
149 | When dynamic ftrace is configured in (see the | |
150 | section below "dynamic ftrace"), the code is dynamically | |
151 | modified (code text rewrite) to disable calling of the | |
152 | function profiler (mcount). This lets tracing be configured | |
153 | in with practically no overhead in performance. This also | |
154 | has a side effect of enabling or disabling specific functions | |
155 | to be traced. Echoing names of functions into this file | |
156 | will limit the trace to only those functions. | |
157 | ||
07271aa4 CD |
158 | This interface also allows for commands to be used. See the |
159 | "Filter commands" section for more details. | |
160 | ||
5752674e IM |
161 | set_ftrace_notrace: |
162 | ||
163 | This has an effect opposite to that of | |
164 | set_ftrace_filter. Any function that is added here will not | |
165 | be traced. If a function exists in both set_ftrace_filter | |
166 | and set_ftrace_notrace, the function will _not_ be traced. | |
167 | ||
168 | set_ftrace_pid: | |
169 | ||
170 | Have the function tracer only trace a single thread. | |
171 | ||
172 | set_graph_function: | |
173 | ||
174 | Set a "trigger" function where tracing should start | |
175 | with the function graph tracer (See the section | |
176 | "dynamic ftrace" for more details). | |
177 | ||
178 | available_filter_functions: | |
179 | ||
180 | This lists the functions that ftrace | |
181 | has processed and can trace. These are the function | |
182 | names that you can pass to "set_ftrace_filter" or | |
183 | "set_ftrace_notrace". (See the section "dynamic ftrace" | |
184 | below for more details.) | |
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185 | |
186 | ||
187 | The Tracers | |
188 | ----------- | |
189 | ||
f2d9c740 | 190 | Here is the list of current tracers that may be configured. |
eb6d42ea | 191 | |
5752674e IM |
192 | "function" |
193 | ||
194 | Function call tracer to trace all kernel functions. | |
195 | ||
bc5c6c04 | 196 | "function_graph" |
5752674e IM |
197 | |
198 | Similar to the function tracer except that the | |
199 | function tracer probes the functions on their entry | |
200 | whereas the function graph tracer traces on both entry | |
201 | and exit of the functions. It then provides the ability | |
202 | to draw a graph of function calls similar to C code | |
203 | source. | |
204 | ||
5752674e IM |
205 | "irqsoff" |
206 | ||
207 | Traces the areas that disable interrupts and saves | |
208 | the trace with the longest max latency. | |
209 | See tracing_max_latency. When a new max is recorded, | |
210 | it replaces the old trace. It is best to view this | |
4a88d44a | 211 | trace with the latency-format option enabled. |
eb6d42ea | 212 | |
5752674e | 213 | "preemptoff" |
985ec20a | 214 | |
5752674e IM |
215 | Similar to irqsoff but traces and records the amount of |
216 | time for which preemption is disabled. | |
eb6d42ea | 217 | |
5752674e | 218 | "preemptirqsoff" |
eb6d42ea | 219 | |
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220 | Similar to irqsoff and preemptoff, but traces and |
221 | records the largest time for which irqs and/or preemption | |
222 | is disabled. | |
eb6d42ea | 223 | |
5752674e | 224 | "wakeup" |
eb6d42ea | 225 | |
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226 | Traces and records the max latency that it takes for |
227 | the highest priority task to get scheduled after | |
228 | it has been woken up. | |
cdfb0d30 GL |
229 | Traces all tasks as an average developer would expect. |
230 | ||
231 | "wakeup_rt" | |
232 | ||
233 | Traces and records the max latency that it takes for just | |
234 | RT tasks (as the current "wakeup" does). This is useful | |
235 | for those interested in wake up timings of RT tasks. | |
eb6d42ea | 236 | |
5752674e | 237 | "hw-branch-tracer" |
eb6d42ea | 238 | |
5752674e IM |
239 | Uses the BTS CPU feature on x86 CPUs to traces all |
240 | branches executed. | |
241 | ||
242 | "nop" | |
243 | ||
244 | This is the "trace nothing" tracer. To remove all | |
245 | tracers from tracing simply echo "nop" into | |
246 | current_tracer. | |
e2ea5399 | 247 | |
eb6d42ea SR |
248 | |
249 | Examples of using the tracer | |
250 | ---------------------------- | |
251 | ||
5752674e IM |
252 | Here are typical examples of using the tracers when controlling |
253 | them only with the debugfs interface (without using any | |
254 | user-land utilities). | |
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255 | |
256 | Output format: | |
257 | -------------- | |
258 | ||
f2d9c740 | 259 | Here is an example of the output format of the file "trace" |
eb6d42ea SR |
260 | |
261 | -------- | |
9b803c0f | 262 | # tracer: function |
eb6d42ea SR |
263 | # |
264 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
265 | # | | | | | | |
266 | bash-4251 [01] 10152.583854: path_put <-path_walk | |
267 | bash-4251 [01] 10152.583855: dput <-path_put | |
268 | bash-4251 [01] 10152.583855: _atomic_dec_and_lock <-dput | |
269 | -------- | |
270 | ||
5752674e IM |
271 | A header is printed with the tracer name that is represented by |
272 | the trace. In this case the tracer is "function". Then a header | |
273 | showing the format. Task name "bash", the task PID "4251", the | |
274 | CPU that it was running on "01", the timestamp in <secs>.<usecs> | |
275 | format, the function name that was traced "path_put" and the | |
276 | parent function that called this function "path_walk". The | |
277 | timestamp is the time at which the function was entered. | |
eb6d42ea | 278 | |
eb6d42ea SR |
279 | Latency trace format |
280 | -------------------- | |
281 | ||
4a88d44a AT |
282 | When the latency-format option is enabled, the trace file gives |
283 | somewhat more information to see why a latency happened. | |
5752674e | 284 | Here is a typical trace. |
eb6d42ea SR |
285 | |
286 | # tracer: irqsoff | |
287 | # | |
288 | irqsoff latency trace v1.1.5 on 2.6.26-rc8 | |
289 | -------------------------------------------------------------------- | |
290 | latency: 97 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
291 | ----------------- | |
292 | | task: swapper-0 (uid:0 nice:0 policy:0 rt_prio:0) | |
293 | ----------------- | |
294 | => started at: apic_timer_interrupt | |
295 | => ended at: do_softirq | |
296 | ||
297 | # _------=> CPU# | |
298 | # / _-----=> irqs-off | |
299 | # | / _----=> need-resched | |
300 | # || / _---=> hardirq/softirq | |
301 | # ||| / _--=> preempt-depth | |
302 | # |||| / | |
303 | # ||||| delay | |
304 | # cmd pid ||||| time | caller | |
305 | # \ / ||||| \ | / | |
306 | <idle>-0 0d..1 0us+: trace_hardirqs_off_thunk (apic_timer_interrupt) | |
307 | <idle>-0 0d.s. 97us : __do_softirq (do_softirq) | |
308 | <idle>-0 0d.s1 98us : trace_hardirqs_on (do_softirq) | |
309 | ||
310 | ||
5752674e IM |
311 | This shows that the current tracer is "irqsoff" tracing the time |
312 | for which interrupts were disabled. It gives the trace version | |
313 | and the version of the kernel upon which this was executed on | |
314 | (2.6.26-rc8). Then it displays the max latency in microsecs (97 | |
315 | us). The number of trace entries displayed and the total number | |
316 | recorded (both are three: #3/3). The type of preemption that was | |
317 | used (PREEMPT). VP, KP, SP, and HP are always zero and are | |
318 | reserved for later use. #P is the number of online CPUS (#P:2). | |
eb6d42ea | 319 | |
5752674e IM |
320 | The task is the process that was running when the latency |
321 | occurred. (swapper pid: 0). | |
eb6d42ea | 322 | |
5752674e IM |
323 | The start and stop (the functions in which the interrupts were |
324 | disabled and enabled respectively) that caused the latencies: | |
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325 | |
326 | apic_timer_interrupt is where the interrupts were disabled. | |
327 | do_softirq is where they were enabled again. | |
328 | ||
329 | The next lines after the header are the trace itself. The header | |
330 | explains which is which. | |
331 | ||
332 | cmd: The name of the process in the trace. | |
333 | ||
334 | pid: The PID of that process. | |
335 | ||
f2d9c740 | 336 | CPU#: The CPU which the process was running on. |
eb6d42ea SR |
337 | |
338 | irqs-off: 'd' interrupts are disabled. '.' otherwise. | |
9244489a SR |
339 | Note: If the architecture does not support a way to |
340 | read the irq flags variable, an 'X' will always | |
341 | be printed here. | |
eb6d42ea SR |
342 | |
343 | need-resched: 'N' task need_resched is set, '.' otherwise. | |
344 | ||
345 | hardirq/softirq: | |
f2d9c740 | 346 | 'H' - hard irq occurred inside a softirq. |
eb6d42ea SR |
347 | 'h' - hard irq is running |
348 | 's' - soft irq is running | |
349 | '.' - normal context. | |
350 | ||
351 | preempt-depth: The level of preempt_disabled | |
352 | ||
353 | The above is mostly meaningful for kernel developers. | |
354 | ||
4a88d44a AT |
355 | time: When the latency-format option is enabled, the trace file |
356 | output includes a timestamp relative to the start of the | |
357 | trace. This differs from the output when latency-format | |
358 | is disabled, which includes an absolute timestamp. | |
eb6d42ea SR |
359 | |
360 | delay: This is just to help catch your eye a bit better. And | |
5752674e IM |
361 | needs to be fixed to be only relative to the same CPU. |
362 | The marks are determined by the difference between this | |
363 | current trace and the next trace. | |
364 | '!' - greater than preempt_mark_thresh (default 100) | |
365 | '+' - greater than 1 microsecond | |
366 | ' ' - less than or equal to 1 microsecond. | |
eb6d42ea SR |
367 | |
368 | The rest is the same as the 'trace' file. | |
369 | ||
370 | ||
ee6bce52 SR |
371 | trace_options |
372 | ------------- | |
eb6d42ea | 373 | |
5752674e IM |
374 | The trace_options file is used to control what gets printed in |
375 | the trace output. To see what is available, simply cat the file: | |
eb6d42ea | 376 | |
156f5a78 | 377 | cat trace_options |
