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1 | ftrace - Function Tracer |
2 | ======================== | |
3 | ||
4 | Copyright 2008 Red Hat Inc. | |
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5 | Author: Steven Rostedt <srostedt@redhat.com> |
6 | License: The GNU Free Documentation License, Version 1.2 | |
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7 | Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton, |
8 | John Kacur, and David Teigland. | |
eb6d42ea | 9 | |
f2d9c740 | 10 | Written for: 2.6.27-rc1 |
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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. | |
17 | It can be used for debugging or analyzing latencies and performance | |
18 | issues that take place outside of user-space. | |
19 | ||
20 | Although ftrace is the function tracer, it also includes an | |
21 | infrastructure that allows for other types of tracing. Some of the | |
f2d9c740 | 22 | tracers that are currently in ftrace include a tracer to trace |
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23 | context switches, the time it takes for a high priority task to |
24 | run after it was woken up, the time interrupts are disabled, and | |
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25 | more (ftrace allows for tracer plugins, which means that the list of |
26 | tracers can always grow). | |
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27 | |
28 | ||
29 | The File System | |
30 | --------------- | |
31 | ||
32 | Ftrace uses the debugfs file system to hold the control files as well | |
33 | as the files to display output. | |
34 | ||
35 | To mount the debugfs system: | |
36 | ||
37 | # mkdir /debug | |
38 | # mount -t debugfs nodev /debug | |
39 | ||
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40 | (Note: it is more common to mount at /sys/kernel/debug, but for simplicity |
41 | this document will use /debug) | |
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42 | |
43 | That's it! (assuming that you have ftrace configured into your kernel) | |
44 | ||
45 | After mounting the debugfs, you can see a directory called | |
46 | "tracing". This directory contains the control and output files | |
47 | of ftrace. Here is a list of some of the key files: | |
48 | ||
49 | ||
50 | Note: all time values are in microseconds. | |
51 | ||
52 | current_tracer : This is used to set or display the current tracer | |
53 | that is configured. | |
54 | ||
55 | available_tracers : This holds the different types of tracers that | |
a41eebab | 56 | have been compiled into the kernel. The tracers |
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57 | listed here can be configured by echoing their name |
58 | into current_tracer. | |
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59 | |
60 | tracing_enabled : This sets or displays whether the current_tracer | |
61 | is activated and tracing or not. Echo 0 into this | |
f2d9c740 | 62 | file to disable the tracer or 1 to enable it. |
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63 | |
64 | trace : This file holds the output of the trace in a human readable | |
f2d9c740 | 65 | format (described below). |
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66 | |
67 | latency_trace : This file shows the same trace but the information | |
68 | is organized more to display possible latencies | |
f2d9c740 | 69 | in the system (described below). |
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70 | |
71 | trace_pipe : The output is the same as the "trace" file but this | |
72 | file is meant to be streamed with live tracing. | |
73 | Reads from this file will block until new data | |
74 | is retrieved. Unlike the "trace" and "latency_trace" | |
75 | files, this file is a consumer. This means reading | |
76 | from this file causes sequential reads to display | |
77 | more current data. Once data is read from this | |
78 | file, it is consumed, and will not be read | |
79 | again with a sequential read. The "trace" and | |
80 | "latency_trace" files are static, and if the | |
f2d9c740 | 81 | tracer is not adding more data, they will display |
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82 | the same information every time they are read. |
83 | ||
84 | iter_ctrl : This file lets the user control the amount of data | |
85 | that is displayed in one of the above output | |
86 | files. | |
87 | ||
88 | trace_max_latency : Some of the tracers record the max latency. | |
89 | For example, the time interrupts are disabled. | |
90 | This time is saved in this file. The max trace | |
91 | will also be stored, and displayed by either | |
92 | "trace" or "latency_trace". A new max trace will | |
93 | only be recorded if the latency is greater than | |
94 | the value in this file. (in microseconds) | |
95 | ||
96 | trace_entries : This sets or displays the number of trace | |
97 | entries each CPU buffer can hold. The tracer buffers | |
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98 | are the same size for each CPU. The displayed number |
99 | is the size of the CPU buffer and not total size. The | |
100 | trace buffers are allocated in pages (blocks of memory | |
101 | that the kernel uses for allocation, usually 4 KB in size). | |
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102 | Since each entry is smaller than a page, if the last |
103 | allocated page has room for more entries than were | |
104 | requested, the rest of the page is used to allocate | |
105 | entries. | |
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106 | |
107 | This can only be updated when the current_tracer | |
108 | is set to "none". | |
109 | ||
110 | NOTE: It is planned on changing the allocated buffers | |
111 | from being the number of possible CPUS to | |
112 | the number of online CPUS. | |
113 | ||
114 | tracing_cpumask : This is a mask that lets the user only trace | |
115 | on specified CPUS. The format is a hex string | |
116 | representing the CPUS. | |
117 | ||
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118 | set_ftrace_filter : When dynamic ftrace is configured in (see the |
119 | section below "dynamic ftrace"), the code is dynamically | |
120 | modified (code text rewrite) to disable calling of the | |
121 | function profiler (mcount). This lets tracing be configured | |
