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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. | |
eb6d42ea | 10 | |
42ec632e | 11 | Written for: 2.6.28-rc2 |
eb6d42ea SR |
12 | |
13 | Introduction | |
14 | ------------ | |
15 | ||
16 | Ftrace is an internal tracer designed to help out developers and | |
17 | designers of systems to find what is going on inside the kernel. | |
5752674e IM |
18 | It can be used for debugging or analyzing latencies and |
19 | performance issues that take place outside of user-space. | |
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20 | |
21 | Although ftrace is the function tracer, it also includes an | |
5752674e IM |
22 | infrastructure that allows for other types of tracing. Some of |
23 | the tracers that are currently in ftrace include a tracer to | |
24 | trace context switches, the time it takes for a high priority | |
25 | task to run after it was woken up, the time interrupts are | |
26 | disabled, and more (ftrace allows for tracer plugins, which | |
27 | means that the list of tracers can always grow). | |
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28 | |
29 | ||
30 | The File System | |
31 | --------------- | |
32 | ||
5752674e IM |
33 | Ftrace uses the debugfs file system to hold the control files as |
34 | well as the files to display output. | |
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35 | |
36 | To mount the debugfs system: | |
37 | ||
38 | # mkdir /debug | |
39 | # mount -t debugfs nodev /debug | |
40 | ||
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41 | ( Note: it is more common to mount at /sys/kernel/debug, but for |
42 | simplicity this document will use /debug) | |
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43 | |
44 | That's it! (assuming that you have ftrace configured into your kernel) | |
45 | ||
46 | After mounting the debugfs, you can see a directory called | |
47 | "tracing". This directory contains the control and output files | |
48 | of ftrace. Here is a list of some of the key files: | |
49 | ||
50 | ||
51 | Note: all time values are in microseconds. | |
52 | ||
5752674e IM |
53 | current_tracer: |
54 | ||
55 | This is used to set or display the current tracer | |
56 | that is configured. | |
57 | ||
58 | available_tracers: | |
59 | ||
60 | This holds the different types of tracers that | |
61 | have been compiled into the kernel. The | |
62 | tracers listed here can be configured by | |
63 | echoing their name into current_tracer. | |
64 | ||
65 | tracing_enabled: | |
66 | ||
67 | This sets or displays whether the current_tracer | |
68 | is activated and tracing or not. Echo 0 into this | |
69 | file to disable the tracer or 1 to enable it. | |
70 | ||
71 | trace: | |
72 | ||
73 | This file holds the output of the trace in a human | |
74 | readable format (described below). | |
75 | ||
76 | latency_trace: | |
77 | ||
78 | This file shows the same trace but the information | |
79 | is organized more to display possible latencies | |
80 | in the system (described below). | |
81 | ||
82 | trace_pipe: | |
83 | ||
84 | The output is the same as the "trace" file but this | |
85 | file is meant to be streamed with live tracing. | |
86 | Reads from this file will block until new data | |
87 | is retrieved. Unlike the "trace" and "latency_trace" | |
88 | files, this file is a consumer. This means reading | |
89 | from this file causes sequential reads to display | |
90 | more current data. Once data is read from this | |
91 | file, it is consumed, and will not be read | |
92 | again with a sequential read. The "trace" and | |
93 | "latency_trace" files are static, and if the | |
94 | tracer is not adding more data, they will display | |
95 | the same information every time they are read. | |
96 | ||
97 | trace_options: | |
98 | ||
99 | This file lets the user control the amount of data | |
100 | that is displayed in one of the above output | |
101 | files. | |
102 | ||
42b40b3d | 103 | tracing_max_latency: |
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104 | |
105 | Some of the tracers record the max latency. | |
106 | For example, the time interrupts are disabled. | |
107 | This time is saved in this file. The max trace | |
108 | will also be stored, and displayed by either | |
109 | "trace" or "latency_trace". A new max trace will | |
110 | only be recorded if the latency is greater than | |
111 | the value in this file. (in microseconds) | |
112 | ||
113 | buffer_size_kb: | |
114 | ||
115 | This sets or displays the number of kilobytes each CPU | |
116 | buffer can hold. The tracer buffers are the same size | |
117 | for each CPU. The displayed number is the size of the | |
118 | CPU buffer and not total size of all buffers. The | |
119 | trace buffers are allocated in pages (blocks of memory | |
120 | that the kernel uses for allocation, usually 4 KB in size). | |
121 | If the last page allocated has room for more bytes | |
122 | than requested, the rest of the page will be used, | |
123 | making the actual allocation bigger than requested. | |
124 | ( Note, the size may not be a multiple of the page size | |
125 | due to buffer managment overhead. ) | |
126 | ||
127 | This can only be updated when the current_tracer | |
128 | is set to "nop". | |
129 | ||
130 | tracing_cpumask: | |
131 | ||
132 | This is a mask that lets the user only trace | |
133 | on specified CPUS. The format is a hex string | |
134 | representing the CPUS. | |
135 | ||
136 | set_ftrace_filter: | |
137 | ||
138 | When dynamic ftrace is configured in (see the | |
139 | section below "dynamic ftrace"), the code is dynamically | |
140 | modified (code text rewrite) to disable calling of the | |
141 | function profiler (mcount). This lets tracing be configured | |
142 | in with practically no overhead in performance. This also | |
143 | has a side effect of enabling or disabling specific functions | |
144 | to be traced. Echoing names of functions into this file | |
145 | will limit the trace to only those functions. | |
146 | ||
147 | set_ftrace_notrace: | |
148 | ||
149 | This has an effect opposite to that of | |
150 | set_ftrace_filter. Any function that is added here will not | |
151 | be traced. If a function exists in both set_ftrace_filter | |
152 | and set_ftrace_notrace, the function will _not_ be traced. | |
153 | ||
154 | set_ftrace_pid: | |
155 | ||
156 | Have the function tracer only trace a single thread. | |
157 | ||
158 | set_graph_function: | |
159 | ||
160 | Set a "trigger" function where tracing should start | |
161 | with the function graph tracer (See the section | |
162 | "dynamic ftrace" for more details). | |
163 | ||
164 | available_filter_functions: | |
165 | ||
166 | This lists the functions that ftrace | |
167 | has processed and can trace. These are the function | |
168 | names that you can pass to "set_ftrace_filter" or | |
169 | "set_ftrace_notrace". (See the section "dynamic ftrace" | |
170 | below for more details.) | |
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171 | |
172 | ||
173 | The Tracers | |
174 | ----------- | |
175 | ||
f2d9c740 | 176 | Here is the list of current tracers that may be configured. |
eb6d42ea | 177 | |
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178 | "function" |
179 | ||
180 | Function call tracer to trace all kernel functions. | |
181 | ||
182 | "function_graph_tracer" | |
183 | ||
184 | Similar to the function tracer except that the | |
185 | function tracer probes the functions on their entry | |
186 | whereas the function graph tracer traces on both entry | |
187 | and exit of the functions. It then provides the ability | |
188 | to draw a graph of function calls similar to C code | |
189 | source. | |
190 | ||
191 | "sched_switch" | |
192 | ||
193 | Traces the context switches and wakeups between tasks. | |
194 | ||
195 | "irqsoff" | |
196 | ||
197 | Traces the areas that disable interrupts and saves | |
198 | the trace with the longest max latency. | |
199 | See tracing_max_latency. When a new max is recorded, | |
200 | it replaces the old trace. It is best to view this | |
201 | trace via the latency_trace file. | |
eb6d42ea | 202 | |
5752674e | 203 | "preemptoff" |
985ec20a | 204 | |
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205 | Similar to irqsoff but traces and records the amount of |
206 | time for which preemption is disabled. | |
eb6d42ea | 207 | |
5752674e | 208 | "preemptirqsoff" |
eb6d42ea | 209 | |
5752674e IM |
210 | Similar to irqsoff and preemptoff, but traces and |
211 | records the largest time for which irqs and/or preemption | |
212 | is disabled. | |
eb6d42ea | 213 | |
5752674e | 214 | "wakeup" |
eb6d42ea | 215 | |
5752674e IM |
216 | Traces and records the max latency that it takes for |
217 | the highest priority task to get scheduled after | |
218 | it has been woken up. | |
eb6d42ea | 219 | |
5752674e | 220 | "hw-branch-tracer" |
eb6d42ea | 221 | |
5752674e IM |
222 | Uses the BTS CPU feature on x86 CPUs to traces all |
223 | branches executed. | |
224 | ||
225 | "nop" | |
226 | ||
227 | This is the "trace nothing" tracer. To remove all | |
228 | tracers from tracing simply echo "nop" into | |
229 | current_tracer. | |
e2ea5399 | 230 | |
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231 | |
232 | Examples of using the tracer | |
233 | ---------------------------- | |
234 | ||
5752674e IM |
235 | Here are typical examples of using the tracers when controlling |
236 | them only with the debugfs interface (without using any | |
237 | user-land utilities). | |
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238 | |
239 | Output format: | |
240 | -------------- | |
241 | ||
f2d9c740 | 242 | Here is an example of the output format of the file "trace" |
eb6d42ea SR |
243 | |
244 | -------- | |
9b803c0f | 245 | # tracer: function |
eb6d42ea SR |
246 | # |
247 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
248 | # | | | | | | |
249 | bash-4251 [01] 10152.583854: path_put <-path_walk | |
250 | bash-4251 [01] 10152.583855: dput <-path_put | |
251 | bash-4251 [01] 10152.583855: _atomic_dec_and_lock <-dput | |
252 | -------- | |
253 | ||
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254 | A header is printed with the tracer name that is represented by |
255 | the trace. In this case the tracer is "function". Then a header | |