eb6d42ea | 378 | print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \ |
5752674e | 379 | noblock nostacktrace nosched-tree nouserstacktrace nosym-userobj |
eb6d42ea | 380 | |
5752674e IM |
381 | To disable one of the options, echo in the option prepended with |
382 | "no". | |
eb6d42ea | 383 | |
156f5a78 | 384 | echo noprint-parent > trace_options |
eb6d42ea SR |
385 | |
386 | To enable an option, leave off the "no". | |
387 | ||
156f5a78 | 388 | echo sym-offset > trace_options |
eb6d42ea SR |
389 | |
390 | Here are the available options: | |
391 | ||
5752674e IM |
392 | print-parent - On function traces, display the calling (parent) |
393 | function as well as the function being traced. | |
eb6d42ea SR |
394 | |
395 | print-parent: | |
396 | bash-4000 [01] 1477.606694: simple_strtoul <-strict_strtoul | |
397 | ||
398 | noprint-parent: | |
399 | bash-4000 [01] 1477.606694: simple_strtoul | |
400 | ||
401 | ||
5752674e IM |
402 | sym-offset - Display not only the function name, but also the |
403 | offset in the function. For example, instead of | |
404 | seeing just "ktime_get", you will see | |
405 | "ktime_get+0xb/0x20". | |
eb6d42ea SR |
406 | |
407 | sym-offset: | |
408 | bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0 | |
409 | ||
5752674e IM |
410 | sym-addr - this will also display the function address as well |
411 | as the function name. | |
eb6d42ea SR |
412 | |
413 | sym-addr: | |
414 | bash-4000 [01] 1477.606694: simple_strtoul <c0339346> | |
415 | ||
4a88d44a AT |
416 | verbose - This deals with the trace file when the |
417 | latency-format option is enabled. | |
eb6d42ea SR |
418 | |
419 | bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \ | |
420 | (+0.000ms): simple_strtoul (strict_strtoul) | |
421 | ||
5752674e IM |
422 | raw - This will display raw numbers. This option is best for |
423 | use with user applications that can translate the raw | |
424 | numbers better than having it done in the kernel. | |
eb6d42ea | 425 | |
5752674e IM |
426 | hex - Similar to raw, but the numbers will be in a hexadecimal |
427 | format. | |
eb6d42ea SR |
428 | |
429 | bin - This will print out the formats in raw binary. | |
430 | ||
431 | block - TBD (needs update) | |
432 | ||
5752674e IM |
433 | stacktrace - This is one of the options that changes the trace |
434 | itself. When a trace is recorded, so is the stack | |
435 | of functions. This allows for back traces of | |
436 | trace sites. | |
eb6d42ea | 437 | |
5752674e IM |
438 | userstacktrace - This option changes the trace. It records a |
439 | stacktrace of the current userspace thread. | |
02b67518 | 440 | |
5752674e IM |
441 | sym-userobj - when user stacktrace are enabled, look up which |
442 | object the address belongs to, and print a | |
443 | relative address. This is especially useful when | |
444 | ASLR is on, otherwise you don't get a chance to | |
445 | resolve the address to object/file/line after | |
446 | the app is no longer running | |
b54d3de9 | 447 | |
5752674e | 448 | The lookup is performed when you read |
4a88d44a | 449 | trace,trace_pipe. Example: |
b54d3de9 TE |
450 | |
451 | a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0 | |
452 | x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6] | |
453 | ||
5752674e IM |
454 | sched-tree - trace all tasks that are on the runqueue, at |
455 | every scheduling event. Will add overhead if | |
456 | there's a lot of tasks running at once. | |
eb6d42ea | 457 | |
4a88d44a AT |
458 | latency-format - This option changes the trace. When |
459 | it is enabled, the trace displays | |
460 | additional information about the | |
461 | latencies, as described in "Latency | |
462 | trace format". | |
eb6d42ea | 463 | |
750912fa DS |
464 | overwrite - This controls what happens when the trace buffer is |
465 | full. If "1" (default), the oldest events are | |
466 | discarded and overwritten. If "0", then the newest | |
467 | events are discarded. | |
468 | ||
eb6d42ea SR |
469 | ftrace_enabled |
470 | -------------- | |
471 | ||
5752674e IM |
472 | The following tracers (listed below) give different output |
473 | depending on whether or not the sysctl ftrace_enabled is set. To | |
474 | set ftrace_enabled, one can either use the sysctl function or | |
475 | set it via the proc file system interface. | |
eb6d42ea SR |
476 | |
477 | sysctl kernel.ftrace_enabled=1 | |
478 | ||
479 | or | |
480 | ||
481 | echo 1 > /proc/sys/kernel/ftrace_enabled | |
482 | ||
5752674e IM |
483 | To disable ftrace_enabled simply replace the '1' with '0' in the |
484 | above commands. | |
eb6d42ea | 485 | |
5752674e IM |
486 | When ftrace_enabled is set the tracers will also record the |
487 | functions that are within the trace. The descriptions of the | |
488 | tracers will also show an example with ftrace enabled. | |
eb6d42ea SR |
489 | |
490 | ||
491 | irqsoff | |
492 | ------- | |
493 | ||
494 | When interrupts are disabled, the CPU can not react to any other | |
495 | external event (besides NMIs and SMIs). This prevents the timer | |
5752674e IM |
496 | interrupt from triggering or the mouse interrupt from letting |
497 | the kernel know of a new mouse event. The result is a latency | |
498 | with the reaction time. | |
eb6d42ea | 499 | |
5752674e IM |
500 | The irqsoff tracer tracks the time for which interrupts are |
501 | disabled. When a new maximum latency is hit, the tracer saves | |
502 | the trace leading up to that latency point so that every time a | |
503 | new maximum is reached, the old saved trace is discarded and the | |
504 | new trace is saved. | |
eb6d42ea | 505 | |
5752674e IM |
506 | To reset the maximum, echo 0 into tracing_max_latency. Here is |
507 | an example: | |
eb6d42ea | 508 | |
156f5a78 | 509 | # echo irqsoff > current_tracer |
4a88d44a | 510 | # echo latency-format > trace_options |
156f5a78 | 511 | # echo 0 > tracing_max_latency |
6752ab4a | 512 | # echo 1 > tracing_on |
eb6d42ea SR |
513 | # ls -ltr |
514 | [...] | |
6752ab4a | 515 | # echo 0 > tracing_on |
4a88d44a | 516 | # cat trace |
eb6d42ea SR |
517 | # tracer: irqsoff |
518 | # | |
f2d9c740 | 519 | irqsoff latency trace v1.1.5 on 2.6.26 |
eb6d42ea | 520 | -------------------------------------------------------------------- |
f2d9c740 | 521 | latency: 12 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) |
eb6d42ea | 522 | ----------------- |
f2d9c740 | 523 | | task: bash-3730 (uid:0 nice:0 policy:0 rt_prio:0) |
eb6d42ea | 524 | ----------------- |
f2d9c740 SR |
525 | => started at: sys_setpgid |
526 | => ended at: sys_setpgid | |
eb6d42ea SR |
527 | |
528 | # _------=> CPU# | |
529 | # / _-----=> irqs-off | |
530 | # | / _----=> need-resched | |
531 | # || / _---=> hardirq/softirq | |
532 | # ||| / _--=> preempt-depth | |
533 | # |||| / | |
534 | # ||||| delay | |
535 | # cmd pid ||||| time | caller | |
536 | # \ / ||||| \ | / | |
f2d9c740 SR |
537 | bash-3730 1d... 0us : _write_lock_irq (sys_setpgid) |
538 | bash-3730 1d..1 1us+: _write_unlock_irq (sys_setpgid) | |
539 | bash-3730 1d..2 14us : trace_hardirqs_on (sys_setpgid) | |
eb6d42ea | 540 | |
eb6d42ea | 541 | |
f2d9c740 | 542 | Here we see that that we had a latency of 12 microsecs (which is |
5752674e IM |
543 | very good). The _write_lock_irq in sys_setpgid disabled |
544 | interrupts. The difference between the 12 and the displayed | |
545 | timestamp 14us occurred because the clock was incremented | |
546 | between the time of recording the max latency and the time of | |
547 | recording the function that had that latency. | |
eb6d42ea | 548 | |
f2d9c740 SR |
549 | Note the above example had ftrace_enabled not set. If we set the |
550 | ftrace_enabled, we get a much larger output: | |
eb6d42ea SR |
551 | |
552 | # tracer: irqsoff | |
553 | # | |
554 | irqsoff latency trace v1.1.5 on 2.6.26-rc8 | |
555 | -------------------------------------------------------------------- | |
556 | latency: 50 us, #101/101, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
557 | ----------------- | |
558 | | task: ls-4339 (uid:0 nice:0 policy:0 rt_prio:0) | |
559 | ----------------- | |
560 | => started at: __alloc_pages_internal | |
561 | => ended at: __alloc_pages_internal | |
562 | ||
563 | # _------=> CPU# | |
564 | # / _-----=> irqs-off | |
565 | # | / _----=> need-resched | |
566 | # || / _---=> hardirq/softirq | |
567 | # ||| / _--=> preempt-depth | |
568 | # |||| / | |
569 | # ||||| delay | |
570 | # cmd pid ||||| time | caller | |
571 | # \ / ||||| \ | / | |
572 | ls-4339 0...1 0us+: get_page_from_freelist (__alloc_pages_internal) | |
573 | ls-4339 0d..1 3us : rmqueue_bulk (get_page_from_freelist) | |
574 | ls-4339 0d..1 3us : _spin_lock (rmqueue_bulk) | |
575 | ls-4339 0d..1 4us : add_preempt_count (_spin_lock) | |
576 | ls-4339 0d..2 4us : __rmqueue (rmqueue_bulk) | |
577 | ls-4339 0d..2 5us : __rmqueue_smallest (__rmqueue) | |
578 | ls-4339 0d..2 5us : __mod_zone_page_state (__rmqueue_smallest) | |
579 | ls-4339 0d..2 6us : __rmqueue (rmqueue_bulk) | |
580 | ls-4339 0d..2 6us : __rmqueue_smallest (__rmqueue) | |
581 | ls-4339 0d..2 7us : __mod_zone_page_state (__rmqueue_smallest) | |
582 | ls-4339 0d..2 7us : __rmqueue (rmqueue_bulk) | |
583 | ls-4339 0d..2 8us : __rmqueue_smallest (__rmqueue) | |
584 | [...] | |
585 | ls-4339 0d..2 46us : __rmqueue_smallest (__rmqueue) | |
586 | ls-4339 0d..2 47us : __mod_zone_page_state (__rmqueue_smallest) | |
587 | ls-4339 0d..2 47us : __rmqueue (rmqueue_bulk) | |
588 | ls-4339 0d..2 48us : __rmqueue_smallest (__rmqueue) | |
589 | ls-4339 0d..2 48us : __mod_zone_page_state (__rmqueue_smallest) | |
590 | ls-4339 0d..2 49us : _spin_unlock (rmqueue_bulk) | |
591 | ls-4339 0d..2 49us : sub_preempt_count (_spin_unlock) | |
592 | ls-4339 0d..1 50us : get_page_from_freelist (__alloc_pages_internal) | |
593 | ls-4339 0d..2 51us : trace_hardirqs_on (__alloc_pages_internal) | |
594 | ||
595 | ||
eb6d42ea SR |
596 | |
597 | Here we traced a 50 microsecond latency. But we also see all the | |
5752674e IM |
598 | functions that were called during that time. Note that by |