122 | in with practically no overhead in performance. This also | |
123 | has a side effect of enabling or disabling specific functions | |
124 | to be traced. Echoing names of functions into this file | |
125 | will limit the trace to only those functions. | |
126 | ||
127 | set_ftrace_notrace: This has an effect opposite to that of | |
128 | set_ftrace_filter. Any function that is added here will not | |
129 | be traced. If a function exists in both set_ftrace_filter | |
130 | and set_ftrace_notrace, the function will _not_ be traced. | |
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131 | |
132 | available_filter_functions : When a function is encountered the first | |
133 | time by the dynamic tracer, it is recorded and | |
134 | later the call is converted into a nop. This file | |
135 | lists the functions that have been recorded | |
136 | by the dynamic tracer and these functions can | |
137 | be used to set the ftrace filter by the above | |
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138 | "set_ftrace_filter" file. (See the section "dynamic ftrace" |
139 | below for more details). | |
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140 | |
141 | ||
142 | The Tracers | |
143 | ----------- | |
144 | ||
f2d9c740 | 145 | Here is the list of current tracers that may be configured. |
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146 | |
147 | ftrace - function tracer that uses mcount to trace all functions. | |
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148 | |
149 | sched_switch - traces the context switches between tasks. | |
150 | ||
f2d9c740 | 151 | irqsoff - traces the areas that disable interrupts and saves |
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152 | the trace with the longest max latency. |
153 | See tracing_max_latency. When a new max is recorded, | |
154 | it replaces the old trace. It is best to view this | |
f2d9c740 | 155 | trace via the latency_trace file. |
eb6d42ea | 156 | |
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157 | preemptoff - Similar to irqsoff but traces and records the amount of |
158 | time for which preemption is disabled. | |
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159 | |
160 | preemptirqsoff - Similar to irqsoff and preemptoff, but traces and | |
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161 | records the largest time for which irqs and/or preemption |
162 | is disabled. | |
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163 | |
164 | wakeup - Traces and records the max latency that it takes for | |
165 | the highest priority task to get scheduled after | |
166 | it has been woken up. | |
167 | ||
168 | none - This is not a tracer. To remove all tracers from tracing | |
169 | simply echo "none" into current_tracer. | |
170 | ||
171 | ||
172 | Examples of using the tracer | |
173 | ---------------------------- | |
174 | ||
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175 | Here are typical examples of using the tracers when controlling them only |
176 | with the debugfs interface (without using any user-land utilities). | |
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177 | |
178 | Output format: | |
179 | -------------- | |
180 | ||
f2d9c740 | 181 | Here is an example of the output format of the file "trace" |
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182 | |
183 | -------- | |
184 | # tracer: ftrace | |
185 | # | |
186 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
187 | # | | | | | | |
188 | bash-4251 [01] 10152.583854: path_put <-path_walk | |
189 | bash-4251 [01] 10152.583855: dput <-path_put | |
190 | bash-4251 [01] 10152.583855: _atomic_dec_and_lock <-dput | |
191 | -------- | |
192 | ||
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193 | A header is printed with the tracer name that is represented by the trace. |
194 | In this case the tracer is "ftrace". Then a header showing the format. Task | |
195 | name "bash", the task PID "4251", the CPU that it was running on | |
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196 | "01", the timestamp in <secs>.<usecs> format, the function name that was |
197 | traced "path_put" and the parent function that called this function | |
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198 | "path_walk". The timestamp is the time at which the function was |
199 | entered. | |
eb6d42ea | 200 | |
f2d9c740 | 201 | The sched_switch tracer also includes tracing of task wakeups and |
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202 | context switches. |
203 | ||
204 | ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 2916:115:S | |
205 | ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 10:115:S | |
206 | ksoftirqd/1-7 [01] 1453.070013: 7:115:R ==> 10:115:R | |
207 | events/1-10 [01] 1453.070013: 10:115:S ==> 2916:115:R | |
208 | kondemand/1-2916 [01] 1453.070013: 2916:115:S ==> 7:115:R | |
209 | ksoftirqd/1-7 [01] 1453.070013: 7:115:S ==> 0:140:R | |
210 | ||
f2d9c740 | 211 | Wake ups are represented by a "+" and the context switches are shown as |
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212 | "==>". The format is: |
213 | ||
214 | Context switches: | |
215 | ||
216 | Previous task Next Task | |
217 | ||
218 | <pid>:<prio>:<state> ==> <pid>:<prio>:<state> | |
219 | ||
220 | Wake ups: | |
221 | ||
222 | Current task Task waking up | |
223 | ||
224 | <pid>:<prio>:<state> + <pid>:<prio>:<state> | |
225 | ||
f2d9c740 | 226 | The prio is the internal kernel priority, which is the inverse of the |
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227 | priority that is usually displayed by user-space tools. Zero represents |
228 | the highest priority (99). Prio 100 starts the "nice" priorities with | |
229 | 100 being equal to nice -20 and 139 being nice 19. The prio "140" is | |
230 | reserved for the idle task which is the lowest priority thread (pid 0). | |
231 | ||
232 | ||
233 | Latency trace format | |
234 | -------------------- | |
235 | ||
236 | For traces that display latency times, the latency_trace file gives | |
f2d9c740 | 237 | somewhat more information to see why a latency happened. Here is a typical |
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238 | trace. |
239 | ||
240 | # tracer: irqsoff | |
241 | # | |
242 | irqsoff latency trace v1.1.5 on 2.6.26-rc8 | |