256 | showing the format. Task name "bash", the task PID "4251", the | |
257 | CPU that it was running on "01", the timestamp in <secs>.<usecs> | |
258 | format, the function name that was traced "path_put" and the | |
259 | parent function that called this function "path_walk". The | |
260 | timestamp is the time at which the function was entered. | |
eb6d42ea | 261 | |
5752674e IM |
262 | The sched_switch tracer also includes tracing of task wakeups |
263 | and context switches. | |
eb6d42ea SR |
264 | |
265 | ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 2916:115:S | |
266 | ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 10:115:S | |
267 | ksoftirqd/1-7 [01] 1453.070013: 7:115:R ==> 10:115:R | |
268 | events/1-10 [01] 1453.070013: 10:115:S ==> 2916:115:R | |
269 | kondemand/1-2916 [01] 1453.070013: 2916:115:S ==> 7:115:R | |
270 | ksoftirqd/1-7 [01] 1453.070013: 7:115:S ==> 0:140:R | |
271 | ||
5752674e IM |
272 | Wake ups are represented by a "+" and the context switches are |
273 | shown as "==>". The format is: | |
eb6d42ea SR |
274 | |
275 | Context switches: | |
276 | ||
277 | Previous task Next Task | |
278 | ||
279 | <pid>:<prio>:<state> ==> <pid>:<prio>:<state> | |
280 | ||
281 | Wake ups: | |
282 | ||
283 | Current task Task waking up | |
284 | ||
285 | <pid>:<prio>:<state> + <pid>:<prio>:<state> | |
286 | ||
5752674e IM |
287 | The prio is the internal kernel priority, which is the inverse |
288 | of the priority that is usually displayed by user-space tools. | |
289 | Zero represents the highest priority (99). Prio 100 starts the | |
290 | "nice" priorities with 100 being equal to nice -20 and 139 being | |
291 | nice 19. The prio "140" is reserved for the idle task which is | |
292 | the lowest priority thread (pid 0). | |
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293 | |
294 | ||
295 | Latency trace format | |
296 | -------------------- | |
297 | ||
5752674e IM |
298 | For traces that display latency times, the latency_trace file |
299 | gives somewhat more information to see why a latency happened. | |
300 | Here is a typical trace. | |
eb6d42ea SR |
301 | |
302 | # tracer: irqsoff | |
303 | # | |
304 | irqsoff latency trace v1.1.5 on 2.6.26-rc8 | |
305 | -------------------------------------------------------------------- | |
306 | latency: 97 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
307 | ----------------- | |
308 | | task: swapper-0 (uid:0 nice:0 policy:0 rt_prio:0) | |
309 | ----------------- | |
310 | => started at: apic_timer_interrupt | |
311 | => ended at: do_softirq | |
312 | ||
313 | # _------=> CPU# | |
314 | # / _-----=> irqs-off | |
315 | # | / _----=> need-resched | |
316 | # || / _---=> hardirq/softirq | |
317 | # ||| / _--=> preempt-depth | |
318 | # |||| / | |
319 | # ||||| delay | |
320 | # cmd pid ||||| time | caller | |
321 | # \ / ||||| \ | / | |
322 | <idle>-0 0d..1 0us+: trace_hardirqs_off_thunk (apic_timer_interrupt) | |
323 | <idle>-0 0d.s. 97us : __do_softirq (do_softirq) | |
324 | <idle>-0 0d.s1 98us : trace_hardirqs_on (do_softirq) | |
325 | ||
326 | ||
5752674e IM |
327 | This shows that the current tracer is "irqsoff" tracing the time |
328 | for which interrupts were disabled. It gives the trace version | |
329 | and the version of the kernel upon which this was executed on | |
330 | (2.6.26-rc8). Then it displays the max latency in microsecs (97 | |
331 | us). The number of trace entries displayed and the total number | |
332 | recorded (both are three: #3/3). The type of preemption that was | |
333 | used (PREEMPT). VP, KP, SP, and HP are always zero and are | |
334 | reserved for later use. #P is the number of online CPUS (#P:2). | |
eb6d42ea | 335 | |
5752674e IM |
336 | The task is the process that was running when the latency |
337 | occurred. (swapper pid: 0). | |
eb6d42ea | 338 | |
5752674e IM |
339 | The start and stop (the functions in which the interrupts were |
340 | disabled and enabled respectively) that caused the latencies: | |
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341 | |
342 | apic_timer_interrupt is where the interrupts were disabled. | |
343 | do_softirq is where they were enabled again. | |
344 | ||
345 | The next lines after the header are the trace itself. The header | |
346 | explains which is which. | |
347 | ||
348 | cmd: The name of the process in the trace. | |
349 | ||
350 | pid: The PID of that process. | |
351 | ||
f2d9c740 | 352 | CPU#: The CPU which the process was running on. |
eb6d42ea SR |
353 | |
354 | irqs-off: 'd' interrupts are disabled. '.' otherwise. | |
9244489a SR |
355 | Note: If the architecture does not support a way to |
356 | read the irq flags variable, an 'X' will always | |
357 | be printed here. | |
eb6d42ea SR |
358 | |
359 | need-resched: 'N' task need_resched is set, '.' otherwise. | |
360 | ||
361 | hardirq/softirq: | |
f2d9c740 | 362 | 'H' - hard irq occurred inside a softirq. |
eb6d42ea SR |
363 | 'h' - hard irq is running |
364 | 's' - soft irq is running | |
365 | '.' - normal context. | |
366 | ||
367 | preempt-depth: The level of preempt_disabled | |
368 | ||
369 | The above is mostly meaningful for kernel developers. | |
370 | ||
a41eebab | 371 | time: This differs from the trace file output. The trace file output |
f2d9c740 | 372 | includes an absolute timestamp. The timestamp used by the |
a41eebab | 373 | latency_trace file is relative to the start of the trace. |
eb6d42ea SR |
374 | |
375 | delay: This is just to help catch your eye a bit better. And | |
5752674e IM |
376 | needs to be fixed to be only relative to the same CPU. |
377 | The marks are determined by the difference between this | |
378 | current trace and the next trace. | |
379 | '!' - greater than preempt_mark_thresh (default 100) | |
380 | '+' - greater than 1 microsecond | |
381 | ' ' - less than or equal to 1 microsecond. | |
eb6d42ea SR |
382 | |
383 | The rest is the same as the 'trace' file. | |
384 | ||
385 | ||
ee6bce52 SR |
386 | trace_options |
387 | ------------- | |
eb6d42ea | 388 | |
5752674e IM |
389 | The trace_options file is used to control what gets printed in |
390 | the trace output. To see what is available, simply cat the file: | |
eb6d42ea | 391 | |
ee6bce52 | 392 | cat /debug/tracing/trace_options |
eb6d42ea | 393 | print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \ |
5752674e | 394 | noblock nostacktrace nosched-tree nouserstacktrace nosym-userobj |
eb6d42ea | 395 | |
5752674e IM |
396 | To disable one of the options, echo in the option prepended with |
397 | "no". | |
eb6d42ea | 398 | |
ee6bce52 | 399 | echo noprint-parent > /debug/tracing/trace_options |
eb6d42ea SR |
400 | |
401 | To enable an option, leave off the "no". | |
402 | ||
ee6bce52 | 403 | echo sym-offset > /debug/tracing/trace_options |
eb6d42ea SR |
404 | |
405 | Here are the available options: | |
406 | ||
5752674e IM |
407 | print-parent - On function traces, display the calling (parent) |
408 | function as well as the function being traced. | |
eb6d42ea SR |
409 | |
410 | print-parent: | |
411 | bash-4000 [01] 1477.606694: simple_strtoul <-strict_strtoul | |
412 | ||
413 | noprint-parent: | |
414 | bash-4000 [01] 1477.606694: simple_strtoul | |
415 | ||
416 | ||
5752674e IM |
417 | sym-offset - Display not only the function name, but also the |
418 | offset in the function. For example, instead of | |
419 | seeing just "ktime_get", you will see | |
420 | "ktime_get+0xb/0x20". | |
eb6d42ea SR |
421 | |
422 | sym-offset: | |
423 | bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0 | |
424 | ||
5752674e IM |
425 | sym-addr - this will also display the function address as well |
426 | as the function name. | |
eb6d42ea SR |
427 | |
428 | sym-addr: | |
429 | bash-4000 [01] 1477.606694: simple_strtoul <c0339346> | |
430 | ||
431 | verbose - This deals with the latency_trace file. | |
432 | ||
433 | bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \ | |
434 | (+0.000ms): simple_strtoul (strict_strtoul) | |
435 | ||
5752674e IM |
436 | raw - This will display raw numbers. This option is best for |
437 | use with user applications that can translate the raw | |
438 | numbers better than having it done in the kernel. | |
eb6d42ea | 439 | |
5752674e IM |
440 | hex - Similar to raw, but the numbers will be in a hexadecimal |
441 | format. | |
eb6d42ea SR |
442 | |
443 | bin - This will print out the formats in raw binary. | |
444 | ||
445 | block - TBD (needs update) | |
446 | ||
5752674e IM |
447 | stacktrace - This is one of the options that changes the trace |
448 | itself. When a trace is recorded, so is the stack | |
449 | of functions. This allows for back traces of | |
450 | trace sites. | |
eb6d42ea | 451 | |
5752674e IM |
452 | userstacktrace - This option changes the trace. It records a |
453 | stacktrace of the current userspace thread. | |
02b67518 | 454 | |
5752674e IM |
455 | sym-userobj - when user stacktrace are enabled, look up which |
456 | object the address belongs to, and print a | |
457 | relative address. This is especially useful when | |
458 | ASLR is on, otherwise you don't get a chance to | |
459 | resolve the address to object/file/line after | |
460 | the app is no longer running | |
b54d3de9 | 461 | |
5752674e IM |
462 | The lookup is performed when you read |
463 | trace,trace_pipe,latency_trace. Example: | |
b54d3de9 TE |
464 | |
465 | a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0 | |
466 | x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6] | |
467 | ||
5752674e IM |
468 | sched-tree - trace all tasks that are on the runqueue, at |
469 | every scheduling event. Will add overhead if | |
470 | there's a lot of tasks running at once. | |
eb6d42ea SR |
471 | |
472 | ||
473 | sched_switch | |
474 | ------------ | |
475 | ||
f2d9c740 | 476 | This tracer simply records schedule switches. Here is an example |
a41eebab | 477 | of how to use it. |
eb6d42ea SR |
478 | |
479 | # echo sched_switch > /debug/tracing/current_tracer | |
480 | # echo 1 > /debug/tracing/tracing_enabled | |
481 | # sleep 1 | |
482 | # echo 0 > /debug/tracing/tracing_enabled | |
483 | # cat /debug/tracing/trace | |
484 | ||
485 | # tracer: sched_switch | |
486 | # | |
487 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
488 | # | | | | | | |
489 | bash-3997 [01] 240.132281: 3997:120:R + 4055:120:R | |
490 | bash-3997 [01] 240.132284: 3997:120:R ==> 4055:120:R | |