599 | enabling function tracing, we incur an added overhead. This | |
600 | overhead may extend the latency times. But nevertheless, this | |
601 | trace has provided some very helpful debugging information. | |
eb6d42ea SR |
602 | |
603 | ||
604 | preemptoff | |
605 | ---------- | |
606 | ||
5752674e IM |
607 | When preemption is disabled, we may be able to receive |
608 | interrupts but the task cannot be preempted and a higher | |
609 | priority task must wait for preemption to be enabled again | |
610 | before it can preempt a lower priority task. | |
eb6d42ea | 611 | |
a41eebab | 612 | The preemptoff tracer traces the places that disable preemption. |
5752674e IM |
613 | Like the irqsoff tracer, it records the maximum latency for |
614 | which preemption was disabled. The control of preemptoff tracer | |
615 | is much like the irqsoff tracer. | |
eb6d42ea | 616 | |
156f5a78 | 617 | # echo preemptoff > current_tracer |
4a88d44a | 618 | # echo latency-format > trace_options |
156f5a78 | 619 | # echo 0 > tracing_max_latency |
6752ab4a | 620 | # echo 1 > tracing_on |
eb6d42ea SR |
621 | # ls -ltr |
622 | [...] | |
6752ab4a | 623 | # echo 0 > tracing_on |
4a88d44a | 624 | # cat trace |
eb6d42ea SR |
625 | # tracer: preemptoff |
626 | # | |
627 | preemptoff latency trace v1.1.5 on 2.6.26-rc8 | |
628 | -------------------------------------------------------------------- | |
629 | latency: 29 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
630 | ----------------- | |
631 | | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) | |
632 | ----------------- | |
633 | => started at: do_IRQ | |
634 | => ended at: __do_softirq | |
635 | ||
636 | # _------=> CPU# | |
637 | # / _-----=> irqs-off | |
638 | # | / _----=> need-resched | |
639 | # || / _---=> hardirq/softirq | |
640 | # ||| / _--=> preempt-depth | |
641 | # |||| / | |
642 | # ||||| delay | |
643 | # cmd pid ||||| time | caller | |
644 | # \ / ||||| \ | / | |
645 | sshd-4261 0d.h. 0us+: irq_enter (do_IRQ) | |
646 | sshd-4261 0d.s. 29us : _local_bh_enable (__do_softirq) | |
647 | sshd-4261 0d.s1 30us : trace_preempt_on (__do_softirq) | |
648 | ||
649 | ||
5752674e IM |
650 | This has some more changes. Preemption was disabled when an |
651 | interrupt came in (notice the 'h'), and was enabled while doing | |
652 | a softirq. (notice the 's'). But we also see that interrupts | |
653 | have been disabled when entering the preempt off section and | |
654 | leaving it (the 'd'). We do not know if interrupts were enabled | |
655 | in the mean time. | |
eb6d42ea SR |
656 | |
657 | # tracer: preemptoff | |
658 | # | |
659 | preemptoff latency trace v1.1.5 on 2.6.26-rc8 | |
660 | -------------------------------------------------------------------- | |
661 | latency: 63 us, #87/87, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
662 | ----------------- | |
663 | | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) | |
664 | ----------------- | |
665 | => started at: remove_wait_queue | |
666 | => ended at: __do_softirq | |
667 | ||
668 | # _------=> CPU# | |
669 | # / _-----=> irqs-off | |
670 | # | / _----=> need-resched | |
671 | # || / _---=> hardirq/softirq | |
672 | # ||| / _--=> preempt-depth | |
673 | # |||| / | |
674 | # ||||| delay | |
675 | # cmd pid ||||| time | caller | |
676 | # \ / ||||| \ | / | |
677 | sshd-4261 0d..1 0us : _spin_lock_irqsave (remove_wait_queue) | |
678 | sshd-4261 0d..1 1us : _spin_unlock_irqrestore (remove_wait_queue) | |
679 | sshd-4261 0d..1 2us : do_IRQ (common_interrupt) | |
680 | sshd-4261 0d..1 2us : irq_enter (do_IRQ) | |
681 | sshd-4261 0d..1 2us : idle_cpu (irq_enter) | |
682 | sshd-4261 0d..1 3us : add_preempt_count (irq_enter) | |
683 | sshd-4261 0d.h1 3us : idle_cpu (irq_enter) | |
684 | sshd-4261 0d.h. 4us : handle_fasteoi_irq (do_IRQ) | |
685 | [...] | |
686 | sshd-4261 0d.h. 12us : add_preempt_count (_spin_lock) | |
687 | sshd-4261 0d.h1 12us : ack_ioapic_quirk_irq (handle_fasteoi_irq) | |
688 | sshd-4261 0d.h1 13us : move_native_irq (ack_ioapic_quirk_irq) | |
689 | sshd-4261 0d.h1 13us : _spin_unlock (handle_fasteoi_irq) | |
690 | sshd-4261 0d.h1 14us : sub_preempt_count (_spin_unlock) | |
691 | sshd-4261 0d.h1 14us : irq_exit (do_IRQ) | |
692 | sshd-4261 0d.h1 15us : sub_preempt_count (irq_exit) | |
693 | sshd-4261 0d..2 15us : do_softirq (irq_exit) | |
694 | sshd-4261 0d... 15us : __do_softirq (do_softirq) | |
695 | sshd-4261 0d... 16us : __local_bh_disable (__do_softirq) | |
696 | sshd-4261 0d... 16us+: add_preempt_count (__local_bh_disable) | |
697 | sshd-4261 0d.s4 20us : add_preempt_count (__local_bh_disable) | |
698 | sshd-4261 0d.s4 21us : sub_preempt_count (local_bh_enable) | |
699 | sshd-4261 0d.s5 21us : sub_preempt_count (local_bh_enable) | |
700 | [...] | |
701 | sshd-4261 0d.s6 41us : add_preempt_count (__local_bh_disable) | |
702 | sshd-4261 0d.s6 42us : sub_preempt_count (local_bh_enable) | |
703 | sshd-4261 0d.s7 42us : sub_preempt_count (local_bh_enable) | |
704 | sshd-4261 0d.s5 43us : add_preempt_count (__local_bh_disable) | |
705 | sshd-4261 0d.s5 43us : sub_preempt_count (local_bh_enable_ip) | |
706 | sshd-4261 0d.s6 44us : sub_preempt_count (local_bh_enable_ip) | |
707 | sshd-4261 0d.s5 44us : add_preempt_count (__local_bh_disable) | |
708 | sshd-4261 0d.s5 45us : sub_preempt_count (local_bh_enable) | |
709 | [...] | |
710 | sshd-4261 0d.s. 63us : _local_bh_enable (__do_softirq) | |
711 | sshd-4261 0d.s1 64us : trace_preempt_on (__do_softirq) | |
712 | ||
713 | ||
5752674e IM |
714 | The above is an example of the preemptoff trace with |
715 | ftrace_enabled set. Here we see that interrupts were disabled | |
716 | the entire time. The irq_enter code lets us know that we entered | |
717 | an interrupt 'h'. Before that, the functions being traced still | |
718 | show that it is not in an interrupt, but we can see from the | |
719 | functions themselves that this is not the case. | |
eb6d42ea | 720 | |
5752674e IM |
721 | Notice that __do_softirq when called does not have a |
722 | preempt_count. It may seem that we missed a preempt enabling. | |
723 | What really happened is that the preempt count is held on the | |
724 | thread's stack and we switched to the softirq stack (4K stacks | |
725 | in effect). The code does not copy the preempt count, but | |
726 | because interrupts are disabled, we do not need to worry about | |
727 | it. Having a tracer like this is good for letting people know | |
728 | what really happens inside the kernel. | |
eb6d42ea SR |
729 | |
730 | ||
731 | preemptirqsoff | |
732 | -------------- | |
733 | ||
5752674e IM |
734 | Knowing the locations that have interrupts disabled or |
735 | preemption disabled for the longest times is helpful. But | |
736 | sometimes we would like to know when either preemption and/or | |
737 | interrupts are disabled. | |
eb6d42ea | 738 | |
f2d9c740 | 739 | Consider the following code: |
eb6d42ea SR |
740 | |
741 | local_irq_disable(); | |
742 | call_function_with_irqs_off(); | |
743 | preempt_disable(); | |
744 | call_function_with_irqs_and_preemption_off(); | |
745 | local_irq_enable(); | |
746 | call_function_with_preemption_off(); | |
747 | preempt_enable(); | |
748 | ||
749 | The irqsoff tracer will record the total length of | |
750 | call_function_with_irqs_off() and | |
751 | call_function_with_irqs_and_preemption_off(). | |
752 | ||
753 | The preemptoff tracer will record the total length of | |
754 | call_function_with_irqs_and_preemption_off() and | |
755 | call_function_with_preemption_off(). | |
756 | ||
5752674e IM |
757 | But neither will trace the time that interrupts and/or |
758 | preemption is disabled. This total time is the time that we can | |
759 | not schedule. To record this time, use the preemptirqsoff | |
760 | tracer. | |
eb6d42ea | 761 | |
5752674e IM |
762 | Again, using this trace is much like the irqsoff and preemptoff |
763 | tracers. | |
eb6d42ea | 764 | |
156f5a78 | 765 | # echo preemptirqsoff > current_tracer |
4a88d44a | 766 | # echo latency-format > trace_options |
156f5a78 | 767 | # echo 0 > tracing_max_latency |
6752ab4a | 768 | # echo 1 > tracing_on |
eb6d42ea SR |
769 | # ls -ltr |
770 | [...] | |
6752ab4a | 771 | # echo 0 > tracing_on |
4a88d44a | 772 | # cat trace |
eb6d42ea SR |
773 | # tracer: preemptirqsoff |
774 | # | |
775 | preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8 | |
776 | -------------------------------------------------------------------- | |
777 | latency: 293 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
778 | ----------------- | |
779 | | task: ls-4860 (uid:0 nice:0 policy:0 rt_prio:0) | |
780 | ----------------- | |
781 | => started at: apic_timer_interrupt | |
782 | => ended at: __do_softirq | |
783 | ||
784 | # _------=> CPU# | |
785 | # / _-----=> irqs-off | |
786 | # | / _----=> need-resched | |
787 | # || / _---=> hardirq/softirq | |
788 | # ||| / _--=> preempt-depth | |
789 | # |||| / | |
790 | # ||||| delay | |
791 | # cmd pid ||||| time | caller | |
792 | # \ / ||||| \ | / | |
793 | ls-4860 0d... 0us!: trace_hardirqs_off_thunk (apic_timer_interrupt) | |
794 | ls-4860 0d.s. 294us : _local_bh_enable (__do_softirq) | |
795 | ls-4860 0d.s1 294us : trace_preempt_on (__do_softirq) | |
796 | ||
797 | ||
eb6d42ea SR |
798 | |
799 | The trace_hardirqs_off_thunk is called from assembly on x86 when | |
5752674e IM |
800 | interrupts are disabled in the assembly code. Without the |
801 | function tracing, we do not know if interrupts were enabled | |
802 | within the preemption points. We do see that it started with | |
803 | preemption enabled. | |
eb6d42ea SR |
804 | |
805 | Here is a trace with ftrace_enabled set: | |
806 | ||
807 | ||
808 | # tracer: preemptirqsoff | |
809 | # | |
810 | preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8 | |
811 | -------------------------------------------------------------------- | |
812 | latency: 105 us, #183/183, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
813 | ----------------- | |