243 | -------------------------------------------------------------------- | |
244 | latency: 97 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
245 | ----------------- | |
246 | | task: swapper-0 (uid:0 nice:0 policy:0 rt_prio:0) | |
247 | ----------------- | |
248 | => started at: apic_timer_interrupt | |
249 | => ended at: do_softirq | |
250 | ||
251 | # _------=> CPU# | |
252 | # / _-----=> irqs-off | |
253 | # | / _----=> need-resched | |
254 | # || / _---=> hardirq/softirq | |
255 | # ||| / _--=> preempt-depth | |
256 | # |||| / | |
257 | # ||||| delay | |
258 | # cmd pid ||||| time | caller | |
259 | # \ / ||||| \ | / | |
260 | <idle>-0 0d..1 0us+: trace_hardirqs_off_thunk (apic_timer_interrupt) | |
261 | <idle>-0 0d.s. 97us : __do_softirq (do_softirq) | |
262 | <idle>-0 0d.s1 98us : trace_hardirqs_on (do_softirq) | |
263 | ||
264 | ||
eb6d42ea | 265 | |
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266 | This shows that the current tracer is "irqsoff" tracing the time for which |
267 | interrupts were disabled. It gives the trace version and the version | |
268 | of the kernel upon which this was executed on (2.6.26-rc8). Then it displays | |
269 | the max latency in microsecs (97 us). The number of trace entries displayed | |
270 | and the total number recorded (both are three: #3/3). The type of | |
eb6d42ea | 271 | preemption that was used (PREEMPT). VP, KP, SP, and HP are always zero |
f2d9c740 | 272 | and are reserved for later use. #P is the number of online CPUS (#P:2). |
eb6d42ea | 273 | |
f2d9c740 | 274 | The task is the process that was running when the latency occurred. |
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275 | (swapper pid: 0). |
276 | ||
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277 | The start and stop (the functions in which the interrupts were disabled and |
278 | enabled respectively) that caused the latencies: | |
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279 | |
280 | apic_timer_interrupt is where the interrupts were disabled. | |
281 | do_softirq is where they were enabled again. | |
282 | ||
283 | The next lines after the header are the trace itself. The header | |
284 | explains which is which. | |
285 | ||
286 | cmd: The name of the process in the trace. | |
287 | ||
288 | pid: The PID of that process. | |
289 | ||
f2d9c740 | 290 | CPU#: The CPU which the process was running on. |
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291 | |
292 | irqs-off: 'd' interrupts are disabled. '.' otherwise. | |
293 | ||
294 | need-resched: 'N' task need_resched is set, '.' otherwise. | |
295 | ||
296 | hardirq/softirq: | |
f2d9c740 | 297 | 'H' - hard irq occurred inside a softirq. |
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298 | 'h' - hard irq is running |
299 | 's' - soft irq is running | |
300 | '.' - normal context. | |
301 | ||
302 | preempt-depth: The level of preempt_disabled | |
303 | ||
304 | The above is mostly meaningful for kernel developers. | |
305 | ||
a41eebab | 306 | time: This differs from the trace file output. The trace file output |
f2d9c740 | 307 | includes an absolute timestamp. The timestamp used by the |
a41eebab | 308 | latency_trace file is relative to the start of the trace. |
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309 | |
310 | delay: This is just to help catch your eye a bit better. And | |
311 | needs to be fixed to be only relative to the same CPU. | |
a41eebab | 312 | The marks are determined by the difference between this |
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313 | current trace and the next trace. |
314 | '!' - greater than preempt_mark_thresh (default 100) | |
315 | '+' - greater than 1 microsecond | |
316 | ' ' - less than or equal to 1 microsecond. | |
317 | ||
318 | The rest is the same as the 'trace' file. | |
319 | ||
320 | ||
321 | iter_ctrl | |
322 | --------- | |
323 | ||
324 | The iter_ctrl file is used to control what gets printed in the trace | |
325 | output. To see what is available, simply cat the file: | |
326 | ||
327 | cat /debug/tracing/iter_ctrl | |
328 | print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \ | |
329 | noblock nostacktrace nosched-tree | |
330 | ||
a41eebab | 331 | To disable one of the options, echo in the option prepended with "no". |
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332 | |
333 | echo noprint-parent > /debug/tracing/iter_ctrl | |
334 | ||
335 | To enable an option, leave off the "no". | |
336 | ||
a41eebab | 337 | echo sym-offset > /debug/tracing/iter_ctrl |
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338 | |
339 | Here are the available options: | |
340 | ||
341 | print-parent - On function traces, display the calling function | |
342 | as well as the function being traced. | |
343 | ||
344 | print-parent: | |
345 | bash-4000 [01] 1477.606694: simple_strtoul <-strict_strtoul | |
346 | ||
347 | noprint-parent: | |
348 | bash-4000 [01] 1477.606694: simple_strtoul | |
349 | ||
350 | ||
351 | sym-offset - Display not only the function name, but also the offset | |
352 | in the function. For example, instead of seeing just | |
a41eebab | 353 | "ktime_get", you will see "ktime_get+0xb/0x20". |
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354 | |
355 | sym-offset: | |
356 | bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0 | |
357 | ||
358 | sym-addr - this will also display the function address as well as | |
359 | the function name. | |
360 | ||
361 | sym-addr: | |
362 | bash-4000 [01] 1477.606694: simple_strtoul <c0339346> | |
363 | ||
364 | verbose - This deals with the latency_trace file. | |
365 | ||
366 | bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \ | |
367 | (+0.000ms): simple_strtoul (strict_strtoul) | |
368 | ||
369 | raw - This will display raw numbers. This option is best for use with | |
370 | user applications that can translate the raw numbers better than | |
371 | having it done in the kernel. | |
372 | ||
a41eebab | 373 | hex - Similar to raw, but the numbers will be in a hexadecimal format. |
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374 | |
375 | bin - This will print out the formats in raw binary. | |
376 | ||
377 | block - TBD (needs update) | |
378 | ||
379 | stacktrace - This is one of the options that changes the trace itself. | |