491 | sleep-4055 [01] 240.132371: 4055:120:S ==> 3997:120:R | |
492 | bash-3997 [01] 240.132454: 3997:120:R + 4055:120:S | |
493 | bash-3997 [01] 240.132457: 3997:120:R ==> 4055:120:R | |
494 | sleep-4055 [01] 240.132460: 4055:120:D ==> 3997:120:R | |
495 | bash-3997 [01] 240.132463: 3997:120:R + 4055:120:D | |
496 | bash-3997 [01] 240.132465: 3997:120:R ==> 4055:120:R | |
497 | <idle>-0 [00] 240.132589: 0:140:R + 4:115:S | |
498 | <idle>-0 [00] 240.132591: 0:140:R ==> 4:115:R | |
499 | ksoftirqd/0-4 [00] 240.132595: 4:115:S ==> 0:140:R | |
500 | <idle>-0 [00] 240.132598: 0:140:R + 4:115:S | |
501 | <idle>-0 [00] 240.132599: 0:140:R ==> 4:115:R | |
502 | ksoftirqd/0-4 [00] 240.132603: 4:115:S ==> 0:140:R | |
503 | sleep-4055 [01] 240.133058: 4055:120:S ==> 3997:120:R | |
504 | [...] | |
505 | ||
506 | ||
5752674e IM |
507 | As we have discussed previously about this format, the header |
508 | shows the name of the trace and points to the options. The | |
509 | "FUNCTION" is a misnomer since here it represents the wake ups | |
510 | and context switches. | |
eb6d42ea | 511 | |
5752674e IM |
512 | The sched_switch file only lists the wake ups (represented with |
513 | '+') and context switches ('==>') with the previous task or | |
514 | current task first followed by the next task or task waking up. | |
515 | The format for both of these is PID:KERNEL-PRIO:TASK-STATE. | |
516 | Remember that the KERNEL-PRIO is the inverse of the actual | |
517 | priority with zero (0) being the highest priority and the nice | |
518 | values starting at 100 (nice -20). Below is a quick chart to map | |
519 | the kernel priority to user land priorities. | |
eb6d42ea SR |
520 | |
521 | Kernel priority: 0 to 99 ==> user RT priority 99 to 0 | |
522 | Kernel priority: 100 to 139 ==> user nice -20 to 19 | |
523 | Kernel priority: 140 ==> idle task priority | |
524 | ||
525 | The task states are: | |
526 | ||
527 | R - running : wants to run, may not actually be running | |
528 | S - sleep : process is waiting to be woken up (handles signals) | |
f2d9c740 SR |
529 | D - disk sleep (uninterruptible sleep) : process must be woken up |
530 | (ignores signals) | |
eb6d42ea SR |
531 | T - stopped : process suspended |
532 | t - traced : process is being traced (with something like gdb) | |
533 | Z - zombie : process waiting to be cleaned up | |
534 | X - unknown | |
535 | ||
536 | ||
537 | ftrace_enabled | |
538 | -------------- | |
539 | ||
5752674e IM |
540 | The following tracers (listed below) give different output |
541 | depending on whether or not the sysctl ftrace_enabled is set. To | |
542 | set ftrace_enabled, one can either use the sysctl function or | |
543 | set it via the proc file system interface. | |
eb6d42ea SR |
544 | |
545 | sysctl kernel.ftrace_enabled=1 | |
546 | ||
547 | or | |
548 | ||
549 | echo 1 > /proc/sys/kernel/ftrace_enabled | |
550 | ||
5752674e IM |
551 | To disable ftrace_enabled simply replace the '1' with '0' in the |
552 | above commands. | |
eb6d42ea | 553 | |
5752674e IM |
554 | When ftrace_enabled is set the tracers will also record the |
555 | functions that are within the trace. The descriptions of the | |
556 | tracers will also show an example with ftrace enabled. | |
eb6d42ea SR |
557 | |
558 | ||
559 | irqsoff | |
560 | ------- | |
561 | ||
562 | When interrupts are disabled, the CPU can not react to any other | |
563 | external event (besides NMIs and SMIs). This prevents the timer | |
5752674e IM |
564 | interrupt from triggering or the mouse interrupt from letting |
565 | the kernel know of a new mouse event. The result is a latency | |
566 | with the reaction time. | |
eb6d42ea | 567 | |
5752674e IM |
568 | The irqsoff tracer tracks the time for which interrupts are |
569 | disabled. When a new maximum latency is hit, the tracer saves | |
570 | the trace leading up to that latency point so that every time a | |
571 | new maximum is reached, the old saved trace is discarded and the | |
572 | new trace is saved. | |
eb6d42ea | 573 | |
5752674e IM |
574 | To reset the maximum, echo 0 into tracing_max_latency. Here is |
575 | an example: | |
eb6d42ea SR |
576 | |
577 | # echo irqsoff > /debug/tracing/current_tracer | |
578 | # echo 0 > /debug/tracing/tracing_max_latency | |
579 | # echo 1 > /debug/tracing/tracing_enabled | |
580 | # ls -ltr | |
581 | [...] | |
582 | # echo 0 > /debug/tracing/tracing_enabled | |
583 | # cat /debug/tracing/latency_trace | |
584 | # tracer: irqsoff | |
585 | # | |
f2d9c740 | 586 | irqsoff latency trace v1.1.5 on 2.6.26 |
eb6d42ea | 587 | -------------------------------------------------------------------- |
f2d9c740 | 588 | latency: 12 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) |
eb6d42ea | 589 | ----------------- |
f2d9c740 | 590 | | task: bash-3730 (uid:0 nice:0 policy:0 rt_prio:0) |
eb6d42ea | 591 | ----------------- |
f2d9c740 SR |
592 | => started at: sys_setpgid |
593 | => ended at: sys_setpgid | |
eb6d42ea SR |
594 | |
595 | # _------=> CPU# | |
596 | # / _-----=> irqs-off | |
597 | # | / _----=> need-resched | |
598 | # || / _---=> hardirq/softirq | |
599 | # ||| / _--=> preempt-depth | |
600 | # |||| / | |
601 | # ||||| delay | |
602 | # cmd pid ||||| time | caller | |
603 | # \ / ||||| \ | / | |
f2d9c740 SR |
604 | bash-3730 1d... 0us : _write_lock_irq (sys_setpgid) |
605 | bash-3730 1d..1 1us+: _write_unlock_irq (sys_setpgid) | |
606 | bash-3730 1d..2 14us : trace_hardirqs_on (sys_setpgid) | |
eb6d42ea | 607 | |
eb6d42ea | 608 | |
f2d9c740 | 609 | Here we see that that we had a latency of 12 microsecs (which is |
5752674e IM |
610 | very good). The _write_lock_irq in sys_setpgid disabled |
611 | interrupts. The difference between the 12 and the displayed | |
612 | timestamp 14us occurred because the clock was incremented | |
613 | between the time of recording the max latency and the time of | |
614 | recording the function that had that latency. | |
eb6d42ea | 615 | |
f2d9c740 SR |
616 | Note the above example had ftrace_enabled not set. If we set the |
617 | ftrace_enabled, we get a much larger output: | |
eb6d42ea SR |
618 | |
619 | # tracer: irqsoff | |
620 | # | |
621 | irqsoff latency trace v1.1.5 on 2.6.26-rc8 | |
622 | -------------------------------------------------------------------- | |
623 | latency: 50 us, #101/101, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
624 | ----------------- | |
625 | | task: ls-4339 (uid:0 nice:0 policy:0 rt_prio:0) | |
626 | ----------------- | |
627 | => started at: __alloc_pages_internal | |
628 | => ended at: __alloc_pages_internal | |
629 | ||
630 | # _------=> CPU# | |
631 | # / _-----=> irqs-off | |
632 | # | / _----=> need-resched | |
633 | # || / _---=> hardirq/softirq | |
634 | # ||| / _--=> preempt-depth | |
635 | # |||| / | |
636 | # ||||| delay | |
637 | # cmd pid ||||| time | caller | |
638 | # \ / ||||| \ | / | |
639 | ls-4339 0...1 0us+: get_page_from_freelist (__alloc_pages_internal) | |
640 | ls-4339 0d..1 3us : rmqueue_bulk (get_page_from_freelist) | |
641 | ls-4339 0d..1 3us : _spin_lock (rmqueue_bulk) | |
642 | ls-4339 0d..1 4us : add_preempt_count (_spin_lock) | |
643 | ls-4339 0d..2 4us : __rmqueue (rmqueue_bulk) | |
644 | ls-4339 0d..2 5us : __rmqueue_smallest (__rmqueue) | |
645 | ls-4339 0d..2 5us : __mod_zone_page_state (__rmqueue_smallest) | |
646 | ls-4339 0d..2 6us : __rmqueue (rmqueue_bulk) | |
647 | ls-4339 0d..2 6us : __rmqueue_smallest (__rmqueue) | |
648 | ls-4339 0d..2 7us : __mod_zone_page_state (__rmqueue_smallest) | |
649 | ls-4339 0d..2 7us : __rmqueue (rmqueue_bulk) | |
650 | ls-4339 0d..2 8us : __rmqueue_smallest (__rmqueue) | |
651 | [...] | |
652 | ls-4339 0d..2 46us : __rmqueue_smallest (__rmqueue) | |
653 | ls-4339 0d..2 47us : __mod_zone_page_state (__rmqueue_smallest) | |
654 | ls-4339 0d..2 47us : __rmqueue (rmqueue_bulk) | |
655 | ls-4339 0d..2 48us : __rmqueue_smallest (__rmqueue) | |
656 | ls-4339 0d..2 48us : __mod_zone_page_state (__rmqueue_smallest) | |
657 | ls-4339 0d..2 49us : _spin_unlock (rmqueue_bulk) | |
658 | ls-4339 0d..2 49us : sub_preempt_count (_spin_unlock) | |
659 | ls-4339 0d..1 50us : get_page_from_freelist (__alloc_pages_internal) | |
660 | ls-4339 0d..2 51us : trace_hardirqs_on (__alloc_pages_internal) | |
661 | ||
662 | ||
eb6d42ea SR |
663 | |
664 | Here we traced a 50 microsecond latency. But we also see all the | |
5752674e IM |
665 | functions that were called during that time. Note that by |
666 | enabling function tracing, we incur an added overhead. This | |
667 | overhead may extend the latency times. But nevertheless, this | |
668 | trace has provided some very helpful debugging information. | |
eb6d42ea SR |
669 | |
670 | ||
671 | preemptoff | |
672 | ---------- | |
673 | ||
5752674e IM |
674 | When preemption is disabled, we may be able to receive |
675 | interrupts but the task cannot be preempted and a higher | |
676 | priority task must wait for preemption to be enabled again | |
677 | before it can preempt a lower priority task. | |
eb6d42ea | 678 | |
a41eebab | 679 | The preemptoff tracer traces the places that disable preemption. |
5752674e IM |
680 | Like the irqsoff tracer, it records the maximum latency for |
681 | which preemption was disabled. The control of preemptoff tracer | |
682 | is much like the irqsoff tracer. | |
eb6d42ea SR |
683 | |
684 | # echo preemptoff > /debug/tracing/current_tracer | |
685 | # echo 0 > /debug/tracing/tracing_max_latency | |
686 | # echo 1 > /debug/tracing/tracing_enabled | |
687 | # ls -ltr | |
688 | [...] | |
689 | # echo 0 > /debug/tracing/tracing_enabled | |
690 | # cat /debug/tracing/latency_trace | |
691 | # tracer: preemptoff | |
692 | # | |
693 | preemptoff latency trace v1.1.5 on 2.6.26-rc8 | |
694 | -------------------------------------------------------------------- | |
695 | latency: 29 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
696 | ----------------- | |
697 | | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) | |
698 | ----------------- | |
699 | => started at: do_IRQ | |
700 | => ended at: __do_softirq | |
701 | ||
702 | # _------=> CPU# | |
703 | # / _-----=> irqs-off | |
704 | # | / _----=> need-resched | |
705 | # || / _---=> hardirq/softirq | |
706 | # ||| / _--=> preempt-depth | |
707 | # |||| / | |
708 | # ||||| delay | |
709 | # cmd pid ||||| time | caller | |
710 | # \ / ||||| \ | / | |