814 | | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) | |
815 | ----------------- | |
816 | => started at: write_chan | |
817 | => ended at: __do_softirq | |
818 | ||
819 | # _------=> CPU# | |
820 | # / _-----=> irqs-off | |
821 | # | / _----=> need-resched | |
822 | # || / _---=> hardirq/softirq | |
823 | # ||| / _--=> preempt-depth | |
824 | # |||| / | |
825 | # ||||| delay | |
826 | # cmd pid ||||| time | caller | |
827 | # \ / ||||| \ | / | |
828 | ls-4473 0.N.. 0us : preempt_schedule (write_chan) | |
829 | ls-4473 0dN.1 1us : _spin_lock (schedule) | |
830 | ls-4473 0dN.1 2us : add_preempt_count (_spin_lock) | |
831 | ls-4473 0d..2 2us : put_prev_task_fair (schedule) | |
832 | [...] | |
833 | ls-4473 0d..2 13us : set_normalized_timespec (ktime_get_ts) | |
834 | ls-4473 0d..2 13us : __switch_to (schedule) | |
835 | sshd-4261 0d..2 14us : finish_task_switch (schedule) | |
836 | sshd-4261 0d..2 14us : _spin_unlock_irq (finish_task_switch) | |
837 | sshd-4261 0d..1 15us : add_preempt_count (_spin_lock_irqsave) | |
838 | sshd-4261 0d..2 16us : _spin_unlock_irqrestore (hrtick_set) | |
839 | sshd-4261 0d..2 16us : do_IRQ (common_interrupt) | |
840 | sshd-4261 0d..2 17us : irq_enter (do_IRQ) | |
841 | sshd-4261 0d..2 17us : idle_cpu (irq_enter) | |
842 | sshd-4261 0d..2 18us : add_preempt_count (irq_enter) | |
843 | sshd-4261 0d.h2 18us : idle_cpu (irq_enter) | |
844 | sshd-4261 0d.h. 18us : handle_fasteoi_irq (do_IRQ) | |
845 | sshd-4261 0d.h. 19us : _spin_lock (handle_fasteoi_irq) | |
846 | sshd-4261 0d.h. 19us : add_preempt_count (_spin_lock) | |
847 | sshd-4261 0d.h1 20us : _spin_unlock (handle_fasteoi_irq) | |
848 | sshd-4261 0d.h1 20us : sub_preempt_count (_spin_unlock) | |
849 | [...] | |
850 | sshd-4261 0d.h1 28us : _spin_unlock (handle_fasteoi_irq) | |
851 | sshd-4261 0d.h1 29us : sub_preempt_count (_spin_unlock) | |
852 | sshd-4261 0d.h2 29us : irq_exit (do_IRQ) | |
853 | sshd-4261 0d.h2 29us : sub_preempt_count (irq_exit) | |
854 | sshd-4261 0d..3 30us : do_softirq (irq_exit) | |
855 | sshd-4261 0d... 30us : __do_softirq (do_softirq) | |
856 | sshd-4261 0d... 31us : __local_bh_disable (__do_softirq) | |
857 | sshd-4261 0d... 31us+: add_preempt_count (__local_bh_disable) | |
858 | sshd-4261 0d.s4 34us : add_preempt_count (__local_bh_disable) | |
859 | [...] | |
860 | sshd-4261 0d.s3 43us : sub_preempt_count (local_bh_enable_ip) | |
861 | sshd-4261 0d.s4 44us : sub_preempt_count (local_bh_enable_ip) | |
862 | sshd-4261 0d.s3 44us : smp_apic_timer_interrupt (apic_timer_interrupt) | |
863 | sshd-4261 0d.s3 45us : irq_enter (smp_apic_timer_interrupt) | |
864 | sshd-4261 0d.s3 45us : idle_cpu (irq_enter) | |
865 | sshd-4261 0d.s3 46us : add_preempt_count (irq_enter) | |
866 | sshd-4261 0d.H3 46us : idle_cpu (irq_enter) | |
867 | sshd-4261 0d.H3 47us : hrtimer_interrupt (smp_apic_timer_interrupt) | |
868 | sshd-4261 0d.H3 47us : ktime_get (hrtimer_interrupt) | |
869 | [...] | |
870 | sshd-4261 0d.H3 81us : tick_program_event (hrtimer_interrupt) | |
871 | sshd-4261 0d.H3 82us : ktime_get (tick_program_event) | |
872 | sshd-4261 0d.H3 82us : ktime_get_ts (ktime_get) | |
873 | sshd-4261 0d.H3 83us : getnstimeofday (ktime_get_ts) | |
874 | sshd-4261 0d.H3 83us : set_normalized_timespec (ktime_get_ts) | |
875 | sshd-4261 0d.H3 84us : clockevents_program_event (tick_program_event) | |
876 | sshd-4261 0d.H3 84us : lapic_next_event (clockevents_program_event) | |
877 | sshd-4261 0d.H3 85us : irq_exit (smp_apic_timer_interrupt) | |
878 | sshd-4261 0d.H3 85us : sub_preempt_count (irq_exit) | |
879 | sshd-4261 0d.s4 86us : sub_preempt_count (irq_exit) | |
880 | sshd-4261 0d.s3 86us : add_preempt_count (__local_bh_disable) | |
881 | [...] | |
882 | sshd-4261 0d.s1 98us : sub_preempt_count (net_rx_action) | |
883 | sshd-4261 0d.s. 99us : add_preempt_count (_spin_lock_irq) | |
884 | sshd-4261 0d.s1 99us+: _spin_unlock_irq (run_timer_softirq) | |
885 | sshd-4261 0d.s. 104us : _local_bh_enable (__do_softirq) | |
886 | sshd-4261 0d.s. 104us : sub_preempt_count (_local_bh_enable) | |
887 | sshd-4261 0d.s. 105us : _local_bh_enable (__do_softirq) | |
888 | sshd-4261 0d.s1 105us : trace_preempt_on (__do_softirq) | |
889 | ||
890 | ||
5752674e IM |
891 | This is a very interesting trace. It started with the preemption |
892 | of the ls task. We see that the task had the "need_resched" bit | |
893 | set via the 'N' in the trace. Interrupts were disabled before | |
894 | the spin_lock at the beginning of the trace. We see that a | |
895 | schedule took place to run sshd. When the interrupts were | |
896 | enabled, we took an interrupt. On return from the interrupt | |
897 | handler, the softirq ran. We took another interrupt while | |
898 | running the softirq as we see from the capital 'H'. | |
eb6d42ea SR |
899 | |
900 | ||
901 | wakeup | |
902 | ------ | |
903 | ||
5752674e IM |
904 | In a Real-Time environment it is very important to know the |
905 | wakeup time it takes for the highest priority task that is woken | |
906 | up to the time that it executes. This is also known as "schedule | |
907 | latency". I stress the point that this is about RT tasks. It is | |
908 | also important to know the scheduling latency of non-RT tasks, | |
909 | but the average schedule latency is better for non-RT tasks. | |
910 | Tools like LatencyTop are more appropriate for such | |
911 | measurements. | |
eb6d42ea | 912 | |
a41eebab | 913 | Real-Time environments are interested in the worst case latency. |
5752674e IM |
914 | That is the longest latency it takes for something to happen, |
915 | and not the average. We can have a very fast scheduler that may | |
916 | only have a large latency once in a while, but that would not | |
917 | work well with Real-Time tasks. The wakeup tracer was designed | |
918 | to record the worst case wakeups of RT tasks. Non-RT tasks are | |
919 | not recorded because the tracer only records one worst case and | |
920 | tracing non-RT tasks that are unpredictable will overwrite the | |
921 | worst case latency of RT tasks. | |
922 | ||
923 | Since this tracer only deals with RT tasks, we will run this | |
924 | slightly differently than we did with the previous tracers. | |
925 | Instead of performing an 'ls', we will run 'sleep 1' under | |
926 | 'chrt' which changes the priority of the task. | |
eb6d42ea | 927 | |
156f5a78 | 928 | # echo wakeup > current_tracer |
4a88d44a | 929 | # echo latency-format > trace_options |
156f5a78 | 930 | # echo 0 > tracing_max_latency |
6752ab4a | 931 | # echo 1 > tracing_on |
eb6d42ea | 932 | # chrt -f 5 sleep 1 |
6752ab4a | 933 | # echo 0 > tracing_on |
4a88d44a | 934 | # cat trace |
eb6d42ea SR |
935 | # tracer: wakeup |
936 | # | |
937 | wakeup latency trace v1.1.5 on 2.6.26-rc8 | |
938 | -------------------------------------------------------------------- | |
939 | latency: 4 us, #2/2, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
940 | ----------------- | |
941 | | task: sleep-4901 (uid:0 nice:0 policy:1 rt_prio:5) | |
942 | ----------------- | |
943 | ||
944 | # _------=> CPU# | |
945 | # / _-----=> irqs-off | |
946 | # | / _----=> need-resched | |
947 | # || / _---=> hardirq/softirq | |
948 | # ||| / _--=> preempt-depth | |
949 | # |||| / | |
950 | # ||||| delay | |
951 | # cmd pid ||||| time | caller | |
952 | # \ / ||||| \ | / | |
953 | <idle>-0 1d.h4 0us+: try_to_wake_up (wake_up_process) | |
954 | <idle>-0 1d..4 4us : schedule (cpu_idle) | |
955 | ||
956 | ||
5752674e IM |
957 | Running this on an idle system, we see that it only took 4 |
958 | microseconds to perform the task switch. Note, since the trace | |
959 | marker in the schedule is before the actual "switch", we stop | |
960 | the tracing when the recorded task is about to schedule in. This | |
961 | may change if we add a new marker at the end of the scheduler. | |
eb6d42ea | 962 | |
5752674e IM |
963 | Notice that the recorded task is 'sleep' with the PID of 4901 |
964 | and it has an rt_prio of 5. This priority is user-space priority | |
965 | and not the internal kernel priority. The policy is 1 for | |
966 | SCHED_FIFO and 2 for SCHED_RR. | |
eb6d42ea SR |
967 | |
968 | Doing the same with chrt -r 5 and ftrace_enabled set. | |
969 | ||
970 | # tracer: wakeup | |
971 | # | |
972 | wakeup latency trace v1.1.5 on 2.6.26-rc8 | |
973 | -------------------------------------------------------------------- | |
974 | latency: 50 us, #60/60, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
975 | ----------------- | |
976 | | task: sleep-4068 (uid:0 nice:0 policy:2 rt_prio:5) | |
977 | ----------------- | |
978 | ||
979 | # _------=> CPU# | |
980 | # / _-----=> irqs-off | |
981 | # | / _----=> need-resched | |
982 | # || / _---=> hardirq/softirq | |
983 | # ||| / _--=> preempt-depth | |
984 | # |||| / | |
985 | # ||||| delay | |
986 | # cmd pid ||||| time | caller | |
987 | # \ / ||||| \ | / | |
988 | ksoftirq-7 1d.H3 0us : try_to_wake_up (wake_up_process) | |
989 | ksoftirq-7 1d.H4 1us : sub_preempt_count (marker_probe_cb) | |
990 | ksoftirq-7 1d.H3 2us : check_preempt_wakeup (try_to_wake_up) | |
991 | ksoftirq-7 1d.H3 3us : update_curr (check_preempt_wakeup) | |
992 | ksoftirq-7 1d.H3 4us : calc_delta_mine (update_curr) | |
993 | ksoftirq-7 1d.H3 5us : __resched_task (check_preempt_wakeup) | |
994 | ksoftirq-7 1d.H3 6us : task_wake_up_rt (try_to_wake_up) | |
995 | ksoftirq-7 1d.H3 7us : _spin_unlock_irqrestore (try_to_wake_up) | |
996 | [...] | |
997 | ksoftirq-7 1d.H2 17us : irq_exit (smp_apic_timer_interrupt) | |
998 | ksoftirq-7 1d.H2 18us : sub_preempt_count (irq_exit) | |
999 | ksoftirq-7 1d.s3 19us : sub_preempt_count (irq_exit) | |
1000 | ksoftirq-7 1..s2 20us : rcu_process_callbacks (__do_softirq) | |
1001 | [...] | |
1002 | ksoftirq-7 1..s2 26us : __rcu_process_callbacks (rcu_process_callbacks) | |
1003 | ksoftirq-7 1d.s2 27us : _local_bh_enable (__do_softirq) | |
1004 | ksoftirq-7 1d.s2 28us : sub_preempt_count (_local_bh_enable) | |
1005 | ksoftirq-7 1.N.3 29us : sub_preempt_count (ksoftirqd) | |