380 | When a trace is recorded, so is the stack of functions. | |
381 | This allows for back traces of trace sites. | |
382 | ||
383 | sched-tree - TBD (any users??) | |
384 | ||
385 | ||
386 | sched_switch | |
387 | ------------ | |
388 | ||
f2d9c740 | 389 | This tracer simply records schedule switches. Here is an example |
a41eebab | 390 | of how to use it. |
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391 | |
392 | # echo sched_switch > /debug/tracing/current_tracer | |
393 | # echo 1 > /debug/tracing/tracing_enabled | |
394 | # sleep 1 | |
395 | # echo 0 > /debug/tracing/tracing_enabled | |
396 | # cat /debug/tracing/trace | |
397 | ||
398 | # tracer: sched_switch | |
399 | # | |
400 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
401 | # | | | | | | |
402 | bash-3997 [01] 240.132281: 3997:120:R + 4055:120:R | |
403 | bash-3997 [01] 240.132284: 3997:120:R ==> 4055:120:R | |
404 | sleep-4055 [01] 240.132371: 4055:120:S ==> 3997:120:R | |
405 | bash-3997 [01] 240.132454: 3997:120:R + 4055:120:S | |
406 | bash-3997 [01] 240.132457: 3997:120:R ==> 4055:120:R | |
407 | sleep-4055 [01] 240.132460: 4055:120:D ==> 3997:120:R | |
408 | bash-3997 [01] 240.132463: 3997:120:R + 4055:120:D | |
409 | bash-3997 [01] 240.132465: 3997:120:R ==> 4055:120:R | |
410 | <idle>-0 [00] 240.132589: 0:140:R + 4:115:S | |
411 | <idle>-0 [00] 240.132591: 0:140:R ==> 4:115:R | |
412 | ksoftirqd/0-4 [00] 240.132595: 4:115:S ==> 0:140:R | |
413 | <idle>-0 [00] 240.132598: 0:140:R + 4:115:S | |
414 | <idle>-0 [00] 240.132599: 0:140:R ==> 4:115:R | |
415 | ksoftirqd/0-4 [00] 240.132603: 4:115:S ==> 0:140:R | |
416 | sleep-4055 [01] 240.133058: 4055:120:S ==> 3997:120:R | |
417 | [...] | |
418 | ||
419 | ||
420 | As we have discussed previously about this format, the header shows | |
421 | the name of the trace and points to the options. The "FUNCTION" | |
422 | is a misnomer since here it represents the wake ups and context | |
423 | switches. | |
424 | ||
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425 | The sched_switch file only lists the wake ups (represented with '+') |
426 | and context switches ('==>') with the previous task or current task | |
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427 | first followed by the next task or task waking up. The format for both |
428 | of these is PID:KERNEL-PRIO:TASK-STATE. Remember that the KERNEL-PRIO | |
429 | is the inverse of the actual priority with zero (0) being the highest | |
430 | priority and the nice values starting at 100 (nice -20). Below is | |
431 | a quick chart to map the kernel priority to user land priorities. | |
432 | ||
433 | Kernel priority: 0 to 99 ==> user RT priority 99 to 0 | |
434 | Kernel priority: 100 to 139 ==> user nice -20 to 19 | |
435 | Kernel priority: 140 ==> idle task priority | |
436 | ||
437 | The task states are: | |
438 | ||
439 | R - running : wants to run, may not actually be running | |
440 | S - sleep : process is waiting to be woken up (handles signals) | |
f2d9c740 SR |
441 | D - disk sleep (uninterruptible sleep) : process must be woken up |
442 | (ignores signals) | |
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443 | T - stopped : process suspended |
444 | t - traced : process is being traced (with something like gdb) | |
445 | Z - zombie : process waiting to be cleaned up | |
446 | X - unknown | |
447 | ||
448 | ||
449 | ftrace_enabled | |
450 | -------------- | |
451 | ||
f2d9c740 SR |
452 | The following tracers (listed below) give different output depending |
453 | on whether or not the sysctl ftrace_enabled is set. To set ftrace_enabled, | |
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454 | one can either use the sysctl function or set it via the proc |
455 | file system interface. | |
456 | ||
457 | sysctl kernel.ftrace_enabled=1 | |
458 | ||
459 | or | |
460 | ||
461 | echo 1 > /proc/sys/kernel/ftrace_enabled | |
462 | ||
463 | To disable ftrace_enabled simply replace the '1' with '0' in | |
464 | the above commands. | |
465 | ||
466 | When ftrace_enabled is set the tracers will also record the functions | |
467 | that are within the trace. The descriptions of the tracers | |
468 | will also show an example with ftrace enabled. | |
469 | ||
470 | ||
471 | irqsoff | |
472 | ------- | |
473 | ||
474 | When interrupts are disabled, the CPU can not react to any other | |
475 | external event (besides NMIs and SMIs). This prevents the timer | |
476 | interrupt from triggering or the mouse interrupt from letting the | |
477 | kernel know of a new mouse event. The result is a latency with the | |
478 | reaction time. | |
479 | ||
f2d9c740 SR |
480 | The irqsoff tracer tracks the time for which interrupts are disabled. |
481 | When a new maximum latency is hit, the tracer saves the trace leading up | |
482 | to that latency point so that every time a new maximum is reached, the old | |
483 | saved trace is discarded and the new trace is saved. | |
eb6d42ea | 484 | |
f2d9c740 | 485 | To reset the maximum, echo 0 into tracing_max_latency. Here is an |
eb6d42ea SR |
486 | example: |
487 | ||
488 | # echo irqsoff > /debug/tracing/current_tracer | |
489 | # echo 0 > /debug/tracing/tracing_max_latency | |
490 | # echo 1 > /debug/tracing/tracing_enabled | |
491 | # ls -ltr | |
492 | [...] | |
493 | # echo 0 > /debug/tracing/tracing_enabled | |
494 | # cat /debug/tracing/latency_trace | |
495 | # tracer: irqsoff | |
496 | # | |
f2d9c740 | 497 | irqsoff latency trace v1.1.5 on 2.6.26 |
eb6d42ea | 498 | -------------------------------------------------------------------- |
f2d9c740 | 499 | latency: 12 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) |
eb6d42ea | 500 | ----------------- |
f2d9c740 | 501 | | task: bash-3730 (uid:0 nice:0 policy:0 rt_prio:0) |
eb6d42ea | 502 | ----------------- |
f2d9c740 SR |
503 | => started at: sys_setpgid |
504 | => ended at: sys_setpgid | |
eb6d42ea SR |
505 | |
506 | # _------=> CPU# | |
507 | # / _-----=> irqs-off | |
508 | # | / _----=> need-resched | |
509 | # || / _---=> hardirq/softirq | |
510 | # ||| / _--=> preempt-depth | |
511 | # |||| / | |
512 | # ||||| delay | |
513 | # cmd pid ||||| time | caller | |
514 | # \ / ||||| \ | / | |
f2d9c740 SR |
515 | bash-3730 1d... 0us : _write_lock_irq (sys_setpgid) |
516 | bash-3730 1d..1 1us+: _write_unlock_irq (sys_setpgid) | |