711 | sshd-4261 0d.h. 0us+: irq_enter (do_IRQ) | |
712 | sshd-4261 0d.s. 29us : _local_bh_enable (__do_softirq) | |
713 | sshd-4261 0d.s1 30us : trace_preempt_on (__do_softirq) | |
714 | ||
715 | ||
5752674e IM |
716 | This has some more changes. Preemption was disabled when an |
717 | interrupt came in (notice the 'h'), and was enabled while doing | |
718 | a softirq. (notice the 's'). But we also see that interrupts | |
719 | have been disabled when entering the preempt off section and | |
720 | leaving it (the 'd'). We do not know if interrupts were enabled | |
721 | in the mean time. | |
eb6d42ea SR |
722 | |
723 | # tracer: preemptoff | |
724 | # | |
725 | preemptoff latency trace v1.1.5 on 2.6.26-rc8 | |
726 | -------------------------------------------------------------------- | |
727 | latency: 63 us, #87/87, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
728 | ----------------- | |
729 | | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) | |
730 | ----------------- | |
731 | => started at: remove_wait_queue | |
732 | => ended at: __do_softirq | |
733 | ||
734 | # _------=> CPU# | |
735 | # / _-----=> irqs-off | |
736 | # | / _----=> need-resched | |
737 | # || / _---=> hardirq/softirq | |
738 | # ||| / _--=> preempt-depth | |
739 | # |||| / | |
740 | # ||||| delay | |
741 | # cmd pid ||||| time | caller | |
742 | # \ / ||||| \ | / | |
743 | sshd-4261 0d..1 0us : _spin_lock_irqsave (remove_wait_queue) | |
744 | sshd-4261 0d..1 1us : _spin_unlock_irqrestore (remove_wait_queue) | |
745 | sshd-4261 0d..1 2us : do_IRQ (common_interrupt) | |
746 | sshd-4261 0d..1 2us : irq_enter (do_IRQ) | |
747 | sshd-4261 0d..1 2us : idle_cpu (irq_enter) | |
748 | sshd-4261 0d..1 3us : add_preempt_count (irq_enter) | |
749 | sshd-4261 0d.h1 3us : idle_cpu (irq_enter) | |
750 | sshd-4261 0d.h. 4us : handle_fasteoi_irq (do_IRQ) | |
751 | [...] | |
752 | sshd-4261 0d.h. 12us : add_preempt_count (_spin_lock) | |
753 | sshd-4261 0d.h1 12us : ack_ioapic_quirk_irq (handle_fasteoi_irq) | |
754 | sshd-4261 0d.h1 13us : move_native_irq (ack_ioapic_quirk_irq) | |
755 | sshd-4261 0d.h1 13us : _spin_unlock (handle_fasteoi_irq) | |
756 | sshd-4261 0d.h1 14us : sub_preempt_count (_spin_unlock) | |
757 | sshd-4261 0d.h1 14us : irq_exit (do_IRQ) | |
758 | sshd-4261 0d.h1 15us : sub_preempt_count (irq_exit) | |
759 | sshd-4261 0d..2 15us : do_softirq (irq_exit) | |
760 | sshd-4261 0d... 15us : __do_softirq (do_softirq) | |
761 | sshd-4261 0d... 16us : __local_bh_disable (__do_softirq) | |
762 | sshd-4261 0d... 16us+: add_preempt_count (__local_bh_disable) | |
763 | sshd-4261 0d.s4 20us : add_preempt_count (__local_bh_disable) | |
764 | sshd-4261 0d.s4 21us : sub_preempt_count (local_bh_enable) | |
765 | sshd-4261 0d.s5 21us : sub_preempt_count (local_bh_enable) | |
766 | [...] | |
767 | sshd-4261 0d.s6 41us : add_preempt_count (__local_bh_disable) | |
768 | sshd-4261 0d.s6 42us : sub_preempt_count (local_bh_enable) | |
769 | sshd-4261 0d.s7 42us : sub_preempt_count (local_bh_enable) | |
770 | sshd-4261 0d.s5 43us : add_preempt_count (__local_bh_disable) | |
771 | sshd-4261 0d.s5 43us : sub_preempt_count (local_bh_enable_ip) | |
772 | sshd-4261 0d.s6 44us : sub_preempt_count (local_bh_enable_ip) | |
773 | sshd-4261 0d.s5 44us : add_preempt_count (__local_bh_disable) | |
774 | sshd-4261 0d.s5 45us : sub_preempt_count (local_bh_enable) | |
775 | [...] | |
776 | sshd-4261 0d.s. 63us : _local_bh_enable (__do_softirq) | |
777 | sshd-4261 0d.s1 64us : trace_preempt_on (__do_softirq) | |
778 | ||
779 | ||
5752674e IM |
780 | The above is an example of the preemptoff trace with |
781 | ftrace_enabled set. Here we see that interrupts were disabled | |
782 | the entire time. The irq_enter code lets us know that we entered | |
783 | an interrupt 'h'. Before that, the functions being traced still | |
784 | show that it is not in an interrupt, but we can see from the | |
785 | functions themselves that this is not the case. | |
eb6d42ea | 786 | |
5752674e IM |
787 | Notice that __do_softirq when called does not have a |
788 | preempt_count. It may seem that we missed a preempt enabling. | |
789 | What really happened is that the preempt count is held on the | |
790 | thread's stack and we switched to the softirq stack (4K stacks | |
791 | in effect). The code does not copy the preempt count, but | |
792 | because interrupts are disabled, we do not need to worry about | |
793 | it. Having a tracer like this is good for letting people know | |
794 | what really happens inside the kernel. | |
eb6d42ea SR |
795 | |
796 | ||
797 | preemptirqsoff | |
798 | -------------- | |
799 | ||
5752674e IM |
800 | Knowing the locations that have interrupts disabled or |
801 | preemption disabled for the longest times is helpful. But | |
802 | sometimes we would like to know when either preemption and/or | |
803 | interrupts are disabled. | |
eb6d42ea | 804 | |
f2d9c740 | 805 | Consider the following code: |
eb6d42ea SR |
806 | |
807 | local_irq_disable(); | |
808 | call_function_with_irqs_off(); | |
809 | preempt_disable(); | |
810 | call_function_with_irqs_and_preemption_off(); | |
811 | local_irq_enable(); | |
812 | call_function_with_preemption_off(); | |
813 | preempt_enable(); | |
814 | ||
815 | The irqsoff tracer will record the total length of | |
816 | call_function_with_irqs_off() and | |
817 | call_function_with_irqs_and_preemption_off(). | |
818 | ||
819 | The preemptoff tracer will record the total length of | |
820 | call_function_with_irqs_and_preemption_off() and | |
821 | call_function_with_preemption_off(). | |
822 | ||
5752674e IM |
823 | But neither will trace the time that interrupts and/or |
824 | preemption is disabled. This total time is the time that we can | |
825 | not schedule. To record this time, use the preemptirqsoff | |
826 | tracer. | |
eb6d42ea | 827 | |
5752674e IM |
828 | Again, using this trace is much like the irqsoff and preemptoff |
829 | tracers. | |
eb6d42ea | 830 | |
a41eebab | 831 | # echo preemptirqsoff > /debug/tracing/current_tracer |
eb6d42ea SR |
832 | # echo 0 > /debug/tracing/tracing_max_latency |
833 | # echo 1 > /debug/tracing/tracing_enabled | |
834 | # ls -ltr | |
835 | [...] | |
836 | # echo 0 > /debug/tracing/tracing_enabled | |
837 | # cat /debug/tracing/latency_trace | |
838 | # tracer: preemptirqsoff | |
839 | # | |
840 | preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8 | |
841 | -------------------------------------------------------------------- | |
842 | latency: 293 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
843 | ----------------- | |
844 | | task: ls-4860 (uid:0 nice:0 policy:0 rt_prio:0) | |
845 | ----------------- | |
846 | => started at: apic_timer_interrupt | |
847 | => ended at: __do_softirq | |
848 | ||
849 | # _------=> CPU# | |
850 | # / _-----=> irqs-off | |
851 | # | / _----=> need-resched | |
852 | # || / _---=> hardirq/softirq | |
853 | # ||| / _--=> preempt-depth | |
854 | # |||| / | |
855 | # ||||| delay | |
856 | # cmd pid ||||| time | caller | |
857 | # \ / ||||| \ | / | |
858 | ls-4860 0d... 0us!: trace_hardirqs_off_thunk (apic_timer_interrupt) | |
859 | ls-4860 0d.s. 294us : _local_bh_enable (__do_softirq) | |
860 | ls-4860 0d.s1 294us : trace_preempt_on (__do_softirq) | |
861 | ||
862 | ||
eb6d42ea SR |
863 | |
864 | The trace_hardirqs_off_thunk is called from assembly on x86 when | |
5752674e IM |
865 | interrupts are disabled in the assembly code. Without the |
866 | function tracing, we do not know if interrupts were enabled | |
867 | within the preemption points. We do see that it started with | |
868 | preemption enabled. | |
eb6d42ea SR |
869 | |
870 | Here is a trace with ftrace_enabled set: | |
871 | ||
872 | ||
873 | # tracer: preemptirqsoff | |
874 | # | |
875 | preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8 | |
876 | -------------------------------------------------------------------- | |
877 | latency: 105 us, #183/183, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
878 | ----------------- | |
879 | | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) | |
880 | ----------------- | |
881 | => started at: write_chan | |
882 | => ended at: __do_softirq | |
883 | ||
884 | # _------=> CPU# | |
885 | # / _-----=> irqs-off | |
886 | # | / _----=> need-resched | |
887 | # || / _---=> hardirq/softirq | |
888 | # ||| / _--=> preempt-depth | |
889 | # |||| / | |
890 | # ||||| delay | |
891 | # cmd pid ||||| time | caller | |
892 | # \ / ||||| \ | / | |
893 | ls-4473 0.N.. 0us : preempt_schedule (write_chan) | |
894 | ls-4473 0dN.1 1us : _spin_lock (schedule) | |
895 | ls-4473 0dN.1 2us : add_preempt_count (_spin_lock) | |
896 | ls-4473 0d..2 2us : put_prev_task_fair (schedule) | |
897 | [...] | |
898 | ls-4473 0d..2 13us : set_normalized_timespec (ktime_get_ts) | |
899 | ls-4473 0d..2 13us : __switch_to (schedule) | |
900 | sshd-4261 0d..2 14us : finish_task_switch (schedule) | |
901 | sshd-4261 0d..2 14us : _spin_unlock_irq (finish_task_switch) | |
902 | sshd-4261 0d..1 15us : add_preempt_count (_spin_lock_irqsave) | |
903 | sshd-4261 0d..2 16us : _spin_unlock_irqrestore (hrtick_set) | |
904 | sshd-4261 0d..2 16us : do_IRQ (common_interrupt) | |
905 | sshd-4261 0d..2 17us : irq_enter (do_IRQ) | |
906 | sshd-4261 0d..2 17us : idle_cpu (irq_enter) | |
907 | sshd-4261 0d..2 18us : add_preempt_count (irq_enter) | |
908 | sshd-4261 0d.h2 18us : idle_cpu (irq_enter) | |
909 | sshd-4261 0d.h. 18us : handle_fasteoi_irq (do_IRQ) | |
910 | sshd-4261 0d.h. 19us : _spin_lock (handle_fasteoi_irq) | |
911 | sshd-4261 0d.h. 19us : add_preempt_count (_spin_lock) | |
912 | sshd-4261 0d.h1 20us : _spin_unlock (handle_fasteoi_irq) | |
913 | sshd-4261 0d.h1 20us : sub_preempt_count (_spin_unlock) | |
914 | [...] | |
915 | sshd-4261 0d.h1 28us : _spin_unlock (handle_fasteoi_irq) | |
916 | sshd-4261 0d.h1 29us : sub_preempt_count (_spin_unlock) | |
917 | sshd-4261 0d.h2 29us : irq_exit (do_IRQ) | |
918 | sshd-4261 0d.h2 29us : sub_preempt_count (irq_exit) | |
919 | sshd-4261 0d..3 30us : do_softirq (irq_exit) | |
920 | sshd-4261 0d... 30us : __do_softirq (do_softirq) | |
921 | sshd-4261 0d... 31us : __local_bh_disable (__do_softirq) | |
922 | sshd-4261 0d... 31us+: add_preempt_count (__local_bh_disable) | |
923 | sshd-4261 0d.s4 34us : add_preempt_count (__local_bh_disable) | |
924 | [...] | |
925 | sshd-4261 0d.s3 43us : sub_preempt_count (local_bh_enable_ip) | |