1006 | ksoftirq-7 1.N.2 30us : _cond_resched (ksoftirqd) | |
1007 | ksoftirq-7 1.N.2 31us : __cond_resched (_cond_resched) | |
1008 | ksoftirq-7 1.N.2 32us : add_preempt_count (__cond_resched) | |
1009 | ksoftirq-7 1.N.2 33us : schedule (__cond_resched) | |
1010 | ksoftirq-7 1.N.2 33us : add_preempt_count (schedule) | |
1011 | ksoftirq-7 1.N.3 34us : hrtick_clear (schedule) | |
1012 | ksoftirq-7 1dN.3 35us : _spin_lock (schedule) | |
1013 | ksoftirq-7 1dN.3 36us : add_preempt_count (_spin_lock) | |
1014 | ksoftirq-7 1d..4 37us : put_prev_task_fair (schedule) | |
1015 | ksoftirq-7 1d..4 38us : update_curr (put_prev_task_fair) | |
1016 | [...] | |
1017 | ksoftirq-7 1d..5 47us : _spin_trylock (tracing_record_cmdline) | |
1018 | ksoftirq-7 1d..5 48us : add_preempt_count (_spin_trylock) | |
1019 | ksoftirq-7 1d..6 49us : _spin_unlock (tracing_record_cmdline) | |
1020 | ksoftirq-7 1d..6 49us : sub_preempt_count (_spin_unlock) | |
1021 | ksoftirq-7 1d..4 50us : schedule (__cond_resched) | |
1022 | ||
5752674e IM |
1023 | The interrupt went off while running ksoftirqd. This task runs |
1024 | at SCHED_OTHER. Why did not we see the 'N' set early? This may | |
1025 | be a harmless bug with x86_32 and 4K stacks. On x86_32 with 4K | |
1026 | stacks configured, the interrupt and softirq run with their own | |
1027 | stack. Some information is held on the top of the task's stack | |
1028 | (need_resched and preempt_count are both stored there). The | |
1029 | setting of the NEED_RESCHED bit is done directly to the task's | |
1030 | stack, but the reading of the NEED_RESCHED is done by looking at | |
1031 | the current stack, which in this case is the stack for the hard | |
1032 | interrupt. This hides the fact that NEED_RESCHED has been set. | |
1033 | We do not see the 'N' until we switch back to the task's | |
a41eebab | 1034 | assigned stack. |
eb6d42ea | 1035 | |
9b803c0f SR |
1036 | function |
1037 | -------- | |
eb6d42ea | 1038 | |
9b803c0f | 1039 | This tracer is the function tracer. Enabling the function tracer |
5752674e IM |
1040 | can be done from the debug file system. Make sure the |
1041 | ftrace_enabled is set; otherwise this tracer is a nop. | |
eb6d42ea SR |
1042 | |
1043 | # sysctl kernel.ftrace_enabled=1 | |
156f5a78 | 1044 | # echo function > current_tracer |
6752ab4a | 1045 | # echo 1 > tracing_on |
eb6d42ea | 1046 | # usleep 1 |
6752ab4a | 1047 | # echo 0 > tracing_on |
156f5a78 | 1048 | # cat trace |
9b803c0f | 1049 | # tracer: function |
eb6d42ea SR |
1050 | # |
1051 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1052 | # | | | | | | |
1053 | bash-4003 [00] 123.638713: finish_task_switch <-schedule | |
1054 | bash-4003 [00] 123.638714: _spin_unlock_irq <-finish_task_switch | |
1055 | bash-4003 [00] 123.638714: sub_preempt_count <-_spin_unlock_irq | |
1056 | bash-4003 [00] 123.638715: hrtick_set <-schedule | |
1057 | bash-4003 [00] 123.638715: _spin_lock_irqsave <-hrtick_set | |
1058 | bash-4003 [00] 123.638716: add_preempt_count <-_spin_lock_irqsave | |
1059 | bash-4003 [00] 123.638716: _spin_unlock_irqrestore <-hrtick_set | |
1060 | bash-4003 [00] 123.638717: sub_preempt_count <-_spin_unlock_irqrestore | |
1061 | bash-4003 [00] 123.638717: hrtick_clear <-hrtick_set | |
1062 | bash-4003 [00] 123.638718: sub_preempt_count <-schedule | |
1063 | bash-4003 [00] 123.638718: sub_preempt_count <-preempt_schedule | |
1064 | bash-4003 [00] 123.638719: wait_for_completion <-__stop_machine_run | |
1065 | bash-4003 [00] 123.638719: wait_for_common <-wait_for_completion | |
1066 | bash-4003 [00] 123.638720: _spin_lock_irq <-wait_for_common | |
1067 | bash-4003 [00] 123.638720: add_preempt_count <-_spin_lock_irq | |
1068 | [...] | |
1069 | ||
1070 | ||
5752674e IM |
1071 | Note: function tracer uses ring buffers to store the above |
1072 | entries. The newest data may overwrite the oldest data. | |
1073 | Sometimes using echo to stop the trace is not sufficient because | |
1074 | the tracing could have overwritten the data that you wanted to | |
1075 | record. For this reason, it is sometimes better to disable | |
1076 | tracing directly from a program. This allows you to stop the | |
1077 | tracing at the point that you hit the part that you are | |
1078 | interested in. To disable the tracing directly from a C program, | |
1079 | something like following code snippet can be used: | |
eb6d42ea SR |
1080 | |
1081 | int trace_fd; | |
1082 | [...] | |
1083 | int main(int argc, char *argv[]) { | |
1084 | [...] | |
6752ab4a | 1085 | trace_fd = open(tracing_file("tracing_on"), O_WRONLY); |
eb6d42ea SR |
1086 | [...] |
1087 | if (condition_hit()) { | |
f2d9c740 | 1088 | write(trace_fd, "0", 1); |
eb6d42ea SR |
1089 | } |
1090 | [...] | |
1091 | } | |
1092 | ||
df4fc315 SR |
1093 | |
1094 | Single thread tracing | |
1095 | --------------------- | |
1096 | ||
156f5a78 | 1097 | By writing into set_ftrace_pid you can trace a |
df4fc315 SR |
1098 | single thread. For example: |
1099 | ||
156f5a78 | 1100 | # cat set_ftrace_pid |
df4fc315 | 1101 | no pid |
156f5a78 GL |
1102 | # echo 3111 > set_ftrace_pid |
1103 | # cat set_ftrace_pid | |
df4fc315 | 1104 | 3111 |
156f5a78 GL |
1105 | # echo function > current_tracer |
1106 | # cat trace | head | |
df4fc315 SR |
1107 | # tracer: function |
1108 | # | |
1109 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1110 | # | | | | | | |
1111 | yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return | |
1112 | yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range | |
1113 | yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel | |
1114 | yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel | |
1115 | yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll | |
1116 | yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll | |
156f5a78 GL |
1117 | # echo -1 > set_ftrace_pid |
1118 | # cat trace |head | |
df4fc315 SR |
1119 | # tracer: function |
1120 | # | |
1121 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1122 | # | | | | | | |
1123 | ##### CPU 3 buffer started #### | |
1124 | yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait | |
1125 | yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry | |
1126 | yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry | |
1127 | yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit | |
1128 | yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit | |
1129 | ||
1130 | If you want to trace a function when executing, you could use | |
1131 | something like this simple program: | |
1132 | ||
1133 | #include <stdio.h> | |
1134 | #include <stdlib.h> | |
1135 | #include <sys/types.h> | |
1136 | #include <sys/stat.h> | |
1137 | #include <fcntl.h> | |
1138 | #include <unistd.h> | |
67b394f7 | 1139 | #include <string.h> |
df4fc315 | 1140 | |
156f5a78 GL |
1141 | #define _STR(x) #x |
1142 | #define STR(x) _STR(x) | |
1143 | #define MAX_PATH 256 | |
1144 | ||
1145 | const char *find_debugfs(void) | |
1146 | { | |
1147 | static char debugfs[MAX_PATH+1]; | |
1148 | static int debugfs_found; | |
1149 | char type[100]; | |
1150 | FILE *fp; | |
1151 | ||
1152 | if (debugfs_found) | |
1153 | return debugfs; | |
1154 | ||
1155 | if ((fp = fopen("/proc/mounts","r")) == NULL) { | |
1156 | perror("/proc/mounts"); | |
1157 | return NULL; | |
1158 | } | |
1159 | ||
1160 | while (fscanf(fp, "%*s %" | |
1161 | STR(MAX_PATH) | |
1162 | "s %99s %*s %*d %*d\n", | |
1163 | debugfs, type) == 2) { | |
1164 | if (strcmp(type, "debugfs") == 0) | |
1165 | break; | |
1166 | } | |
1167 | fclose(fp); | |
1168 | ||
1169 | if (strcmp(type, "debugfs") != 0) { | |
1170 | fprintf(stderr, "debugfs not mounted"); | |
1171 | return NULL; | |
1172 | } | |
1173 | ||
67b394f7 | 1174 | strcat(debugfs, "/tracing/"); |
156f5a78 GL |
1175 | debugfs_found = 1; |
1176 | ||
1177 | return debugfs; | |
1178 | } | |
1179 | ||
1180 | const char *tracing_file(const char *file_name) | |
1181 | { | |
1182 | static char trace_file[MAX_PATH+1]; | |
1183 | snprintf(trace_file, MAX_PATH, "%s/%s", find_debugfs(), file_name); | |
1184 | return trace_file; | |
1185 | } | |
1186 | ||
df4fc315 SR |
1187 | int main (int argc, char **argv) |
1188 | { | |
1189 | if (argc < 1) | |
1190 | exit(-1); | |
1191 | ||
1192 | if (fork() > 0) { | |
1193 | int fd, ffd; | |
1194 | char line[64]; | |
1195 | int s; | |
1196 | ||
156f5a78 | 1197 | ffd = open(tracing_file("current_tracer"), O_WRONLY); |
df4fc315 SR |
1198 | if (ffd < 0) |
1199 | exit(-1); | |
1200 | write(ffd, "nop", 3); | |
1201 | ||
156f5a78 | 1202 | fd = open(tracing_file("set_ftrace_pid"), O_WRONLY); |
df4fc315 SR |
1203 | s = sprintf(line, "%d\n", getpid()); |
1204 | write(fd, line, s); | |
1205 | ||
1206 | write(ffd, "function", 8); | |
1207 | ||
1208 | close(fd); | |
1209 | close(ffd); | |
1210 | ||
1211 | execvp(argv[1], argv+1); | |
1212 | } | |
1213 | ||
1214 | return 0; | |
1215 | } | |
1216 | ||
e2ea5399 MM |
1217 | |
1218 | hw-branch-tracer (x86 only) | |
1219 | --------------------------- | |
1220 | ||
1221 | This tracer uses the x86 last branch tracing hardware feature to | |
1222 | collect a branch trace on all cpus with relatively low overhead. | |
1223 | ||
1224 | The tracer uses a fixed-size circular buffer per cpu and only | |
1225 | traces ring 0 branches. The trace file dumps that buffer in the | |
1226 | following format: | |
1227 | ||
1228 | # tracer: hw-branch-tracer | |
1229 | # | |
1230 | # CPU# TO <- FROM | |
1231 | 0 scheduler_tick+0xb5/0x1bf <- task_tick_idle+0x5/0x6 | |
1232 | 2 run_posix_cpu_timers+0x2b/0x72a <- run_posix_cpu_timers+0x25/0x72a | |
1233 | 0 scheduler_tick+0x139/0x1bf <- scheduler_tick+0xed/0x1bf | |
1234 | 0 scheduler_tick+0x17c/0x1bf <- scheduler_tick+0x148/0x1bf | |
1235 | 2 run_posix_cpu_timers+0x9e/0x72a <- run_posix_cpu_timers+0x5e/0x72a | |
1236 | 0 scheduler_tick+0x1b6/0x1bf <- scheduler_tick+0x1aa/0x1bf | |
1237 | ||
1238 | ||
5752674e IM |
1239 | The tracer may be used to dump the trace for the oops'ing cpu on |
1240 | a kernel oops into the system log. To enable this, | |
1241 | ftrace_dump_on_oops must be set. To set ftrace_dump_on_oops, one | |