517 | bash-3730 1d..2 14us : trace_hardirqs_on (sys_setpgid) | |
eb6d42ea | 518 | |
eb6d42ea | 519 | |
f2d9c740 SR |
520 | Here we see that that we had a latency of 12 microsecs (which is |
521 | very good). The _write_lock_irq in sys_setpgid disabled interrupts. | |
522 | The difference between the 12 and the displayed timestamp 14us occurred | |
523 | because the clock was incremented between the time of recording the max | |
524 | latency and the time of recording the function that had that latency. | |
eb6d42ea | 525 | |
f2d9c740 SR |
526 | Note the above example had ftrace_enabled not set. If we set the |
527 | ftrace_enabled, we get a much larger output: | |
eb6d42ea SR |
528 | |
529 | # tracer: irqsoff | |
530 | # | |
531 | irqsoff latency trace v1.1.5 on 2.6.26-rc8 | |
532 | -------------------------------------------------------------------- | |
533 | latency: 50 us, #101/101, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
534 | ----------------- | |
535 | | task: ls-4339 (uid:0 nice:0 policy:0 rt_prio:0) | |
536 | ----------------- | |
537 | => started at: __alloc_pages_internal | |
538 | => ended at: __alloc_pages_internal | |
539 | ||
540 | # _------=> CPU# | |
541 | # / _-----=> irqs-off | |
542 | # | / _----=> need-resched | |
543 | # || / _---=> hardirq/softirq | |
544 | # ||| / _--=> preempt-depth | |
545 | # |||| / | |
546 | # ||||| delay | |
547 | # cmd pid ||||| time | caller | |
548 | # \ / ||||| \ | / | |
549 | ls-4339 0...1 0us+: get_page_from_freelist (__alloc_pages_internal) | |
550 | ls-4339 0d..1 3us : rmqueue_bulk (get_page_from_freelist) | |
551 | ls-4339 0d..1 3us : _spin_lock (rmqueue_bulk) | |
552 | ls-4339 0d..1 4us : add_preempt_count (_spin_lock) | |
553 | ls-4339 0d..2 4us : __rmqueue (rmqueue_bulk) | |
554 | ls-4339 0d..2 5us : __rmqueue_smallest (__rmqueue) | |
555 | ls-4339 0d..2 5us : __mod_zone_page_state (__rmqueue_smallest) | |
556 | ls-4339 0d..2 6us : __rmqueue (rmqueue_bulk) | |
557 | ls-4339 0d..2 6us : __rmqueue_smallest (__rmqueue) | |
558 | ls-4339 0d..2 7us : __mod_zone_page_state (__rmqueue_smallest) | |
559 | ls-4339 0d..2 7us : __rmqueue (rmqueue_bulk) | |
560 | ls-4339 0d..2 8us : __rmqueue_smallest (__rmqueue) | |
561 | [...] | |
562 | ls-4339 0d..2 46us : __rmqueue_smallest (__rmqueue) | |
563 | ls-4339 0d..2 47us : __mod_zone_page_state (__rmqueue_smallest) | |
564 | ls-4339 0d..2 47us : __rmqueue (rmqueue_bulk) | |
565 | ls-4339 0d..2 48us : __rmqueue_smallest (__rmqueue) | |
566 | ls-4339 0d..2 48us : __mod_zone_page_state (__rmqueue_smallest) | |
567 | ls-4339 0d..2 49us : _spin_unlock (rmqueue_bulk) | |
568 | ls-4339 0d..2 49us : sub_preempt_count (_spin_unlock) | |
569 | ls-4339 0d..1 50us : get_page_from_freelist (__alloc_pages_internal) | |
570 | ls-4339 0d..2 51us : trace_hardirqs_on (__alloc_pages_internal) | |
571 | ||
572 | ||
eb6d42ea SR |
573 | |
574 | Here we traced a 50 microsecond latency. But we also see all the | |
a41eebab | 575 | functions that were called during that time. Note that by enabling |
f2d9c740 | 576 | function tracing, we incur an added overhead. This overhead may |
a41eebab SR |
577 | extend the latency times. But nevertheless, this trace has provided |
578 | some very helpful debugging information. | |
eb6d42ea SR |
579 | |
580 | ||
581 | preemptoff | |
582 | ---------- | |
583 | ||
a41eebab SR |
584 | When preemption is disabled, we may be able to receive interrupts but |
585 | the task cannot be preempted and a higher priority task must wait | |
eb6d42ea SR |
586 | for preemption to be enabled again before it can preempt a lower |
587 | priority task. | |
588 | ||
a41eebab | 589 | The preemptoff tracer traces the places that disable preemption. |
f2d9c740 SR |
590 | Like the irqsoff tracer, it records the maximum latency for which preemption |
591 | was disabled. The control of preemptoff tracer is much like the irqsoff | |
592 | tracer. | |
eb6d42ea SR |
593 | |
594 | # echo preemptoff > /debug/tracing/current_tracer | |
595 | # echo 0 > /debug/tracing/tracing_max_latency | |
596 | # echo 1 > /debug/tracing/tracing_enabled | |
597 | # ls -ltr | |
598 | [...] | |
599 | # echo 0 > /debug/tracing/tracing_enabled | |
600 | # cat /debug/tracing/latency_trace | |
601 | # tracer: preemptoff | |
602 | # | |
603 | preemptoff latency trace v1.1.5 on 2.6.26-rc8 | |
604 | -------------------------------------------------------------------- | |
605 | latency: 29 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
606 | ----------------- | |
607 | | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) | |
608 | ----------------- | |
609 | => started at: do_IRQ | |
610 | => ended at: __do_softirq | |
611 | ||
612 | # _------=> CPU# | |
613 | # / _-----=> irqs-off | |
614 | # | / _----=> need-resched | |
615 | # || / _---=> hardirq/softirq | |
616 | # ||| / _--=> preempt-depth | |
617 | # |||| / | |
618 | # ||||| delay | |
619 | # cmd pid ||||| time | caller | |
620 | # \ / ||||| \ | / | |
621 | sshd-4261 0d.h. 0us+: irq_enter (do_IRQ) | |
622 | sshd-4261 0d.s. 29us : _local_bh_enable (__do_softirq) | |
623 | sshd-4261 0d.s1 30us : trace_preempt_on (__do_softirq) | |
624 | ||
625 | ||
eb6d42ea SR |
626 | This has some more changes. Preemption was disabled when an interrupt |
627 | came in (notice the 'h'), and was enabled while doing a softirq. | |
628 | (notice the 's'). But we also see that interrupts have been disabled | |
629 | when entering the preempt off section and leaving it (the 'd'). | |
630 | We do not know if interrupts were enabled in the mean time. | |
631 | ||
632 | # tracer: preemptoff | |
633 | # | |
634 | preemptoff latency trace v1.1.5 on 2.6.26-rc8 | |
635 | -------------------------------------------------------------------- | |
636 | latency: 63 us, #87/87, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
637 | ----------------- | |
638 | | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) | |
639 | ----------------- | |
640 | => started at: remove_wait_queue | |
641 | => ended at: __do_softirq | |
642 | ||
643 | # _------=> CPU# | |
644 | # / _-----=> irqs-off | |
645 | # | / _----=> need-resched | |
646 | # || / _---=> hardirq/softirq | |
647 | # ||| / _--=> preempt-depth | |
648 | # |||| / | |
649 | # ||||| delay | |
650 | # cmd pid ||||| time | caller | |
651 | # \ / ||||| \ | / | |