926 | sshd-4261 0d.s4 44us : sub_preempt_count (local_bh_enable_ip) | |
927 | sshd-4261 0d.s3 44us : smp_apic_timer_interrupt (apic_timer_interrupt) | |
928 | sshd-4261 0d.s3 45us : irq_enter (smp_apic_timer_interrupt) | |
929 | sshd-4261 0d.s3 45us : idle_cpu (irq_enter) | |
930 | sshd-4261 0d.s3 46us : add_preempt_count (irq_enter) | |
931 | sshd-4261 0d.H3 46us : idle_cpu (irq_enter) | |
932 | sshd-4261 0d.H3 47us : hrtimer_interrupt (smp_apic_timer_interrupt) | |
933 | sshd-4261 0d.H3 47us : ktime_get (hrtimer_interrupt) | |
934 | [...] | |
935 | sshd-4261 0d.H3 81us : tick_program_event (hrtimer_interrupt) | |
936 | sshd-4261 0d.H3 82us : ktime_get (tick_program_event) | |
937 | sshd-4261 0d.H3 82us : ktime_get_ts (ktime_get) | |
938 | sshd-4261 0d.H3 83us : getnstimeofday (ktime_get_ts) | |
939 | sshd-4261 0d.H3 83us : set_normalized_timespec (ktime_get_ts) | |
940 | sshd-4261 0d.H3 84us : clockevents_program_event (tick_program_event) | |
941 | sshd-4261 0d.H3 84us : lapic_next_event (clockevents_program_event) | |
942 | sshd-4261 0d.H3 85us : irq_exit (smp_apic_timer_interrupt) | |
943 | sshd-4261 0d.H3 85us : sub_preempt_count (irq_exit) | |
944 | sshd-4261 0d.s4 86us : sub_preempt_count (irq_exit) | |
945 | sshd-4261 0d.s3 86us : add_preempt_count (__local_bh_disable) | |
946 | [...] | |
947 | sshd-4261 0d.s1 98us : sub_preempt_count (net_rx_action) | |
948 | sshd-4261 0d.s. 99us : add_preempt_count (_spin_lock_irq) | |
949 | sshd-4261 0d.s1 99us+: _spin_unlock_irq (run_timer_softirq) | |
950 | sshd-4261 0d.s. 104us : _local_bh_enable (__do_softirq) | |
951 | sshd-4261 0d.s. 104us : sub_preempt_count (_local_bh_enable) | |
952 | sshd-4261 0d.s. 105us : _local_bh_enable (__do_softirq) | |
953 | sshd-4261 0d.s1 105us : trace_preempt_on (__do_softirq) | |
954 | ||
955 | ||
5752674e IM |
956 | This is a very interesting trace. It started with the preemption |
957 | of the ls task. We see that the task had the "need_resched" bit | |
958 | set via the 'N' in the trace. Interrupts were disabled before | |
959 | the spin_lock at the beginning of the trace. We see that a | |
960 | schedule took place to run sshd. When the interrupts were | |
961 | enabled, we took an interrupt. On return from the interrupt | |
962 | handler, the softirq ran. We took another interrupt while | |
963 | running the softirq as we see from the capital 'H'. | |
eb6d42ea SR |
964 | |
965 | ||
966 | wakeup | |
967 | ------ | |
968 | ||
5752674e IM |
969 | In a Real-Time environment it is very important to know the |
970 | wakeup time it takes for the highest priority task that is woken | |
971 | up to the time that it executes. This is also known as "schedule | |
972 | latency". I stress the point that this is about RT tasks. It is | |
973 | also important to know the scheduling latency of non-RT tasks, | |
974 | but the average schedule latency is better for non-RT tasks. | |
975 | Tools like LatencyTop are more appropriate for such | |
976 | measurements. | |
eb6d42ea | 977 | |
a41eebab | 978 | Real-Time environments are interested in the worst case latency. |
5752674e IM |
979 | That is the longest latency it takes for something to happen, |
980 | and not the average. We can have a very fast scheduler that may | |
981 | only have a large latency once in a while, but that would not | |
982 | work well with Real-Time tasks. The wakeup tracer was designed | |
983 | to record the worst case wakeups of RT tasks. Non-RT tasks are | |
984 | not recorded because the tracer only records one worst case and | |
985 | tracing non-RT tasks that are unpredictable will overwrite the | |
986 | worst case latency of RT tasks. | |
987 | ||
988 | Since this tracer only deals with RT tasks, we will run this | |
989 | slightly differently than we did with the previous tracers. | |
990 | Instead of performing an 'ls', we will run 'sleep 1' under | |
991 | 'chrt' which changes the priority of the task. | |
eb6d42ea SR |
992 | |
993 | # echo wakeup > /debug/tracing/current_tracer | |
994 | # echo 0 > /debug/tracing/tracing_max_latency | |
995 | # echo 1 > /debug/tracing/tracing_enabled | |
996 | # chrt -f 5 sleep 1 | |
997 | # echo 0 > /debug/tracing/tracing_enabled | |
998 | # cat /debug/tracing/latency_trace | |
999 | # tracer: wakeup | |
1000 | # | |
1001 | wakeup latency trace v1.1.5 on 2.6.26-rc8 | |
1002 | -------------------------------------------------------------------- | |
1003 | latency: 4 us, #2/2, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
1004 | ----------------- | |
1005 | | task: sleep-4901 (uid:0 nice:0 policy:1 rt_prio:5) | |
1006 | ----------------- | |
1007 | ||
1008 | # _------=> CPU# | |
1009 | # / _-----=> irqs-off | |
1010 | # | / _----=> need-resched | |
1011 | # || / _---=> hardirq/softirq | |
1012 | # ||| / _--=> preempt-depth | |
1013 | # |||| / | |
1014 | # ||||| delay | |
1015 | # cmd pid ||||| time | caller | |
1016 | # \ / ||||| \ | / | |
1017 | <idle>-0 1d.h4 0us+: try_to_wake_up (wake_up_process) | |
1018 | <idle>-0 1d..4 4us : schedule (cpu_idle) | |
1019 | ||
1020 | ||
5752674e IM |
1021 | Running this on an idle system, we see that it only took 4 |
1022 | microseconds to perform the task switch. Note, since the trace | |
1023 | marker in the schedule is before the actual "switch", we stop | |
1024 | the tracing when the recorded task is about to schedule in. This | |
1025 | may change if we add a new marker at the end of the scheduler. | |
eb6d42ea | 1026 | |
5752674e IM |
1027 | Notice that the recorded task is 'sleep' with the PID of 4901 |
1028 | and it has an rt_prio of 5. This priority is user-space priority | |
1029 | and not the internal kernel priority. The policy is 1 for | |
1030 | SCHED_FIFO and 2 for SCHED_RR. | |
eb6d42ea SR |
1031 | |
1032 | Doing the same with chrt -r 5 and ftrace_enabled set. | |
1033 | ||
1034 | # tracer: wakeup | |
1035 | # | |
1036 | wakeup latency trace v1.1.5 on 2.6.26-rc8 | |
1037 | -------------------------------------------------------------------- | |
1038 | latency: 50 us, #60/60, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
1039 | ----------------- | |
1040 | | task: sleep-4068 (uid:0 nice:0 policy:2 rt_prio:5) | |
1041 | ----------------- | |
1042 | ||
1043 | # _------=> CPU# | |
1044 | # / _-----=> irqs-off | |
1045 | # | / _----=> need-resched | |
1046 | # || / _---=> hardirq/softirq | |
1047 | # ||| / _--=> preempt-depth | |
1048 | # |||| / | |
1049 | # ||||| delay | |
1050 | # cmd pid ||||| time | caller | |
1051 | # \ / ||||| \ | / | |
1052 | ksoftirq-7 1d.H3 0us : try_to_wake_up (wake_up_process) | |
1053 | ksoftirq-7 1d.H4 1us : sub_preempt_count (marker_probe_cb) | |
1054 | ksoftirq-7 1d.H3 2us : check_preempt_wakeup (try_to_wake_up) | |
1055 | ksoftirq-7 1d.H3 3us : update_curr (check_preempt_wakeup) | |
1056 | ksoftirq-7 1d.H3 4us : calc_delta_mine (update_curr) | |
1057 | ksoftirq-7 1d.H3 5us : __resched_task (check_preempt_wakeup) | |
1058 | ksoftirq-7 1d.H3 6us : task_wake_up_rt (try_to_wake_up) | |
1059 | ksoftirq-7 1d.H3 7us : _spin_unlock_irqrestore (try_to_wake_up) | |
1060 | [...] | |
1061 | ksoftirq-7 1d.H2 17us : irq_exit (smp_apic_timer_interrupt) | |
1062 | ksoftirq-7 1d.H2 18us : sub_preempt_count (irq_exit) | |
1063 | ksoftirq-7 1d.s3 19us : sub_preempt_count (irq_exit) | |
1064 | ksoftirq-7 1..s2 20us : rcu_process_callbacks (__do_softirq) | |
1065 | [...] | |
1066 | ksoftirq-7 1..s2 26us : __rcu_process_callbacks (rcu_process_callbacks) | |
1067 | ksoftirq-7 1d.s2 27us : _local_bh_enable (__do_softirq) | |
1068 | ksoftirq-7 1d.s2 28us : sub_preempt_count (_local_bh_enable) | |
1069 | ksoftirq-7 1.N.3 29us : sub_preempt_count (ksoftirqd) | |
1070 | ksoftirq-7 1.N.2 30us : _cond_resched (ksoftirqd) | |
1071 | ksoftirq-7 1.N.2 31us : __cond_resched (_cond_resched) | |
1072 | ksoftirq-7 1.N.2 32us : add_preempt_count (__cond_resched) | |
1073 | ksoftirq-7 1.N.2 33us : schedule (__cond_resched) | |
1074 | ksoftirq-7 1.N.2 33us : add_preempt_count (schedule) | |
1075 | ksoftirq-7 1.N.3 34us : hrtick_clear (schedule) | |
1076 | ksoftirq-7 1dN.3 35us : _spin_lock (schedule) | |
1077 | ksoftirq-7 1dN.3 36us : add_preempt_count (_spin_lock) | |
1078 | ksoftirq-7 1d..4 37us : put_prev_task_fair (schedule) | |
1079 | ksoftirq-7 1d..4 38us : update_curr (put_prev_task_fair) | |
1080 | [...] | |
1081 | ksoftirq-7 1d..5 47us : _spin_trylock (tracing_record_cmdline) | |
1082 | ksoftirq-7 1d..5 48us : add_preempt_count (_spin_trylock) | |
1083 | ksoftirq-7 1d..6 49us : _spin_unlock (tracing_record_cmdline) | |
1084 | ksoftirq-7 1d..6 49us : sub_preempt_count (_spin_unlock) | |
1085 | ksoftirq-7 1d..4 50us : schedule (__cond_resched) | |
1086 | ||
5752674e IM |
1087 | The interrupt went off while running ksoftirqd. This task runs |
1088 | at SCHED_OTHER. Why did not we see the 'N' set early? This may | |
1089 | be a harmless bug with x86_32 and 4K stacks. On x86_32 with 4K | |
1090 | stacks configured, the interrupt and softirq run with their own | |
1091 | stack. Some information is held on the top of the task's stack | |
1092 | (need_resched and preempt_count are both stored there). The | |
1093 | setting of the NEED_RESCHED bit is done directly to the task's | |
1094 | stack, but the reading of the NEED_RESCHED is done by looking at | |
1095 | the current stack, which in this case is the stack for the hard | |
1096 | interrupt. This hides the fact that NEED_RESCHED has been set. | |
1097 | We do not see the 'N' until we switch back to the task's | |
a41eebab | 1098 | assigned stack. |
eb6d42ea | 1099 | |
9b803c0f SR |
1100 | function |
1101 | -------- | |
eb6d42ea | 1102 | |
9b803c0f | 1103 | This tracer is the function tracer. Enabling the function tracer |
5752674e IM |
1104 | can be done from the debug file system. Make sure the |
1105 | ftrace_enabled is set; otherwise this tracer is a nop. | |
eb6d42ea SR |
1106 | |
1107 | # sysctl kernel.ftrace_enabled=1 | |
9b803c0f | 1108 | # echo function > /debug/tracing/current_tracer |
eb6d42ea SR |
1109 | # echo 1 > /debug/tracing/tracing_enabled |
1110 | # usleep 1 | |
1111 | # echo 0 > /debug/tracing/tracing_enabled | |
1112 | # cat /debug/tracing/trace | |
9b803c0f | 1113 | # tracer: function |
eb6d42ea SR |
1114 | # |
1115 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1116 | # | | | | | | |
1117 | bash-4003 [00] 123.638713: finish_task_switch <-schedule | |