1242 | can either use the sysctl function or set it via the proc system | |
1243 | interface. | |
e2ea5399 | 1244 | |
cecbca96 | 1245 | sysctl kernel.ftrace_dump_on_oops=n |
e2ea5399 MM |
1246 | |
1247 | or | |
1248 | ||
cecbca96 | 1249 | echo n > /proc/sys/kernel/ftrace_dump_on_oops |
e2ea5399 | 1250 | |
cecbca96 FW |
1251 | If n = 1, ftrace will dump buffers of all CPUs, if n = 2 ftrace will |
1252 | only dump the buffer of the CPU that triggered the oops. | |
e2ea5399 | 1253 | |
5752674e IM |
1254 | Here's an example of such a dump after a null pointer |
1255 | dereference in a kernel module: | |
e2ea5399 MM |
1256 | |
1257 | [57848.105921] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000 | |
1258 | [57848.106019] IP: [<ffffffffa0000006>] open+0x6/0x14 [oops] | |
1259 | [57848.106019] PGD 2354e9067 PUD 2375e7067 PMD 0 | |
1260 | [57848.106019] Oops: 0002 [#1] SMP | |
1261 | [57848.106019] last sysfs file: /sys/devices/pci0000:00/0000:00:1e.0/0000:20:05.0/local_cpus | |
1262 | [57848.106019] Dumping ftrace buffer: | |
1263 | [57848.106019] --------------------------------- | |
1264 | [...] | |
1265 | [57848.106019] 0 chrdev_open+0xe6/0x165 <- cdev_put+0x23/0x24 | |
1266 | [57848.106019] 0 chrdev_open+0x117/0x165 <- chrdev_open+0xfa/0x165 | |
1267 | [57848.106019] 0 chrdev_open+0x120/0x165 <- chrdev_open+0x11c/0x165 | |
1268 | [57848.106019] 0 chrdev_open+0x134/0x165 <- chrdev_open+0x12b/0x165 | |
1269 | [57848.106019] 0 open+0x0/0x14 [oops] <- chrdev_open+0x144/0x165 | |
1270 | [57848.106019] 0 page_fault+0x0/0x30 <- open+0x6/0x14 [oops] | |
1271 | [57848.106019] 0 error_entry+0x0/0x5b <- page_fault+0x4/0x30 | |
1272 | [57848.106019] 0 error_kernelspace+0x0/0x31 <- error_entry+0x59/0x5b | |
1273 | [57848.106019] 0 error_sti+0x0/0x1 <- error_kernelspace+0x2d/0x31 | |
1274 | [57848.106019] 0 page_fault+0x9/0x30 <- error_sti+0x0/0x1 | |
1275 | [57848.106019] 0 do_page_fault+0x0/0x881 <- page_fault+0x1a/0x30 | |
1276 | [...] | |
1277 | [57848.106019] 0 do_page_fault+0x66b/0x881 <- is_prefetch+0x1ee/0x1f2 | |
1278 | [57848.106019] 0 do_page_fault+0x6e0/0x881 <- do_page_fault+0x67a/0x881 | |
1279 | [57848.106019] 0 oops_begin+0x0/0x96 <- do_page_fault+0x6e0/0x881 | |
1280 | [57848.106019] 0 trace_hw_branch_oops+0x0/0x2d <- oops_begin+0x9/0x96 | |
1281 | [...] | |
1282 | [57848.106019] 0 ds_suspend_bts+0x2a/0xe3 <- ds_suspend_bts+0x1a/0xe3 | |
1283 | [57848.106019] --------------------------------- | |
1284 | [57848.106019] CPU 0 | |
1285 | [57848.106019] Modules linked in: oops | |
1286 | [57848.106019] Pid: 5542, comm: cat Tainted: G W 2.6.28 #23 | |
1287 | [57848.106019] RIP: 0010:[<ffffffffa0000006>] [<ffffffffa0000006>] open+0x6/0x14 [oops] | |
1288 | [57848.106019] RSP: 0018:ffff880235457d48 EFLAGS: 00010246 | |
1289 | [...] | |
1290 | ||
1291 | ||
985ec20a FW |
1292 | function graph tracer |
1293 | --------------------------- | |
1294 | ||
5752674e IM |
1295 | This tracer is similar to the function tracer except that it |
1296 | probes a function on its entry and its exit. This is done by | |
1297 | using a dynamically allocated stack of return addresses in each | |
1298 | task_struct. On function entry the tracer overwrites the return | |
1299 | address of each function traced to set a custom probe. Thus the | |
1300 | original return address is stored on the stack of return address | |
1301 | in the task_struct. | |
985ec20a | 1302 | |
5752674e IM |
1303 | Probing on both ends of a function leads to special features |
1304 | such as: | |
985ec20a | 1305 | |
5752674e IM |
1306 | - measure of a function's time execution |
1307 | - having a reliable call stack to draw function calls graph | |
985ec20a FW |
1308 | |
1309 | This tracer is useful in several situations: | |
1310 | ||
5752674e IM |
1311 | - you want to find the reason of a strange kernel behavior and |
1312 | need to see what happens in detail on any areas (or specific | |
1313 | ones). | |
1314 | ||
1315 | - you are experiencing weird latencies but it's difficult to | |
1316 | find its origin. | |
1317 | ||
1318 | - you want to find quickly which path is taken by a specific | |
1319 | function | |
1320 | ||
1321 | - you just want to peek inside a working kernel and want to see | |
1322 | what happens there. | |
985ec20a FW |
1323 | |
1324 | # tracer: function_graph | |
1325 | # | |
1326 | # CPU DURATION FUNCTION CALLS | |
1327 | # | | | | | | | | |
1328 | ||
1329 | 0) | sys_open() { | |
1330 | 0) | do_sys_open() { | |
1331 | 0) | getname() { | |
1332 | 0) | kmem_cache_alloc() { | |
1333 | 0) 1.382 us | __might_sleep(); | |
1334 | 0) 2.478 us | } | |
1335 | 0) | strncpy_from_user() { | |
1336 | 0) | might_fault() { | |
1337 | 0) 1.389 us | __might_sleep(); | |
1338 | 0) 2.553 us | } | |
1339 | 0) 3.807 us | } | |
1340 | 0) 7.876 us | } | |
1341 | 0) | alloc_fd() { | |
1342 | 0) 0.668 us | _spin_lock(); | |
1343 | 0) 0.570 us | expand_files(); | |
1344 | 0) 0.586 us | _spin_unlock(); | |
1345 | ||
1346 | ||
5752674e IM |
1347 | There are several columns that can be dynamically |
1348 | enabled/disabled. You can use every combination of options you | |
1349 | want, depending on your needs. | |
985ec20a | 1350 | |
5752674e IM |
1351 | - The cpu number on which the function executed is default |
1352 | enabled. It is sometimes better to only trace one cpu (see | |
1353 | tracing_cpu_mask file) or you might sometimes see unordered | |
1354 | function calls while cpu tracing switch. | |
985ec20a | 1355 | |
156f5a78 GL |
1356 | hide: echo nofuncgraph-cpu > trace_options |
1357 | show: echo funcgraph-cpu > trace_options | |
985ec20a | 1358 | |
5752674e IM |
1359 | - The duration (function's time of execution) is displayed on |
1360 | the closing bracket line of a function or on the same line | |
1361 | than the current function in case of a leaf one. It is default | |
1362 | enabled. | |
985ec20a | 1363 | |
156f5a78 GL |
1364 | hide: echo nofuncgraph-duration > trace_options |
1365 | show: echo funcgraph-duration > trace_options | |
985ec20a | 1366 | |
5752674e IM |
1367 | - The overhead field precedes the duration field in case of |
1368 | reached duration thresholds. | |
985ec20a | 1369 | |
156f5a78 GL |
1370 | hide: echo nofuncgraph-overhead > trace_options |
1371 | show: echo funcgraph-overhead > trace_options | |
985ec20a FW |
1372 | depends on: funcgraph-duration |
1373 | ||
1374 | ie: | |
1375 | ||
1376 | 0) | up_write() { | |
1377 | 0) 0.646 us | _spin_lock_irqsave(); | |
1378 | 0) 0.684 us | _spin_unlock_irqrestore(); | |
1379 | 0) 3.123 us | } | |
1380 | 0) 0.548 us | fput(); | |
1381 | 0) + 58.628 us | } | |
1382 | ||
1383 | [...] | |
1384 | ||
1385 | 0) | putname() { | |
1386 | 0) | kmem_cache_free() { | |
1387 | 0) 0.518 us | __phys_addr(); | |
1388 | 0) 1.757 us | } | |
1389 | 0) 2.861 us | } | |
1390 | 0) ! 115.305 us | } | |
1391 | 0) ! 116.402 us | } | |
1392 | ||
1393 | + means that the function exceeded 10 usecs. | |
1394 | ! means that the function exceeded 100 usecs. | |
1395 | ||
1396 | ||
5752674e IM |
1397 | - The task/pid field displays the thread cmdline and pid which |
1398 | executed the function. It is default disabled. | |
985ec20a | 1399 | |
156f5a78 GL |
1400 | hide: echo nofuncgraph-proc > trace_options |
1401 | show: echo funcgraph-proc > trace_options | |
985ec20a FW |
1402 | |
1403 | ie: | |
1404 | ||
1405 | # tracer: function_graph | |
1406 | # | |
1407 | # CPU TASK/PID DURATION FUNCTION CALLS | |
1408 | # | | | | | | | | | | |
1409 | 0) sh-4802 | | d_free() { | |
1410 | 0) sh-4802 | | call_rcu() { | |
1411 | 0) sh-4802 | | __call_rcu() { | |
1412 | 0) sh-4802 | 0.616 us | rcu_process_gp_end(); | |
1413 | 0) sh-4802 | 0.586 us | check_for_new_grace_period(); | |
1414 | 0) sh-4802 | 2.899 us | } | |
1415 | 0) sh-4802 | 4.040 us | } | |
1416 | 0) sh-4802 | 5.151 us | } | |
1417 | 0) sh-4802 | + 49.370 us | } | |
1418 | ||
1419 | ||
5752674e IM |
1420 | - The absolute time field is an absolute timestamp given by the |
1421 | system clock since it started. A snapshot of this time is | |
1422 | given on each entry/exit of functions | |
985ec20a | 1423 | |
156f5a78 GL |
1424 | hide: echo nofuncgraph-abstime > trace_options |
1425 | show: echo funcgraph-abstime > trace_options | |
985ec20a FW |
1426 | |
1427 | ie: | |
1428 | ||
1429 | # | |
1430 | # TIME CPU DURATION FUNCTION CALLS | |
1431 | # | | | | | | | | | |
1432 | 360.774522 | 1) 0.541 us | } | |
1433 | 360.774522 | 1) 4.663 us | } | |
1434 | 360.774523 | 1) 0.541 us | __wake_up_bit(); | |
1435 | 360.774524 | 1) 6.796 us | } | |
1436 | 360.774524 | 1) 7.952 us | } | |
1437 | 360.774525 | 1) 9.063 us | } | |
1438 | 360.774525 | 1) 0.615 us | journal_mark_dirty(); | |
1439 | 360.774527 | 1) 0.578 us | __brelse(); | |
1440 | 360.774528 | 1) | reiserfs_prepare_for_journal() { | |
1441 | 360.774528 | 1) | unlock_buffer() { | |
1442 | 360.774529 | 1) | wake_up_bit() { | |
1443 | 360.774529 | 1) | bit_waitqueue() { | |
1444 | 360.774530 | 1) 0.594 us | __phys_addr(); | |
1445 | ||
1446 | ||
5752674e | 1447 | You can put some comments on specific functions by using |
5e1607a0 | 1448 | trace_printk() For example, if you want to put a comment inside |
5752674e | 1449 | the __might_sleep() function, you just have to include |
5e1607a0 | 1450 | <linux/ftrace.h> and call trace_printk() inside __might_sleep() |
985ec20a | 1451 | |
5e1607a0 | 1452 | trace_printk("I'm a comment!\n") |
985ec20a FW |
1453 | |
1454 | will produce: | |
1455 | ||
1456 | 1) | __might_sleep() { | |
1457 | 1) | /* I'm a comment! */ | |
1458 | 1) 1.449 us | } | |
1459 | ||
1460 | ||
5752674e IM |
1461 | You might find other useful features for this tracer in the |
1462 | following "dynamic ftrace" section such as tracing only specific | |
1463 | functions or tasks. | |
985ec20a | 1464 | |
eb6d42ea SR |
1465 | dynamic ftrace |
1466 | -------------- | |
1467 | ||
f2d9c740 | 1468 | If CONFIG_DYNAMIC_FTRACE is set, the system will run with |
eb6d42ea SR |
1469 | virtually no overhead when function tracing is disabled. The way |