652 | sshd-4261 0d..1 0us : _spin_lock_irqsave (remove_wait_queue) | |
653 | sshd-4261 0d..1 1us : _spin_unlock_irqrestore (remove_wait_queue) | |
654 | sshd-4261 0d..1 2us : do_IRQ (common_interrupt) | |
655 | sshd-4261 0d..1 2us : irq_enter (do_IRQ) | |
656 | sshd-4261 0d..1 2us : idle_cpu (irq_enter) | |
657 | sshd-4261 0d..1 3us : add_preempt_count (irq_enter) | |
658 | sshd-4261 0d.h1 3us : idle_cpu (irq_enter) | |
659 | sshd-4261 0d.h. 4us : handle_fasteoi_irq (do_IRQ) | |
660 | [...] | |
661 | sshd-4261 0d.h. 12us : add_preempt_count (_spin_lock) | |
662 | sshd-4261 0d.h1 12us : ack_ioapic_quirk_irq (handle_fasteoi_irq) | |
663 | sshd-4261 0d.h1 13us : move_native_irq (ack_ioapic_quirk_irq) | |
664 | sshd-4261 0d.h1 13us : _spin_unlock (handle_fasteoi_irq) | |
665 | sshd-4261 0d.h1 14us : sub_preempt_count (_spin_unlock) | |
666 | sshd-4261 0d.h1 14us : irq_exit (do_IRQ) | |
667 | sshd-4261 0d.h1 15us : sub_preempt_count (irq_exit) | |
668 | sshd-4261 0d..2 15us : do_softirq (irq_exit) | |
669 | sshd-4261 0d... 15us : __do_softirq (do_softirq) | |
670 | sshd-4261 0d... 16us : __local_bh_disable (__do_softirq) | |
671 | sshd-4261 0d... 16us+: add_preempt_count (__local_bh_disable) | |
672 | sshd-4261 0d.s4 20us : add_preempt_count (__local_bh_disable) | |
673 | sshd-4261 0d.s4 21us : sub_preempt_count (local_bh_enable) | |
674 | sshd-4261 0d.s5 21us : sub_preempt_count (local_bh_enable) | |
675 | [...] | |
676 | sshd-4261 0d.s6 41us : add_preempt_count (__local_bh_disable) | |
677 | sshd-4261 0d.s6 42us : sub_preempt_count (local_bh_enable) | |
678 | sshd-4261 0d.s7 42us : sub_preempt_count (local_bh_enable) | |
679 | sshd-4261 0d.s5 43us : add_preempt_count (__local_bh_disable) | |
680 | sshd-4261 0d.s5 43us : sub_preempt_count (local_bh_enable_ip) | |
681 | sshd-4261 0d.s6 44us : sub_preempt_count (local_bh_enable_ip) | |
682 | sshd-4261 0d.s5 44us : add_preempt_count (__local_bh_disable) | |
683 | sshd-4261 0d.s5 45us : sub_preempt_count (local_bh_enable) | |
684 | [...] | |
685 | sshd-4261 0d.s. 63us : _local_bh_enable (__do_softirq) | |
686 | sshd-4261 0d.s1 64us : trace_preempt_on (__do_softirq) | |
687 | ||
688 | ||
689 | The above is an example of the preemptoff trace with ftrace_enabled | |
690 | set. Here we see that interrupts were disabled the entire time. | |
691 | The irq_enter code lets us know that we entered an interrupt 'h'. | |
692 | Before that, the functions being traced still show that it is not | |
f2d9c740 | 693 | in an interrupt, but we can see from the functions themselves that |
eb6d42ea SR |
694 | this is not the case. |
695 | ||
f2d9c740 SR |
696 | Notice that __do_softirq when called does not have a preempt_count. |
697 | It may seem that we missed a preempt enabling. What really happened | |
698 | is that the preempt count is held on the thread's stack and we | |
eb6d42ea | 699 | switched to the softirq stack (4K stacks in effect). The code |
a41eebab | 700 | does not copy the preempt count, but because interrupts are disabled, |
f2d9c740 SR |
701 | we do not need to worry about it. Having a tracer like this is good |
702 | for letting people know what really happens inside the kernel. | |
eb6d42ea SR |
703 | |
704 | ||
705 | preemptirqsoff | |
706 | -------------- | |
707 | ||
708 | Knowing the locations that have interrupts disabled or preemption | |
709 | disabled for the longest times is helpful. But sometimes we would | |
710 | like to know when either preemption and/or interrupts are disabled. | |
711 | ||
f2d9c740 | 712 | Consider the following code: |
eb6d42ea SR |
713 | |
714 | local_irq_disable(); | |
715 | call_function_with_irqs_off(); | |
716 | preempt_disable(); | |
717 | call_function_with_irqs_and_preemption_off(); | |
718 | local_irq_enable(); | |
719 | call_function_with_preemption_off(); | |
720 | preempt_enable(); | |
721 | ||
722 | The irqsoff tracer will record the total length of | |
723 | call_function_with_irqs_off() and | |
724 | call_function_with_irqs_and_preemption_off(). | |
725 | ||
726 | The preemptoff tracer will record the total length of | |
727 | call_function_with_irqs_and_preemption_off() and | |
728 | call_function_with_preemption_off(). | |
729 | ||
730 | But neither will trace the time that interrupts and/or preemption | |
731 | is disabled. This total time is the time that we can not schedule. | |
732 | To record this time, use the preemptirqsoff tracer. | |
733 | ||
734 | Again, using this trace is much like the irqsoff and preemptoff tracers. | |
735 | ||
a41eebab | 736 | # echo preemptirqsoff > /debug/tracing/current_tracer |
eb6d42ea SR |
737 | # echo 0 > /debug/tracing/tracing_max_latency |
738 | # echo 1 > /debug/tracing/tracing_enabled | |
739 | # ls -ltr | |
740 | [...] | |
741 | # echo 0 > /debug/tracing/tracing_enabled | |
742 | # cat /debug/tracing/latency_trace | |
743 | # tracer: preemptirqsoff | |
744 | # | |
745 | preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8 | |
746 | -------------------------------------------------------------------- | |
747 | latency: 293 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
748 | ----------------- | |
749 | | task: ls-4860 (uid:0 nice:0 policy:0 rt_prio:0) | |
750 | ----------------- | |
751 | => started at: apic_timer_interrupt | |
752 | => ended at: __do_softirq | |
753 | ||
754 | # _------=> CPU# | |
755 | # / _-----=> irqs-off | |
756 | # | / _----=> need-resched | |
757 | # || / _---=> hardirq/softirq | |
758 | # ||| / _--=> preempt-depth | |
759 | # |||| / | |
760 | # ||||| delay | |
761 | # cmd pid ||||| time | caller | |
762 | # \ / ||||| \ | / | |
763 | ls-4860 0d... 0us!: trace_hardirqs_off_thunk (apic_timer_interrupt) | |
764 | ls-4860 0d.s. 294us : _local_bh_enable (__do_softirq) | |
765 | ls-4860 0d.s1 294us : trace_preempt_on (__do_softirq) | |
766 | ||
767 | ||
eb6d42ea SR |
768 | |
769 | The trace_hardirqs_off_thunk is called from assembly on x86 when | |
770 | interrupts are disabled in the assembly code. Without the function | |
f2d9c740 | 771 | tracing, we do not know if interrupts were enabled within the preemption |
eb6d42ea SR |
772 | points. We do see that it started with preemption enabled. |
773 | ||
774 | Here is a trace with ftrace_enabled set: | |
775 | ||
776 | ||
777 | # tracer: preemptirqsoff | |
778 | # | |
779 | preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8 | |