1118 | bash-4003 [00] 123.638714: _spin_unlock_irq <-finish_task_switch | |
1119 | bash-4003 [00] 123.638714: sub_preempt_count <-_spin_unlock_irq | |
1120 | bash-4003 [00] 123.638715: hrtick_set <-schedule | |
1121 | bash-4003 [00] 123.638715: _spin_lock_irqsave <-hrtick_set | |
1122 | bash-4003 [00] 123.638716: add_preempt_count <-_spin_lock_irqsave | |
1123 | bash-4003 [00] 123.638716: _spin_unlock_irqrestore <-hrtick_set | |
1124 | bash-4003 [00] 123.638717: sub_preempt_count <-_spin_unlock_irqrestore | |
1125 | bash-4003 [00] 123.638717: hrtick_clear <-hrtick_set | |
1126 | bash-4003 [00] 123.638718: sub_preempt_count <-schedule | |
1127 | bash-4003 [00] 123.638718: sub_preempt_count <-preempt_schedule | |
1128 | bash-4003 [00] 123.638719: wait_for_completion <-__stop_machine_run | |
1129 | bash-4003 [00] 123.638719: wait_for_common <-wait_for_completion | |
1130 | bash-4003 [00] 123.638720: _spin_lock_irq <-wait_for_common | |
1131 | bash-4003 [00] 123.638720: add_preempt_count <-_spin_lock_irq | |
1132 | [...] | |
1133 | ||
1134 | ||
5752674e IM |
1135 | Note: function tracer uses ring buffers to store the above |
1136 | entries. The newest data may overwrite the oldest data. | |
1137 | Sometimes using echo to stop the trace is not sufficient because | |
1138 | the tracing could have overwritten the data that you wanted to | |
1139 | record. For this reason, it is sometimes better to disable | |
1140 | tracing directly from a program. This allows you to stop the | |
1141 | tracing at the point that you hit the part that you are | |
1142 | interested in. To disable the tracing directly from a C program, | |
1143 | something like following code snippet can be used: | |
eb6d42ea SR |
1144 | |
1145 | int trace_fd; | |
1146 | [...] | |
1147 | int main(int argc, char *argv[]) { | |
1148 | [...] | |
1149 | trace_fd = open("/debug/tracing/tracing_enabled", O_WRONLY); | |
1150 | [...] | |
1151 | if (condition_hit()) { | |
f2d9c740 | 1152 | write(trace_fd, "0", 1); |
eb6d42ea SR |
1153 | } |
1154 | [...] | |
1155 | } | |
1156 | ||
f2d9c740 | 1157 | Note: Here we hard coded the path name. The debugfs mount is not |
5752674e IM |
1158 | guaranteed to be at /debug (and is more commonly at |
1159 | /sys/kernel/debug). For simple one time traces, the above is | |
1160 | sufficent. For anything else, a search through /proc/mounts may | |
1161 | be needed to find where the debugfs file-system is mounted. | |
eb6d42ea | 1162 | |
df4fc315 SR |
1163 | |
1164 | Single thread tracing | |
1165 | --------------------- | |
1166 | ||
1167 | By writing into /debug/tracing/set_ftrace_pid you can trace a | |
1168 | single thread. For example: | |
1169 | ||
1170 | # cat /debug/tracing/set_ftrace_pid | |
1171 | no pid | |
1172 | # echo 3111 > /debug/tracing/set_ftrace_pid | |
1173 | # cat /debug/tracing/set_ftrace_pid | |
1174 | 3111 | |
1175 | # echo function > /debug/tracing/current_tracer | |
1176 | # cat /debug/tracing/trace | head | |
1177 | # tracer: function | |
1178 | # | |
1179 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1180 | # | | | | | | |
1181 | yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return | |
1182 | yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range | |
1183 | yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel | |
1184 | yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel | |
1185 | yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll | |
1186 | yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll | |
1187 | # echo -1 > /debug/tracing/set_ftrace_pid | |
1188 | # cat /debug/tracing/trace |head | |
1189 | # tracer: function | |
1190 | # | |
1191 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1192 | # | | | | | | |
1193 | ##### CPU 3 buffer started #### | |
1194 | yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait | |
1195 | yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry | |
1196 | yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry | |
1197 | yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit | |
1198 | yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit | |
1199 | ||
1200 | If you want to trace a function when executing, you could use | |
1201 | something like this simple program: | |
1202 | ||
1203 | #include <stdio.h> | |
1204 | #include <stdlib.h> | |
1205 | #include <sys/types.h> | |
1206 | #include <sys/stat.h> | |
1207 | #include <fcntl.h> | |
1208 | #include <unistd.h> | |
1209 | ||
1210 | int main (int argc, char **argv) | |
1211 | { | |
1212 | if (argc < 1) | |
1213 | exit(-1); | |
1214 | ||
1215 | if (fork() > 0) { | |
1216 | int fd, ffd; | |
1217 | char line[64]; | |
1218 | int s; | |
1219 | ||
1220 | ffd = open("/debug/tracing/current_tracer", O_WRONLY); | |
1221 | if (ffd < 0) | |
1222 | exit(-1); | |
1223 | write(ffd, "nop", 3); | |
1224 | ||
1225 | fd = open("/debug/tracing/set_ftrace_pid", O_WRONLY); | |
1226 | s = sprintf(line, "%d\n", getpid()); | |
1227 | write(fd, line, s); | |
1228 | ||
1229 | write(ffd, "function", 8); | |
1230 | ||
1231 | close(fd); | |
1232 | close(ffd); | |
1233 | ||
1234 | execvp(argv[1], argv+1); | |
1235 | } | |
1236 | ||
1237 | return 0; | |
1238 | } | |
1239 | ||
e2ea5399 MM |
1240 | |
1241 | hw-branch-tracer (x86 only) | |
1242 | --------------------------- | |
1243 | ||
1244 | This tracer uses the x86 last branch tracing hardware feature to | |
1245 | collect a branch trace on all cpus with relatively low overhead. | |
1246 | ||
1247 | The tracer uses a fixed-size circular buffer per cpu and only | |
1248 | traces ring 0 branches. The trace file dumps that buffer in the | |
1249 | following format: | |
1250 | ||
1251 | # tracer: hw-branch-tracer | |
1252 | # | |
1253 | # CPU# TO <- FROM | |
1254 | 0 scheduler_tick+0xb5/0x1bf <- task_tick_idle+0x5/0x6 | |
1255 | 2 run_posix_cpu_timers+0x2b/0x72a <- run_posix_cpu_timers+0x25/0x72a | |
1256 | 0 scheduler_tick+0x139/0x1bf <- scheduler_tick+0xed/0x1bf | |
1257 | 0 scheduler_tick+0x17c/0x1bf <- scheduler_tick+0x148/0x1bf | |
1258 | 2 run_posix_cpu_timers+0x9e/0x72a <- run_posix_cpu_timers+0x5e/0x72a | |
1259 | 0 scheduler_tick+0x1b6/0x1bf <- scheduler_tick+0x1aa/0x1bf | |
1260 | ||
1261 | ||
5752674e IM |
1262 | The tracer may be used to dump the trace for the oops'ing cpu on |
1263 | a kernel oops into the system log. To enable this, | |
1264 | ftrace_dump_on_oops must be set. To set ftrace_dump_on_oops, one | |
1265 | can either use the sysctl function or set it via the proc system | |
1266 | interface. | |
e2ea5399 MM |
1267 | |
1268 | sysctl kernel.ftrace_dump_on_oops=1 | |
1269 | ||
1270 | or | |
1271 | ||
1272 | echo 1 > /proc/sys/kernel/ftrace_dump_on_oops | |
1273 | ||
1274 | ||
5752674e IM |
1275 | Here's an example of such a dump after a null pointer |
1276 | dereference in a kernel module: | |
e2ea5399 MM |
1277 | |
1278 | [57848.105921] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000 | |
1279 | [57848.106019] IP: [<ffffffffa0000006>] open+0x6/0x14 [oops] | |
1280 | [57848.106019] PGD 2354e9067 PUD 2375e7067 PMD 0 | |
1281 | [57848.106019] Oops: 0002 [#1] SMP | |
1282 | [57848.106019] last sysfs file: /sys/devices/pci0000:00/0000:00:1e.0/0000:20:05.0/local_cpus | |
1283 | [57848.106019] Dumping ftrace buffer: | |
1284 | [57848.106019] --------------------------------- | |
1285 | [...] | |
1286 | [57848.106019] 0 chrdev_open+0xe6/0x165 <- cdev_put+0x23/0x24 | |
1287 | [57848.106019] 0 chrdev_open+0x117/0x165 <- chrdev_open+0xfa/0x165 | |
1288 | [57848.106019] 0 chrdev_open+0x120/0x165 <- chrdev_open+0x11c/0x165 | |
1289 | [57848.106019] 0 chrdev_open+0x134/0x165 <- chrdev_open+0x12b/0x165 | |
1290 | [57848.106019] 0 open+0x0/0x14 [oops] <- chrdev_open+0x144/0x165 | |
1291 | [57848.106019] 0 page_fault+0x0/0x30 <- open+0x6/0x14 [oops] | |
1292 | [57848.106019] 0 error_entry+0x0/0x5b <- page_fault+0x4/0x30 | |
1293 | [57848.106019] 0 error_kernelspace+0x0/0x31 <- error_entry+0x59/0x5b | |
1294 | [57848.106019] 0 error_sti+0x0/0x1 <- error_kernelspace+0x2d/0x31 | |
1295 | [57848.106019] 0 page_fault+0x9/0x30 <- error_sti+0x0/0x1 | |
1296 | [57848.106019] 0 do_page_fault+0x0/0x881 <- page_fault+0x1a/0x30 | |
1297 | [...] | |
1298 | [57848.106019] 0 do_page_fault+0x66b/0x881 <- is_prefetch+0x1ee/0x1f2 | |
1299 | [57848.106019] 0 do_page_fault+0x6e0/0x881 <- do_page_fault+0x67a/0x881 | |
1300 | [57848.106019] 0 oops_begin+0x0/0x96 <- do_page_fault+0x6e0/0x881 | |
1301 | [57848.106019] 0 trace_hw_branch_oops+0x0/0x2d <- oops_begin+0x9/0x96 | |
1302 | [...] | |
1303 | [57848.106019] 0 ds_suspend_bts+0x2a/0xe3 <- ds_suspend_bts+0x1a/0xe3 | |
1304 | [57848.106019] --------------------------------- | |
1305 | [57848.106019] CPU 0 | |
1306 | [57848.106019] Modules linked in: oops | |
1307 | [57848.106019] Pid: 5542, comm: cat Tainted: G W 2.6.28 #23 | |
1308 | [57848.106019] RIP: 0010:[<ffffffffa0000006>] [<ffffffffa0000006>] open+0x6/0x14 [oops] | |
1309 | [57848.106019] RSP: 0018:ffff880235457d48 EFLAGS: 00010246 | |
1310 | [...] | |
1311 | ||
1312 | ||
985ec20a FW |
1313 | function graph tracer |
1314 | --------------------------- | |
1315 | ||
5752674e IM |
1316 | This tracer is similar to the function tracer except that it |
1317 | probes a function on its entry and its exit. This is done by | |
1318 | using a dynamically allocated stack of return addresses in each | |
1319 | task_struct. On function entry the tracer overwrites the return | |
1320 | address of each function traced to set a custom probe. Thus the | |
1321 | original return address is stored on the stack of return address | |
1322 | in the task_struct. | |
985ec20a | 1323 | |
5752674e IM |
1324 | Probing on both ends of a function leads to special features |
1325 | such as: | |
985ec20a | 1326 | |
5752674e IM |
1327 | - measure of a function's time execution |
1328 | - having a reliable call stack to draw function calls graph | |
985ec20a FW |
1329 | |
1330 | This tracer is useful in several situations: | |
1331 | ||
5752674e IM |
1332 | - you want to find the reason of a strange kernel behavior and |
1333 | need to see what happens in detail on any areas (or specific | |
1334 | ones). | |
1335 | ||
1336 | - you are experiencing weird latencies but it's difficult to | |
1337 | find its origin. | |
1338 | ||
1339 | - you want to find quickly which path is taken by a specific | |
1340 | function | |
1341 | ||