1470 | this works is the mcount function call (placed at the start of | |
5752674e IM |
1471 | every kernel function, produced by the -pg switch in gcc), |
1472 | starts of pointing to a simple return. (Enabling FTRACE will | |
1473 | include the -pg switch in the compiling of the kernel.) | |
eb6d42ea | 1474 | |
9b803c0f SR |
1475 | At compile time every C file object is run through the |
1476 | recordmcount.pl script (located in the scripts directory). This | |
1477 | script will process the C object using objdump to find all the | |
5752674e IM |
1478 | locations in the .text section that call mcount. (Note, only the |
1479 | .text section is processed, since processing other sections like | |
1480 | .init.text may cause races due to those sections being freed). | |
9b803c0f | 1481 | |
5752674e IM |
1482 | A new section called "__mcount_loc" is created that holds |
1483 | references to all the mcount call sites in the .text section. | |
1484 | This section is compiled back into the original object. The | |
1485 | final linker will add all these references into a single table. | |
9b803c0f SR |
1486 | |
1487 | On boot up, before SMP is initialized, the dynamic ftrace code | |
5752674e IM |
1488 | scans this table and updates all the locations into nops. It |
1489 | also records the locations, which are added to the | |
1490 | available_filter_functions list. Modules are processed as they | |
1491 | are loaded and before they are executed. When a module is | |
1492 | unloaded, it also removes its functions from the ftrace function | |
1493 | list. This is automatic in the module unload code, and the | |
1494 | module author does not need to worry about it. | |
1495 | ||
1496 | When tracing is enabled, kstop_machine is called to prevent | |
1497 | races with the CPUS executing code being modified (which can | |
3ad2f3fb | 1498 | cause the CPU to do undesirable things), and the nops are |
5752674e IM |
1499 | patched back to calls. But this time, they do not call mcount |
1500 | (which is just a function stub). They now call into the ftrace | |
1501 | infrastructure. | |
eb6d42ea SR |
1502 | |
1503 | One special side-effect to the recording of the functions being | |
f2d9c740 | 1504 | traced is that we can now selectively choose which functions we |
5752674e IM |
1505 | wish to trace and which ones we want the mcount calls to remain |
1506 | as nops. | |
eb6d42ea | 1507 | |
5752674e IM |
1508 | Two files are used, one for enabling and one for disabling the |
1509 | tracing of specified functions. They are: | |
eb6d42ea SR |
1510 | |
1511 | set_ftrace_filter | |
1512 | ||
1513 | and | |
1514 | ||
1515 | set_ftrace_notrace | |
1516 | ||
5752674e IM |
1517 | A list of available functions that you can add to these files is |
1518 | listed in: | |
eb6d42ea SR |
1519 | |
1520 | available_filter_functions | |
1521 | ||
156f5a78 | 1522 | # cat available_filter_functions |
eb6d42ea SR |
1523 | put_prev_task_idle |
1524 | kmem_cache_create | |
1525 | pick_next_task_rt | |
1526 | get_online_cpus | |
1527 | pick_next_task_fair | |
1528 | mutex_lock | |
1529 | [...] | |
1530 | ||
f2d9c740 | 1531 | If I am only interested in sys_nanosleep and hrtimer_interrupt: |
eb6d42ea SR |
1532 | |
1533 | # echo sys_nanosleep hrtimer_interrupt \ | |
156f5a78 | 1534 | > set_ftrace_filter |
6993b1bb | 1535 | # echo function > current_tracer |
6752ab4a | 1536 | # echo 1 > tracing_on |
eb6d42ea | 1537 | # usleep 1 |
6752ab4a | 1538 | # echo 0 > tracing_on |
156f5a78 | 1539 | # cat trace |
eb6d42ea SR |
1540 | # tracer: ftrace |
1541 | # | |
1542 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1543 | # | | | | | | |
1544 | usleep-4134 [00] 1317.070017: hrtimer_interrupt <-smp_apic_timer_interrupt | |
1545 | usleep-4134 [00] 1317.070111: sys_nanosleep <-syscall_call | |
1546 | <idle>-0 [00] 1317.070115: hrtimer_interrupt <-smp_apic_timer_interrupt | |
1547 | ||
f2d9c740 | 1548 | To see which functions are being traced, you can cat the file: |
eb6d42ea | 1549 | |
156f5a78 | 1550 | # cat set_ftrace_filter |
eb6d42ea SR |
1551 | hrtimer_interrupt |
1552 | sys_nanosleep | |
1553 | ||
1554 | ||
5752674e IM |
1555 | Perhaps this is not enough. The filters also allow simple wild |
1556 | cards. Only the following are currently available | |
eb6d42ea | 1557 | |
a41eebab | 1558 | <match>* - will match functions that begin with <match> |
eb6d42ea SR |
1559 | *<match> - will match functions that end with <match> |
1560 | *<match>* - will match functions that have <match> in it | |
1561 | ||
f2d9c740 | 1562 | These are the only wild cards which are supported. |
eb6d42ea SR |
1563 | |
1564 | <match>*<match> will not work. | |
1565 | ||
5752674e IM |
1566 | Note: It is better to use quotes to enclose the wild cards, |
1567 | otherwise the shell may expand the parameters into names | |
1568 | of files in the local directory. | |
c072c249 | 1569 | |
156f5a78 | 1570 | # echo 'hrtimer_*' > set_ftrace_filter |
eb6d42ea SR |
1571 | |
1572 | Produces: | |
1573 | ||
1574 | # tracer: ftrace | |
1575 | # | |
1576 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1577 | # | | | | | | |
1578 | bash-4003 [00] 1480.611794: hrtimer_init <-copy_process | |
1579 | bash-4003 [00] 1480.611941: hrtimer_start <-hrtick_set | |
1580 | bash-4003 [00] 1480.611956: hrtimer_cancel <-hrtick_clear | |
1581 | bash-4003 [00] 1480.611956: hrtimer_try_to_cancel <-hrtimer_cancel | |
1582 | <idle>-0 [00] 1480.612019: hrtimer_get_next_event <-get_next_timer_interrupt | |
1583 | <idle>-0 [00] 1480.612025: hrtimer_get_next_event <-get_next_timer_interrupt | |
1584 | <idle>-0 [00] 1480.612032: hrtimer_get_next_event <-get_next_timer_interrupt | |
1585 | <idle>-0 [00] 1480.612037: hrtimer_get_next_event <-get_next_timer_interrupt | |
1586 | <idle>-0 [00] 1480.612382: hrtimer_get_next_event <-get_next_timer_interrupt | |
1587 | ||
1588 | ||
1589 | Notice that we lost the sys_nanosleep. | |
1590 | ||
156f5a78 | 1591 | # cat set_ftrace_filter |
eb6d42ea SR |
1592 | hrtimer_run_queues |
1593 | hrtimer_run_pending | |
1594 | hrtimer_init | |
1595 | hrtimer_cancel | |
1596 | hrtimer_try_to_cancel | |
1597 | hrtimer_forward | |
1598 | hrtimer_start | |
1599 | hrtimer_reprogram | |
1600 | hrtimer_force_reprogram | |
1601 | hrtimer_get_next_event | |
1602 | hrtimer_interrupt | |
1603 | hrtimer_nanosleep | |
1604 | hrtimer_wakeup | |
1605 | hrtimer_get_remaining | |
1606 | hrtimer_get_res | |
1607 | hrtimer_init_sleeper | |
1608 | ||
1609 | ||
1610 | This is because the '>' and '>>' act just like they do in bash. | |
1611 | To rewrite the filters, use '>' | |
1612 | To append to the filters, use '>>' | |
1613 | ||
5752674e IM |
1614 | To clear out a filter so that all functions will be recorded |
1615 | again: | |
eb6d42ea | 1616 | |
156f5a78 GL |
1617 | # echo > set_ftrace_filter |
1618 | # cat set_ftrace_filter | |
eb6d42ea SR |
1619 | # |
1620 | ||
1621 | Again, now we want to append. | |
1622 | ||
156f5a78 GL |
1623 | # echo sys_nanosleep > set_ftrace_filter |
1624 | # cat set_ftrace_filter | |
eb6d42ea | 1625 | sys_nanosleep |
156f5a78 GL |
1626 | # echo 'hrtimer_*' >> set_ftrace_filter |
1627 | # cat set_ftrace_filter | |
eb6d42ea SR |
1628 | hrtimer_run_queues |
1629 | hrtimer_run_pending | |
1630 | hrtimer_init | |
1631 | hrtimer_cancel | |
1632 | hrtimer_try_to_cancel | |
1633 | hrtimer_forward | |
1634 | hrtimer_start | |
1635 | hrtimer_reprogram | |
1636 | hrtimer_force_reprogram | |
1637 | hrtimer_get_next_event | |
1638 | hrtimer_interrupt | |
1639 | sys_nanosleep | |
1640 | hrtimer_nanosleep | |
1641 | hrtimer_wakeup | |
1642 | hrtimer_get_remaining | |
1643 | hrtimer_get_res | |
1644 | hrtimer_init_sleeper | |
1645 | ||
1646 | ||
5752674e IM |
1647 | The set_ftrace_notrace prevents those functions from being |
1648 | traced. | |
eb6d42ea | 1649 | |
156f5a78 | 1650 | # echo '*preempt*' '*lock*' > set_ftrace_notrace |
eb6d42ea SR |
1651 | |
1652 | Produces: | |
1653 | ||
1654 | # tracer: ftrace | |
1655 | # | |
1656 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1657 | # | | | | | | |
1658 | bash-4043 [01] 115.281644: finish_task_switch <-schedule | |
1659 | bash-4043 [01] 115.281645: hrtick_set <-schedule | |
1660 | bash-4043 [01] 115.281645: hrtick_clear <-hrtick_set | |
1661 | bash-4043 [01] 115.281646: wait_for_completion <-__stop_machine_run | |
1662 | bash-4043 [01] 115.281647: wait_for_common <-wait_for_completion | |
1663 | bash-4043 [01] 115.281647: kthread_stop <-stop_machine_run | |
1664 | bash-4043 [01] 115.281648: init_waitqueue_head <-kthread_stop | |
1665 | bash-4043 [01] 115.281648: wake_up_process <-kthread_stop | |
1666 | bash-4043 [01] 115.281649: try_to_wake_up <-wake_up_process | |
1667 | ||
1668 | We can see that there's no more lock or preempt tracing. | |
1669 | ||
985ec20a | 1670 | |
5752674e IM |
1671 | Dynamic ftrace with the function graph tracer |
1672 | --------------------------------------------- | |
985ec20a | 1673 | |
5752674e IM |
1674 | Although what has been explained above concerns both the |
1675 | function tracer and the function-graph-tracer, there are some | |
1676 | special features only available in the function-graph tracer. | |
985ec20a | 1677 | |
5752674e IM |
1678 | If you want to trace only one function and all of its children, |
1679 | you just have to echo its name into set_graph_function: | |
985ec20a | 1680 | |
5752674e | 1681 | echo __do_fault > set_graph_function |
985ec20a | 1682 | |
5752674e IM |
1683 | will produce the following "expanded" trace of the __do_fault() |
1684 | function: | |
985ec20a FW |
1685 | |
1686 | 0) | __do_fault() { | |
1687 | 0) | filemap_fault() { | |
1688 | 0) | find_lock_page() { | |
1689 | 0) 0.804 us | find_get_page(); | |
1690 | 0) | __might_sleep() { | |
1691 | 0) 1.329 us | } | |
1692 | 0) 3.904 us | } | |
1693 | 0) 4.979 us | } | |
1694 | 0) 0.653 us | _spin_lock(); | |
1695 | 0) 0.578 us | page_add_file_rmap(); | |
1696 | 0) 0.525 us | native_set_pte_at(); | |
1697 | 0) 0.585 us | _spin_unlock(); | |