780 | -------------------------------------------------------------------- | |
781 | latency: 105 us, #183/183, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
782 | ----------------- | |
783 | | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) | |
784 | ----------------- | |
785 | => started at: write_chan | |
786 | => ended at: __do_softirq | |
787 | ||
788 | # _------=> CPU# | |
789 | # / _-----=> irqs-off | |
790 | # | / _----=> need-resched | |
791 | # || / _---=> hardirq/softirq | |
792 | # ||| / _--=> preempt-depth | |
793 | # |||| / | |
794 | # ||||| delay | |
795 | # cmd pid ||||| time | caller | |
796 | # \ / ||||| \ | / | |
797 | ls-4473 0.N.. 0us : preempt_schedule (write_chan) | |
798 | ls-4473 0dN.1 1us : _spin_lock (schedule) | |
799 | ls-4473 0dN.1 2us : add_preempt_count (_spin_lock) | |
800 | ls-4473 0d..2 2us : put_prev_task_fair (schedule) | |
801 | [...] | |
802 | ls-4473 0d..2 13us : set_normalized_timespec (ktime_get_ts) | |
803 | ls-4473 0d..2 13us : __switch_to (schedule) | |
804 | sshd-4261 0d..2 14us : finish_task_switch (schedule) | |
805 | sshd-4261 0d..2 14us : _spin_unlock_irq (finish_task_switch) | |
806 | sshd-4261 0d..1 15us : add_preempt_count (_spin_lock_irqsave) | |
807 | sshd-4261 0d..2 16us : _spin_unlock_irqrestore (hrtick_set) | |
808 | sshd-4261 0d..2 16us : do_IRQ (common_interrupt) | |
809 | sshd-4261 0d..2 17us : irq_enter (do_IRQ) | |
810 | sshd-4261 0d..2 17us : idle_cpu (irq_enter) | |
811 | sshd-4261 0d..2 18us : add_preempt_count (irq_enter) | |
812 | sshd-4261 0d.h2 18us : idle_cpu (irq_enter) | |
813 | sshd-4261 0d.h. 18us : handle_fasteoi_irq (do_IRQ) | |
814 | sshd-4261 0d.h. 19us : _spin_lock (handle_fasteoi_irq) | |
815 | sshd-4261 0d.h. 19us : add_preempt_count (_spin_lock) | |
816 | sshd-4261 0d.h1 20us : _spin_unlock (handle_fasteoi_irq) | |
817 | sshd-4261 0d.h1 20us : sub_preempt_count (_spin_unlock) | |
818 | [...] | |
819 | sshd-4261 0d.h1 28us : _spin_unlock (handle_fasteoi_irq) | |
820 | sshd-4261 0d.h1 29us : sub_preempt_count (_spin_unlock) | |
821 | sshd-4261 0d.h2 29us : irq_exit (do_IRQ) | |
822 | sshd-4261 0d.h2 29us : sub_preempt_count (irq_exit) | |
823 | sshd-4261 0d..3 30us : do_softirq (irq_exit) | |
824 | sshd-4261 0d... 30us : __do_softirq (do_softirq) | |
825 | sshd-4261 0d... 31us : __local_bh_disable (__do_softirq) | |
826 | sshd-4261 0d... 31us+: add_preempt_count (__local_bh_disable) | |
827 | sshd-4261 0d.s4 34us : add_preempt_count (__local_bh_disable) | |
828 | [...] | |
829 | sshd-4261 0d.s3 43us : sub_preempt_count (local_bh_enable_ip) | |
830 | sshd-4261 0d.s4 44us : sub_preempt_count (local_bh_enable_ip) | |
831 | sshd-4261 0d.s3 44us : smp_apic_timer_interrupt (apic_timer_interrupt) | |
832 | sshd-4261 0d.s3 45us : irq_enter (smp_apic_timer_interrupt) | |
833 | sshd-4261 0d.s3 45us : idle_cpu (irq_enter) | |
834 | sshd-4261 0d.s3 46us : add_preempt_count (irq_enter) | |
835 | sshd-4261 0d.H3 46us : idle_cpu (irq_enter) | |
836 | sshd-4261 0d.H3 47us : hrtimer_interrupt (smp_apic_timer_interrupt) | |
837 | sshd-4261 0d.H3 47us : ktime_get (hrtimer_interrupt) | |
838 | [...] | |
839 | sshd-4261 0d.H3 81us : tick_program_event (hrtimer_interrupt) | |
840 | sshd-4261 0d.H3 82us : ktime_get (tick_program_event) | |
841 | sshd-4261 0d.H3 82us : ktime_get_ts (ktime_get) | |
842 | sshd-4261 0d.H3 83us : getnstimeofday (ktime_get_ts) | |
843 | sshd-4261 0d.H3 83us : set_normalized_timespec (ktime_get_ts) | |
844 | sshd-4261 0d.H3 84us : clockevents_program_event (tick_program_event) | |
845 | sshd-4261 0d.H3 84us : lapic_next_event (clockevents_program_event) | |
846 | sshd-4261 0d.H3 85us : irq_exit (smp_apic_timer_interrupt) | |
847 | sshd-4261 0d.H3 85us : sub_preempt_count (irq_exit) | |
848 | sshd-4261 0d.s4 86us : sub_preempt_count (irq_exit) | |
849 | sshd-4261 0d.s3 86us : add_preempt_count (__local_bh_disable) | |
850 | [...] | |
851 | sshd-4261 0d.s1 98us : sub_preempt_count (net_rx_action) | |
852 | sshd-4261 0d.s. 99us : add_preempt_count (_spin_lock_irq) | |
853 | sshd-4261 0d.s1 99us+: _spin_unlock_irq (run_timer_softirq) | |
854 | sshd-4261 0d.s. 104us : _local_bh_enable (__do_softirq) | |
855 | sshd-4261 0d.s. 104us : sub_preempt_count (_local_bh_enable) | |
856 | sshd-4261 0d.s. 105us : _local_bh_enable (__do_softirq) | |
857 | sshd-4261 0d.s1 105us : trace_preempt_on (__do_softirq) | |
858 | ||
859 | ||
860 | This is a very interesting trace. It started with the preemption of | |
861 | the ls task. We see that the task had the "need_resched" bit set | |
f2d9c740 SR |
862 | via the 'N' in the trace. Interrupts were disabled before the spin_lock |
863 | at the beginning of the trace. We see that a schedule took place to run | |
a41eebab SR |
864 | sshd. When the interrupts were enabled, we took an interrupt. |
865 | On return from the interrupt handler, the softirq ran. We took another | |
f2d9c740 | 866 | interrupt while running the softirq as we see from the capital 'H'. |
eb6d42ea SR |
867 | |
868 | ||
869 | wakeup | |
870 | ------ | |
871 | ||
f2d9c740 SR |
872 | In a Real-Time environment it is very important to know the wakeup |
873 | time it takes for the highest priority task that is woken up to the | |
874 | time that it executes. This is also known as "schedule latency". | |
eb6d42ea SR |
875 | I stress the point that this is about RT tasks. It is also important |
876 | to know the scheduling latency of non-RT tasks, but the average | |
877 | schedule latency is better for non-RT tasks. Tools like | |
a41eebab | 878 | LatencyTop are more appropriate for such measurements. |
eb6d42ea | 879 | |
a41eebab | 880 | Real-Time environments are interested in the worst case latency. |
eb6d42ea SR |
881 | That is the longest latency it takes for something to happen, and |
882 | not the average. We can have a very fast scheduler that may only | |
883 | have a large latency once in a while, but that would not work well | |
884 | with Real-Time tasks. The wakeup tracer was designed to record | |
885 | the worst case wakeups of RT tasks. Non-RT tasks are not recorded | |
886 | because the tracer only records one worst case and tracing non-RT | |
887 | tasks that are unpredictable will overwrite the worst case latency | |
888 | of RT tasks. | |
889 | ||
890 | Since this tracer only deals with RT tasks, we will run this slightly | |