1342 | - you just want to peek inside a working kernel and want to see | |
1343 | what happens there. | |
985ec20a FW |
1344 | |
1345 | # tracer: function_graph | |
1346 | # | |
1347 | # CPU DURATION FUNCTION CALLS | |
1348 | # | | | | | | | | |
1349 | ||
1350 | 0) | sys_open() { | |
1351 | 0) | do_sys_open() { | |
1352 | 0) | getname() { | |
1353 | 0) | kmem_cache_alloc() { | |
1354 | 0) 1.382 us | __might_sleep(); | |
1355 | 0) 2.478 us | } | |
1356 | 0) | strncpy_from_user() { | |
1357 | 0) | might_fault() { | |
1358 | 0) 1.389 us | __might_sleep(); | |
1359 | 0) 2.553 us | } | |
1360 | 0) 3.807 us | } | |
1361 | 0) 7.876 us | } | |
1362 | 0) | alloc_fd() { | |
1363 | 0) 0.668 us | _spin_lock(); | |
1364 | 0) 0.570 us | expand_files(); | |
1365 | 0) 0.586 us | _spin_unlock(); | |
1366 | ||
1367 | ||
5752674e IM |
1368 | There are several columns that can be dynamically |
1369 | enabled/disabled. You can use every combination of options you | |
1370 | want, depending on your needs. | |
985ec20a | 1371 | |
5752674e IM |
1372 | - The cpu number on which the function executed is default |
1373 | enabled. It is sometimes better to only trace one cpu (see | |
1374 | tracing_cpu_mask file) or you might sometimes see unordered | |
1375 | function calls while cpu tracing switch. | |
985ec20a FW |
1376 | |
1377 | hide: echo nofuncgraph-cpu > /debug/tracing/trace_options | |
1378 | show: echo funcgraph-cpu > /debug/tracing/trace_options | |
1379 | ||
5752674e IM |
1380 | - The duration (function's time of execution) is displayed on |
1381 | the closing bracket line of a function or on the same line | |
1382 | than the current function in case of a leaf one. It is default | |
1383 | enabled. | |
985ec20a FW |
1384 | |
1385 | hide: echo nofuncgraph-duration > /debug/tracing/trace_options | |
1386 | show: echo funcgraph-duration > /debug/tracing/trace_options | |
1387 | ||
5752674e IM |
1388 | - The overhead field precedes the duration field in case of |
1389 | reached duration thresholds. | |
985ec20a FW |
1390 | |
1391 | hide: echo nofuncgraph-overhead > /debug/tracing/trace_options | |
1392 | show: echo funcgraph-overhead > /debug/tracing/trace_options | |
1393 | depends on: funcgraph-duration | |
1394 | ||
1395 | ie: | |
1396 | ||
1397 | 0) | up_write() { | |
1398 | 0) 0.646 us | _spin_lock_irqsave(); | |
1399 | 0) 0.684 us | _spin_unlock_irqrestore(); | |
1400 | 0) 3.123 us | } | |
1401 | 0) 0.548 us | fput(); | |
1402 | 0) + 58.628 us | } | |
1403 | ||
1404 | [...] | |
1405 | ||
1406 | 0) | putname() { | |
1407 | 0) | kmem_cache_free() { | |
1408 | 0) 0.518 us | __phys_addr(); | |
1409 | 0) 1.757 us | } | |
1410 | 0) 2.861 us | } | |
1411 | 0) ! 115.305 us | } | |
1412 | 0) ! 116.402 us | } | |
1413 | ||
1414 | + means that the function exceeded 10 usecs. | |
1415 | ! means that the function exceeded 100 usecs. | |
1416 | ||
1417 | ||
5752674e IM |
1418 | - The task/pid field displays the thread cmdline and pid which |
1419 | executed the function. It is default disabled. | |
985ec20a FW |
1420 | |
1421 | hide: echo nofuncgraph-proc > /debug/tracing/trace_options | |
1422 | show: echo funcgraph-proc > /debug/tracing/trace_options | |
1423 | ||
1424 | ie: | |
1425 | ||
1426 | # tracer: function_graph | |
1427 | # | |
1428 | # CPU TASK/PID DURATION FUNCTION CALLS | |
1429 | # | | | | | | | | | | |
1430 | 0) sh-4802 | | d_free() { | |
1431 | 0) sh-4802 | | call_rcu() { | |
1432 | 0) sh-4802 | | __call_rcu() { | |
1433 | 0) sh-4802 | 0.616 us | rcu_process_gp_end(); | |
1434 | 0) sh-4802 | 0.586 us | check_for_new_grace_period(); | |
1435 | 0) sh-4802 | 2.899 us | } | |
1436 | 0) sh-4802 | 4.040 us | } | |
1437 | 0) sh-4802 | 5.151 us | } | |
1438 | 0) sh-4802 | + 49.370 us | } | |
1439 | ||
1440 | ||
5752674e IM |
1441 | - The absolute time field is an absolute timestamp given by the |
1442 | system clock since it started. A snapshot of this time is | |
1443 | given on each entry/exit of functions | |
985ec20a FW |
1444 | |
1445 | hide: echo nofuncgraph-abstime > /debug/tracing/trace_options | |
1446 | show: echo funcgraph-abstime > /debug/tracing/trace_options | |
1447 | ||
1448 | ie: | |
1449 | ||
1450 | # | |
1451 | # TIME CPU DURATION FUNCTION CALLS | |
1452 | # | | | | | | | | | |
1453 | 360.774522 | 1) 0.541 us | } | |
1454 | 360.774522 | 1) 4.663 us | } | |
1455 | 360.774523 | 1) 0.541 us | __wake_up_bit(); | |
1456 | 360.774524 | 1) 6.796 us | } | |
1457 | 360.774524 | 1) 7.952 us | } | |
1458 | 360.774525 | 1) 9.063 us | } | |
1459 | 360.774525 | 1) 0.615 us | journal_mark_dirty(); | |
1460 | 360.774527 | 1) 0.578 us | __brelse(); | |
1461 | 360.774528 | 1) | reiserfs_prepare_for_journal() { | |
1462 | 360.774528 | 1) | unlock_buffer() { | |
1463 | 360.774529 | 1) | wake_up_bit() { | |
1464 | 360.774529 | 1) | bit_waitqueue() { | |
1465 | 360.774530 | 1) 0.594 us | __phys_addr(); | |
1466 | ||
1467 | ||
5752674e | 1468 | You can put some comments on specific functions by using |
5e1607a0 | 1469 | trace_printk() For example, if you want to put a comment inside |
5752674e | 1470 | the __might_sleep() function, you just have to include |
5e1607a0 | 1471 | <linux/ftrace.h> and call trace_printk() inside __might_sleep() |
985ec20a | 1472 | |
5e1607a0 | 1473 | trace_printk("I'm a comment!\n") |
985ec20a FW |
1474 | |
1475 | will produce: | |
1476 | ||
1477 | 1) | __might_sleep() { | |
1478 | 1) | /* I'm a comment! */ | |
1479 | 1) 1.449 us | } | |
1480 | ||
1481 | ||
5752674e IM |
1482 | You might find other useful features for this tracer in the |
1483 | following "dynamic ftrace" section such as tracing only specific | |
1484 | functions or tasks. | |
985ec20a | 1485 | |
eb6d42ea SR |
1486 | dynamic ftrace |
1487 | -------------- | |
1488 | ||
f2d9c740 | 1489 | If CONFIG_DYNAMIC_FTRACE is set, the system will run with |
eb6d42ea SR |
1490 | virtually no overhead when function tracing is disabled. The way |
1491 | this works is the mcount function call (placed at the start of | |
5752674e IM |
1492 | every kernel function, produced by the -pg switch in gcc), |
1493 | starts of pointing to a simple return. (Enabling FTRACE will | |
1494 | include the -pg switch in the compiling of the kernel.) | |
eb6d42ea | 1495 | |
9b803c0f SR |
1496 | At compile time every C file object is run through the |
1497 | recordmcount.pl script (located in the scripts directory). This | |
1498 | script will process the C object using objdump to find all the | |
5752674e IM |
1499 | locations in the .text section that call mcount. (Note, only the |
1500 | .text section is processed, since processing other sections like | |
1501 | .init.text may cause races due to those sections being freed). | |
9b803c0f | 1502 | |
5752674e IM |
1503 | A new section called "__mcount_loc" is created that holds |
1504 | references to all the mcount call sites in the .text section. | |
1505 | This section is compiled back into the original object. The | |
1506 | final linker will add all these references into a single table. | |
9b803c0f SR |
1507 | |
1508 | On boot up, before SMP is initialized, the dynamic ftrace code | |
5752674e IM |
1509 | scans this table and updates all the locations into nops. It |
1510 | also records the locations, which are added to the | |
1511 | available_filter_functions list. Modules are processed as they | |
1512 | are loaded and before they are executed. When a module is | |
1513 | unloaded, it also removes its functions from the ftrace function | |
1514 | list. This is automatic in the module unload code, and the | |
1515 | module author does not need to worry about it. | |
1516 | ||
1517 | When tracing is enabled, kstop_machine is called to prevent | |
1518 | races with the CPUS executing code being modified (which can | |
1519 | cause the CPU to do undesireable things), and the nops are | |
1520 | patched back to calls. But this time, they do not call mcount | |
1521 | (which is just a function stub). They now call into the ftrace | |
1522 | infrastructure. | |
eb6d42ea SR |
1523 | |
1524 | One special side-effect to the recording of the functions being | |
f2d9c740 | 1525 | traced is that we can now selectively choose which functions we |
5752674e IM |
1526 | wish to trace and which ones we want the mcount calls to remain |
1527 | as nops. | |
eb6d42ea | 1528 | |
5752674e IM |
1529 | Two files are used, one for enabling and one for disabling the |
1530 | tracing of specified functions. They are: | |
eb6d42ea SR |
1531 | |
1532 | set_ftrace_filter | |
1533 | ||
1534 | and | |
1535 | ||
1536 | set_ftrace_notrace | |
1537 | ||
5752674e IM |
1538 | A list of available functions that you can add to these files is |
1539 | listed in: | |
eb6d42ea SR |
1540 | |
1541 | available_filter_functions | |
1542 | ||
1543 | # cat /debug/tracing/available_filter_functions | |
1544 | put_prev_task_idle | |
1545 | kmem_cache_create | |
1546 | pick_next_task_rt | |
1547 | get_online_cpus | |
1548 | pick_next_task_fair | |
1549 | mutex_lock | |
1550 | [...] | |
1551 | ||
f2d9c740 | 1552 | If I am only interested in sys_nanosleep and hrtimer_interrupt: |
eb6d42ea SR |
1553 | |
1554 | # echo sys_nanosleep hrtimer_interrupt \ | |
1555 | > /debug/tracing/set_ftrace_filter | |
1556 | # echo ftrace > /debug/tracing/current_tracer | |
1557 | # echo 1 > /debug/tracing/tracing_enabled | |
1558 | # usleep 1 | |
1559 | # echo 0 > /debug/tracing/tracing_enabled | |
1560 | # cat /debug/tracing/trace | |
1561 | # tracer: ftrace | |
1562 | # | |
1563 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1564 | # | | | | | | |
1565 | usleep-4134 [00] 1317.070017: hrtimer_interrupt <-smp_apic_timer_interrupt | |
1566 | usleep-4134 [00] 1317.070111: sys_nanosleep <-syscall_call | |
1567 | <idle>-0 [00] 1317.070115: hrtimer_interrupt <-smp_apic_timer_interrupt | |
1568 | ||
f2d9c740 | 1569 | To see which functions are being traced, you can cat the file: |
eb6d42ea SR |
1570 | |
1571 | # cat /debug/tracing/set_ftrace_filter | |
1572 | hrtimer_interrupt | |
1573 | sys_nanosleep | |
1574 | ||
1575 | ||
5752674e IM |
1576 | Perhaps this is not enough. The filters also allow simple wild |
1577 | cards. Only the following are currently available | |
eb6d42ea | 1578 | |
a41eebab | 1579 | <match>* - will match functions that begin with <match> |
eb6d42ea SR |
1580 | *<match> - will match functions that end with <match> |