1698 | 0) | unlock_page() { | |
1699 | 0) 0.541 us | page_waitqueue(); | |
1700 | 0) 0.639 us | __wake_up_bit(); | |
1701 | 0) 2.786 us | } | |
1702 | 0) + 14.237 us | } | |
1703 | 0) | __do_fault() { | |
1704 | 0) | filemap_fault() { | |
1705 | 0) | find_lock_page() { | |
1706 | 0) 0.698 us | find_get_page(); | |
1707 | 0) | __might_sleep() { | |
1708 | 0) 1.412 us | } | |
1709 | 0) 3.950 us | } | |
1710 | 0) 5.098 us | } | |
1711 | 0) 0.631 us | _spin_lock(); | |
1712 | 0) 0.571 us | page_add_file_rmap(); | |
1713 | 0) 0.526 us | native_set_pte_at(); | |
1714 | 0) 0.586 us | _spin_unlock(); | |
1715 | 0) | unlock_page() { | |
1716 | 0) 0.533 us | page_waitqueue(); | |
1717 | 0) 0.638 us | __wake_up_bit(); | |
1718 | 0) 2.793 us | } | |
1719 | 0) + 14.012 us | } | |
1720 | ||
5752674e | 1721 | You can also expand several functions at once: |
985ec20a | 1722 | |
5752674e IM |
1723 | echo sys_open > set_graph_function |
1724 | echo sys_close >> set_graph_function | |
985ec20a | 1725 | |
5752674e IM |
1726 | Now if you want to go back to trace all functions you can clear |
1727 | this special filter via: | |
985ec20a | 1728 | |
5752674e | 1729 | echo > set_graph_function |
985ec20a FW |
1730 | |
1731 | ||
07271aa4 CD |
1732 | Filter commands |
1733 | --------------- | |
1734 | ||
1735 | A few commands are supported by the set_ftrace_filter interface. | |
1736 | Trace commands have the following format: | |
1737 | ||
1738 | <function>:<command>:<parameter> | |
1739 | ||
1740 | The following commands are supported: | |
1741 | ||
1742 | - mod | |
1743 | This command enables function filtering per module. The | |
1744 | parameter defines the module. For example, if only the write* | |
1745 | functions in the ext3 module are desired, run: | |
1746 | ||
1747 | echo 'write*:mod:ext3' > set_ftrace_filter | |
1748 | ||
1749 | This command interacts with the filter in the same way as | |
1750 | filtering based on function names. Thus, adding more functions | |
1751 | in a different module is accomplished by appending (>>) to the | |
1752 | filter file. Remove specific module functions by prepending | |
1753 | '!': | |
1754 | ||
1755 | echo '!writeback*:mod:ext3' >> set_ftrace_filter | |
1756 | ||
1757 | - traceon/traceoff | |
1758 | These commands turn tracing on and off when the specified | |
1759 | functions are hit. The parameter determines how many times the | |
1760 | tracing system is turned on and off. If unspecified, there is | |
1761 | no limit. For example, to disable tracing when a schedule bug | |
1762 | is hit the first 5 times, run: | |
1763 | ||
1764 | echo '__schedule_bug:traceoff:5' > set_ftrace_filter | |
1765 | ||
1766 | These commands are cumulative whether or not they are appended | |
1767 | to set_ftrace_filter. To remove a command, prepend it by '!' | |
1768 | and drop the parameter: | |
1769 | ||
1770 | echo '!__schedule_bug:traceoff' > set_ftrace_filter | |
1771 | ||
1772 | ||
eb6d42ea SR |
1773 | trace_pipe |
1774 | ---------- | |
1775 | ||
5752674e IM |
1776 | The trace_pipe outputs the same content as the trace file, but |
1777 | the effect on the tracing is different. Every read from | |
1778 | trace_pipe is consumed. This means that subsequent reads will be | |
1779 | different. The trace is live. | |
eb6d42ea | 1780 | |
156f5a78 GL |
1781 | # echo function > current_tracer |
1782 | # cat trace_pipe > /tmp/trace.out & | |
eb6d42ea | 1783 | [1] 4153 |
6752ab4a | 1784 | # echo 1 > tracing_on |
eb6d42ea | 1785 | # usleep 1 |
6752ab4a | 1786 | # echo 0 > tracing_on |
156f5a78 | 1787 | # cat trace |
9b803c0f | 1788 | # tracer: function |
eb6d42ea SR |
1789 | # |
1790 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1791 | # | | | | | | |
1792 | ||
1793 | # | |
1794 | # cat /tmp/trace.out | |
1795 | bash-4043 [00] 41.267106: finish_task_switch <-schedule | |
1796 | bash-4043 [00] 41.267106: hrtick_set <-schedule | |
1797 | bash-4043 [00] 41.267107: hrtick_clear <-hrtick_set | |
1798 | bash-4043 [00] 41.267108: wait_for_completion <-__stop_machine_run | |
1799 | bash-4043 [00] 41.267108: wait_for_common <-wait_for_completion | |
1800 | bash-4043 [00] 41.267109: kthread_stop <-stop_machine_run | |
1801 | bash-4043 [00] 41.267109: init_waitqueue_head <-kthread_stop | |
1802 | bash-4043 [00] 41.267110: wake_up_process <-kthread_stop | |
1803 | bash-4043 [00] 41.267110: try_to_wake_up <-wake_up_process | |
1804 | bash-4043 [00] 41.267111: select_task_rq_rt <-try_to_wake_up | |
1805 | ||
1806 | ||
5752674e IM |
1807 | Note, reading the trace_pipe file will block until more input is |
1808 | added. By changing the tracer, trace_pipe will issue an EOF. We | |
1809 | needed to set the function tracer _before_ we "cat" the | |
1810 | trace_pipe file. | |
eb6d42ea SR |
1811 | |
1812 | ||
1813 | trace entries | |
1814 | ------------- | |
1815 | ||
5752674e IM |
1816 | Having too much or not enough data can be troublesome in |
1817 | diagnosing an issue in the kernel. The file buffer_size_kb is | |
1818 | used to modify the size of the internal trace buffers. The | |
1819 | number listed is the number of entries that can be recorded per | |
1820 | CPU. To know the full size, multiply the number of possible CPUS | |
1821 | with the number of entries. | |
eb6d42ea | 1822 | |
156f5a78 | 1823 | # cat buffer_size_kb |
1696b2b0 | 1824 | 1408 (units kilobytes) |
eb6d42ea | 1825 | |
5752674e IM |
1826 | Note, to modify this, you must have tracing completely disabled. |
1827 | To do that, echo "nop" into the current_tracer. If the | |
1828 | current_tracer is not set to "nop", an EINVAL error will be | |
1829 | returned. | |
eb6d42ea | 1830 | |
156f5a78 GL |
1831 | # echo nop > current_tracer |
1832 | # echo 10000 > buffer_size_kb | |
1833 | # cat buffer_size_kb | |
1696b2b0 | 1834 | 10000 (units kilobytes) |
eb6d42ea | 1835 | |
5752674e IM |
1836 | The number of pages which will be allocated is limited to a |
1837 | percentage of available memory. Allocating too much will produce | |
1838 | an error. | |
eb6d42ea | 1839 | |
156f5a78 | 1840 | # echo 1000000000000 > buffer_size_kb |
eb6d42ea | 1841 | -bash: echo: write error: Cannot allocate memory |
156f5a78 | 1842 | # cat buffer_size_kb |
eb6d42ea SR |
1843 | 85 |
1844 | ||
c1043fcd HT |
1845 | Snapshot |
1846 | -------- | |
1847 | CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature | |
1848 | available to all non latency tracers. (Latency tracers which | |
1849 | record max latency, such as "irqsoff" or "wakeup", can't use | |
1850 | this feature, since those are already using the snapshot | |
1851 | mechanism internally.) | |
1852 | ||
1853 | Snapshot preserves a current trace buffer at a particular point | |
1854 | in time without stopping tracing. Ftrace swaps the current | |
1855 | buffer with a spare buffer, and tracing continues in the new | |
1856 | current (=previous spare) buffer. | |
1857 | ||
1858 | The following debugfs files in "tracing" are related to this | |
1859 | feature: | |
1860 | ||
1861 | snapshot: | |
1862 | ||
1863 | This is used to take a snapshot and to read the output | |
1864 | of the snapshot. Echo 1 into this file to allocate a | |
1865 | spare buffer and to take a snapshot (swap), then read | |
1866 | the snapshot from this file in the same format as | |
1867 | "trace" (described above in the section "The File | |
1868 | System"). Both reads snapshot and tracing are executable | |
1869 | in parallel. When the spare buffer is allocated, echoing | |
1870 | 0 frees it, and echoing else (positive) values clear the | |
1871 | snapshot contents. | |
1872 | More details are shown in the table below. | |
1873 | ||
1874 | status\input | 0 | 1 | else | | |
1875 | --------------+------------+------------+------------+ | |
1876 | not allocated |(do nothing)| alloc+swap | EINVAL | | |
1877 | --------------+------------+------------+------------+ | |
1878 | allocated | free | swap | clear | | |
1879 | --------------+------------+------------+------------+ | |
1880 | ||
1881 | Here is an example of using the snapshot feature. | |
1882 | ||
1883 | # echo 1 > events/sched/enable | |
1884 | # echo 1 > snapshot | |
1885 | # cat snapshot | |
1886 | # tracer: nop | |
1887 | # | |
1888 | # entries-in-buffer/entries-written: 71/71 #P:8 | |
1889 | # | |
1890 | # _-----=> irqs-off | |
1891 | # / _----=> need-resched | |
1892 | # | / _---=> hardirq/softirq | |
1893 | # || / _--=> preempt-depth | |
1894 | # ||| / delay | |
1895 | # TASK-PID CPU# |||| TIMESTAMP FUNCTION | |
1896 | # | | | |||| | | | |
1897 | <idle>-0 [005] d... 2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120 | |
1898 | sleep-2242 [005] d... 2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120 prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120 | |
1899 | [...] | |
1900 | <idle>-0 [002] d... 2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120 | |
1901 | ||
1902 | # cat trace | |
1903 | # tracer: nop | |
1904 | # | |
1905 | # entries-in-buffer/entries-written: 77/77 #P:8 | |
1906 | # | |
1907 | # _-----=> irqs-off | |
1908 | # / _----=> need-resched | |
1909 | # | / _---=> hardirq/softirq | |
1910 | # || / _--=> preempt-depth | |
1911 | # ||| / delay | |
1912 | # TASK-PID CPU# |||| TIMESTAMP FUNCTION | |
1913 | # | | | |||| | | | |
1914 | <idle>-0 [007] d... 2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120 | |
1915 | snapshot-test-2-2229 [002] d... 2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120 | |
1916 | [...] | |
1917 | ||
1918 | ||
1919 | If you try to use this snapshot feature when current tracer is | |
1920 | one of the latency tracers, you will get the following results. | |
1921 | ||
1922 | # echo wakeup > current_tracer | |
1923 | # echo 1 > snapshot | |
1924 | bash: echo: write error: Device or resource busy | |
1925 | # cat snapshot | |
1926 | cat: snapshot: Device or resource busy | |
1927 | ||
5752674e IM |
1928 | ----------- |
1929 | ||
1930 | More details can be found in the source code, in the | |
baf20b3e | 1931 | kernel/trace/*.c files. |