1581 | *<match>* - will match functions that have <match> in it | |
1582 | ||
f2d9c740 | 1583 | These are the only wild cards which are supported. |
eb6d42ea SR |
1584 | |
1585 | <match>*<match> will not work. | |
1586 | ||
5752674e IM |
1587 | Note: It is better to use quotes to enclose the wild cards, |
1588 | otherwise the shell may expand the parameters into names | |
1589 | of files in the local directory. | |
c072c249 | 1590 | |
1591 | # echo 'hrtimer_*' > /debug/tracing/set_ftrace_filter | |
eb6d42ea SR |
1592 | |
1593 | Produces: | |
1594 | ||
1595 | # tracer: ftrace | |
1596 | # | |
1597 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1598 | # | | | | | | |
1599 | bash-4003 [00] 1480.611794: hrtimer_init <-copy_process | |
1600 | bash-4003 [00] 1480.611941: hrtimer_start <-hrtick_set | |
1601 | bash-4003 [00] 1480.611956: hrtimer_cancel <-hrtick_clear | |
1602 | bash-4003 [00] 1480.611956: hrtimer_try_to_cancel <-hrtimer_cancel | |
1603 | <idle>-0 [00] 1480.612019: hrtimer_get_next_event <-get_next_timer_interrupt | |
1604 | <idle>-0 [00] 1480.612025: hrtimer_get_next_event <-get_next_timer_interrupt | |
1605 | <idle>-0 [00] 1480.612032: hrtimer_get_next_event <-get_next_timer_interrupt | |
1606 | <idle>-0 [00] 1480.612037: hrtimer_get_next_event <-get_next_timer_interrupt | |
1607 | <idle>-0 [00] 1480.612382: hrtimer_get_next_event <-get_next_timer_interrupt | |
1608 | ||
1609 | ||
1610 | Notice that we lost the sys_nanosleep. | |
1611 | ||
1612 | # cat /debug/tracing/set_ftrace_filter | |
1613 | hrtimer_run_queues | |
1614 | hrtimer_run_pending | |
1615 | hrtimer_init | |
1616 | hrtimer_cancel | |
1617 | hrtimer_try_to_cancel | |
1618 | hrtimer_forward | |
1619 | hrtimer_start | |
1620 | hrtimer_reprogram | |
1621 | hrtimer_force_reprogram | |
1622 | hrtimer_get_next_event | |
1623 | hrtimer_interrupt | |
1624 | hrtimer_nanosleep | |
1625 | hrtimer_wakeup | |
1626 | hrtimer_get_remaining | |
1627 | hrtimer_get_res | |
1628 | hrtimer_init_sleeper | |
1629 | ||
1630 | ||
1631 | This is because the '>' and '>>' act just like they do in bash. | |
1632 | To rewrite the filters, use '>' | |
1633 | To append to the filters, use '>>' | |
1634 | ||
5752674e IM |
1635 | To clear out a filter so that all functions will be recorded |
1636 | again: | |
eb6d42ea SR |
1637 | |
1638 | # echo > /debug/tracing/set_ftrace_filter | |
1639 | # cat /debug/tracing/set_ftrace_filter | |
1640 | # | |
1641 | ||
1642 | Again, now we want to append. | |
1643 | ||
1644 | # echo sys_nanosleep > /debug/tracing/set_ftrace_filter | |
1645 | # cat /debug/tracing/set_ftrace_filter | |
1646 | sys_nanosleep | |
c072c249 | 1647 | # echo 'hrtimer_*' >> /debug/tracing/set_ftrace_filter |
eb6d42ea SR |
1648 | # cat /debug/tracing/set_ftrace_filter |
1649 | hrtimer_run_queues | |
1650 | hrtimer_run_pending | |
1651 | hrtimer_init | |
1652 | hrtimer_cancel | |
1653 | hrtimer_try_to_cancel | |
1654 | hrtimer_forward | |
1655 | hrtimer_start | |
1656 | hrtimer_reprogram | |
1657 | hrtimer_force_reprogram | |
1658 | hrtimer_get_next_event | |
1659 | hrtimer_interrupt | |
1660 | sys_nanosleep | |
1661 | hrtimer_nanosleep | |
1662 | hrtimer_wakeup | |
1663 | hrtimer_get_remaining | |
1664 | hrtimer_get_res | |
1665 | hrtimer_init_sleeper | |
1666 | ||
1667 | ||
5752674e IM |
1668 | The set_ftrace_notrace prevents those functions from being |
1669 | traced. | |
eb6d42ea SR |
1670 | |
1671 | # echo '*preempt*' '*lock*' > /debug/tracing/set_ftrace_notrace | |
1672 | ||
1673 | Produces: | |
1674 | ||
1675 | # tracer: ftrace | |
1676 | # | |
1677 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1678 | # | | | | | | |
1679 | bash-4043 [01] 115.281644: finish_task_switch <-schedule | |
1680 | bash-4043 [01] 115.281645: hrtick_set <-schedule | |
1681 | bash-4043 [01] 115.281645: hrtick_clear <-hrtick_set | |
1682 | bash-4043 [01] 115.281646: wait_for_completion <-__stop_machine_run | |
1683 | bash-4043 [01] 115.281647: wait_for_common <-wait_for_completion | |
1684 | bash-4043 [01] 115.281647: kthread_stop <-stop_machine_run | |
1685 | bash-4043 [01] 115.281648: init_waitqueue_head <-kthread_stop | |
1686 | bash-4043 [01] 115.281648: wake_up_process <-kthread_stop | |
1687 | bash-4043 [01] 115.281649: try_to_wake_up <-wake_up_process | |
1688 | ||
1689 | We can see that there's no more lock or preempt tracing. | |
1690 | ||
985ec20a | 1691 | |
5752674e IM |
1692 | Dynamic ftrace with the function graph tracer |
1693 | --------------------------------------------- | |
985ec20a | 1694 | |
5752674e IM |
1695 | Although what has been explained above concerns both the |
1696 | function tracer and the function-graph-tracer, there are some | |
1697 | special features only available in the function-graph tracer. | |
985ec20a | 1698 | |
5752674e IM |
1699 | If you want to trace only one function and all of its children, |
1700 | you just have to echo its name into set_graph_function: | |
985ec20a | 1701 | |
5752674e | 1702 | echo __do_fault > set_graph_function |
985ec20a | 1703 | |
5752674e IM |
1704 | will produce the following "expanded" trace of the __do_fault() |
1705 | function: | |
985ec20a FW |
1706 | |
1707 | 0) | __do_fault() { | |
1708 | 0) | filemap_fault() { | |
1709 | 0) | find_lock_page() { | |
1710 | 0) 0.804 us | find_get_page(); | |
1711 | 0) | __might_sleep() { | |
1712 | 0) 1.329 us | } | |
1713 | 0) 3.904 us | } | |
1714 | 0) 4.979 us | } | |
1715 | 0) 0.653 us | _spin_lock(); | |
1716 | 0) 0.578 us | page_add_file_rmap(); | |
1717 | 0) 0.525 us | native_set_pte_at(); | |
1718 | 0) 0.585 us | _spin_unlock(); | |
1719 | 0) | unlock_page() { | |
1720 | 0) 0.541 us | page_waitqueue(); | |
1721 | 0) 0.639 us | __wake_up_bit(); | |
1722 | 0) 2.786 us | } | |
1723 | 0) + 14.237 us | } | |
1724 | 0) | __do_fault() { | |
1725 | 0) | filemap_fault() { | |
1726 | 0) | find_lock_page() { | |
1727 | 0) 0.698 us | find_get_page(); | |
1728 | 0) | __might_sleep() { | |
1729 | 0) 1.412 us | } | |
1730 | 0) 3.950 us | } | |
1731 | 0) 5.098 us | } | |
1732 | 0) 0.631 us | _spin_lock(); | |
1733 | 0) 0.571 us | page_add_file_rmap(); | |
1734 | 0) 0.526 us | native_set_pte_at(); | |
1735 | 0) 0.586 us | _spin_unlock(); | |
1736 | 0) | unlock_page() { | |
1737 | 0) 0.533 us | page_waitqueue(); | |
1738 | 0) 0.638 us | __wake_up_bit(); | |
1739 | 0) 2.793 us | } | |
1740 | 0) + 14.012 us | } | |
1741 | ||
5752674e | 1742 | You can also expand several functions at once: |
985ec20a | 1743 | |
5752674e IM |
1744 | echo sys_open > set_graph_function |
1745 | echo sys_close >> set_graph_function | |
985ec20a | 1746 | |
5752674e IM |
1747 | Now if you want to go back to trace all functions you can clear |
1748 | this special filter via: | |
985ec20a | 1749 | |
5752674e | 1750 | echo > set_graph_function |
985ec20a FW |
1751 | |
1752 | ||
eb6d42ea SR |
1753 | trace_pipe |
1754 | ---------- | |
1755 | ||
5752674e IM |
1756 | The trace_pipe outputs the same content as the trace file, but |
1757 | the effect on the tracing is different. Every read from | |
1758 | trace_pipe is consumed. This means that subsequent reads will be | |
1759 | different. The trace is live. | |
eb6d42ea | 1760 | |
9b803c0f | 1761 | # echo function > /debug/tracing/current_tracer |
eb6d42ea SR |
1762 | # cat /debug/tracing/trace_pipe > /tmp/trace.out & |
1763 | [1] 4153 | |
1764 | # echo 1 > /debug/tracing/tracing_enabled | |
1765 | # usleep 1 | |
1766 | # echo 0 > /debug/tracing/tracing_enabled | |
1767 | # cat /debug/tracing/trace | |
9b803c0f | 1768 | # tracer: function |
eb6d42ea SR |
1769 | # |
1770 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1771 | # | | | | | | |
1772 | ||
1773 | # | |
1774 | # cat /tmp/trace.out | |
1775 | bash-4043 [00] 41.267106: finish_task_switch <-schedule | |
1776 | bash-4043 [00] 41.267106: hrtick_set <-schedule | |
1777 | bash-4043 [00] 41.267107: hrtick_clear <-hrtick_set | |
1778 | bash-4043 [00] 41.267108: wait_for_completion <-__stop_machine_run | |
1779 | bash-4043 [00] 41.267108: wait_for_common <-wait_for_completion | |
1780 | bash-4043 [00] 41.267109: kthread_stop <-stop_machine_run | |
1781 | bash-4043 [00] 41.267109: init_waitqueue_head <-kthread_stop | |
1782 | bash-4043 [00] 41.267110: wake_up_process <-kthread_stop | |
1783 | bash-4043 [00] 41.267110: try_to_wake_up <-wake_up_process | |
1784 | bash-4043 [00] 41.267111: select_task_rq_rt <-try_to_wake_up | |
1785 | ||
1786 | ||
5752674e IM |
1787 | Note, reading the trace_pipe file will block until more input is |
1788 | added. By changing the tracer, trace_pipe will issue an EOF. We | |
1789 | needed to set the function tracer _before_ we "cat" the | |
1790 | trace_pipe file. | |
eb6d42ea SR |
1791 | |
1792 | ||
1793 | trace entries | |
1794 | ------------- | |
1795 | ||
5752674e IM |
1796 | Having too much or not enough data can be troublesome in |
1797 | diagnosing an issue in the kernel. The file buffer_size_kb is | |
1798 | used to modify the size of the internal trace buffers. The | |
1799 | number listed is the number of entries that can be recorded per | |
1800 | CPU. To know the full size, multiply the number of possible CPUS | |
1801 | with the number of entries. | |
eb6d42ea | 1802 | |
a94c80e7 | 1803 | # cat /debug/tracing/buffer_size_kb |
1696b2b0 | 1804 | 1408 (units kilobytes) |
eb6d42ea | 1805 | |
5752674e IM |
1806 | Note, to modify this, you must have tracing completely disabled. |
1807 | To do that, echo "nop" into the current_tracer. If the | |
1808 | current_tracer is not set to "nop", an EINVAL error will be | |
1809 | returned. | |
eb6d42ea | 1810 | |
9b803c0f | 1811 | # echo nop > /debug/tracing/current_tracer |
1696b2b0 | 1812 | # echo 10000 > /debug/tracing/buffer_size_kb |
a94c80e7 | 1813 | # cat /debug/tracing/buffer_size_kb |
1696b2b0 | 1814 | 10000 (units kilobytes) |
eb6d42ea | 1815 | |
5752674e IM |
1816 | The number of pages which will be allocated is limited to a |
1817 | percentage of available memory. Allocating too much will produce | |
1818 | an error. | |
eb6d42ea | 1819 | |
a94c80e7 | 1820 | # echo 1000000000000 > /debug/tracing/buffer_size_kb |
eb6d42ea | 1821 | -bash: echo: write error: Cannot allocate memory |
a94c80e7 | 1822 | # cat /debug/tracing/buffer_size_kb |
eb6d42ea SR |
1823 | 85 |
1824 | ||
5752674e IM |
1825 | ----------- |
1826 | ||
1827 | More details can be found in the source code, in the | |
1828 | kernel/tracing/*.c files. |