a41eebab SR |
891 | differently than we did with the previous tracers. Instead of performing |
892 | an 'ls', we will run 'sleep 1' under 'chrt' which changes the | |
eb6d42ea SR |
893 | priority of the task. |
894 | ||
895 | # echo wakeup > /debug/tracing/current_tracer | |
896 | # echo 0 > /debug/tracing/tracing_max_latency | |
897 | # echo 1 > /debug/tracing/tracing_enabled | |
898 | # chrt -f 5 sleep 1 | |
899 | # echo 0 > /debug/tracing/tracing_enabled | |
900 | # cat /debug/tracing/latency_trace | |
901 | # tracer: wakeup | |
902 | # | |
903 | wakeup latency trace v1.1.5 on 2.6.26-rc8 | |
904 | -------------------------------------------------------------------- | |
905 | latency: 4 us, #2/2, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
906 | ----------------- | |
907 | | task: sleep-4901 (uid:0 nice:0 policy:1 rt_prio:5) | |
908 | ----------------- | |
909 | ||
910 | # _------=> CPU# | |
911 | # / _-----=> irqs-off | |
912 | # | / _----=> need-resched | |
913 | # || / _---=> hardirq/softirq | |
914 | # ||| / _--=> preempt-depth | |
915 | # |||| / | |
916 | # ||||| delay | |
917 | # cmd pid ||||| time | caller | |
918 | # \ / ||||| \ | / | |
919 | <idle>-0 1d.h4 0us+: try_to_wake_up (wake_up_process) | |
920 | <idle>-0 1d..4 4us : schedule (cpu_idle) | |
921 | ||
922 | ||
eb6d42ea | 923 | |
a41eebab | 924 | Running this on an idle system, we see that it only took 4 microseconds |
eb6d42ea | 925 | to perform the task switch. Note, since the trace marker in the |
a41eebab | 926 | schedule is before the actual "switch", we stop the tracing when |
eb6d42ea SR |
927 | the recorded task is about to schedule in. This may change if |
928 | we add a new marker at the end of the scheduler. | |
929 | ||
930 | Notice that the recorded task is 'sleep' with the PID of 4901 and it | |
931 | has an rt_prio of 5. This priority is user-space priority and not | |
932 | the internal kernel priority. The policy is 1 for SCHED_FIFO and 2 | |
933 | for SCHED_RR. | |
934 | ||
935 | Doing the same with chrt -r 5 and ftrace_enabled set. | |
936 | ||
937 | # tracer: wakeup | |
938 | # | |
939 | wakeup latency trace v1.1.5 on 2.6.26-rc8 | |
940 | -------------------------------------------------------------------- | |
941 | latency: 50 us, #60/60, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
942 | ----------------- | |
943 | | task: sleep-4068 (uid:0 nice:0 policy:2 rt_prio:5) | |
944 | ----------------- | |
945 | ||
946 | # _------=> CPU# | |
947 | # / _-----=> irqs-off | |
948 | # | / _----=> need-resched | |
949 | # || / _---=> hardirq/softirq | |
950 | # ||| / _--=> preempt-depth | |
951 | # |||| / | |
952 | # ||||| delay | |
953 | # cmd pid ||||| time | caller | |
954 | # \ / ||||| \ | / | |
955 | ksoftirq-7 1d.H3 0us : try_to_wake_up (wake_up_process) | |
956 | ksoftirq-7 1d.H4 1us : sub_preempt_count (marker_probe_cb) | |
957 | ksoftirq-7 1d.H3 2us : check_preempt_wakeup (try_to_wake_up) | |
958 | ksoftirq-7 1d.H3 3us : update_curr (check_preempt_wakeup) | |
959 | ksoftirq-7 1d.H3 4us : calc_delta_mine (update_curr) | |
960 | ksoftirq-7 1d.H3 5us : __resched_task (check_preempt_wakeup) | |
961 | ksoftirq-7 1d.H3 6us : task_wake_up_rt (try_to_wake_up) | |
962 | ksoftirq-7 1d.H3 7us : _spin_unlock_irqrestore (try_to_wake_up) | |
963 | [...] | |
964 | ksoftirq-7 1d.H2 17us : irq_exit (smp_apic_timer_interrupt) | |
965 | ksoftirq-7 1d.H2 18us : sub_preempt_count (irq_exit) | |
966 | ksoftirq-7 1d.s3 19us : sub_preempt_count (irq_exit) | |
967 | ksoftirq-7 1..s2 20us : rcu_process_callbacks (__do_softirq) | |
968 | [...] | |
969 | ksoftirq-7 1..s2 26us : __rcu_process_callbacks (rcu_process_callbacks) | |
970 | ksoftirq-7 1d.s2 27us : _local_bh_enable (__do_softirq) | |
971 | ksoftirq-7 1d.s2 28us : sub_preempt_count (_local_bh_enable) | |
972 | ksoftirq-7 1.N.3 29us : sub_preempt_count (ksoftirqd) | |
973 | ksoftirq-7 1.N.2 30us : _cond_resched (ksoftirqd) | |
974 | ksoftirq-7 1.N.2 31us : __cond_resched (_cond_resched) | |
975 | ksoftirq-7 1.N.2 32us : add_preempt_count (__cond_resched) | |
976 | ksoftirq-7 1.N.2 33us : schedule (__cond_resched) | |
977 | ksoftirq-7 1.N.2 33us : add_preempt_count (schedule) | |
978 | ksoftirq-7 1.N.3 34us : hrtick_clear (schedule) | |
979 | ksoftirq-7 1dN.3 35us : _spin_lock (schedule) | |
980 | ksoftirq-7 1dN.3 36us : add_preempt_count (_spin_lock) | |
981 | ksoftirq-7 1d..4 37us : put_prev_task_fair (schedule) | |
982 | ksoftirq-7 1d..4 38us : update_curr (put_prev_task_fair) | |
983 | [...] | |
984 | ksoftirq-7 1d..5 47us : _spin_trylock (tracing_record_cmdline) | |
985 | ksoftirq-7 1d..5 48us : add_preempt_count (_spin_trylock) | |
986 | ksoftirq-7 1d..6 49us : _spin_unlock (tracing_record_cmdline) | |
987 | ksoftirq-7 1d..6 49us : sub_preempt_count (_spin_unlock) | |
988 | ksoftirq-7 1d..4 50us : schedule (__cond_resched) | |
989 | ||
990 | The interrupt went off while running ksoftirqd. This task runs at | |
f2d9c740 | 991 | SCHED_OTHER. Why did not we see the 'N' set early? This may be |
a41eebab | 992 | a harmless bug with x86_32 and 4K stacks. On x86_32 with 4K stacks |
f2d9c740 | 993 | configured, the interrupt and softirq run with their own stack. |
a41eebab SR |
994 | Some information is held on the top of the task's stack (need_resched |
995 | and preempt_count are both stored there). The setting of the NEED_RESCHED | |
996 | bit is done directly to the task's stack, but the reading of the | |
997 | NEED_RESCHED is done by looking at the current stack, which in this case | |
998 | is the stack for the hard interrupt. This hides the fact that NEED_RESCHED | |
f2d9c740 | 999 | has been set. We do not see the 'N' until we switch back to the task's |
a41eebab | 1000 | assigned stack. |
eb6d42ea SR |
1001 | |
1002 | ftrace | |
1003 | ------ | |
1004 | ||
1005 | ftrace is not only the name of the tracing infrastructure, but it | |
1006 | is also a name of one of the tracers. The tracer is the function | |
1007 | tracer. Enabling the function tracer can be done from the | |
1008 | debug file system. Make sure the ftrace_enabled is set otherwise | |
1009 | this tracer is a nop. | |
1010 | ||
1011 | # sysctl kernel.ftrace_enabled=1 | |
1012 | # echo ftrace > /debug/tracing/current_tracer | |
1013 | # echo 1 > /debug/tracing/tracing_enabled | |
1014 | # usleep 1 | |
1015 | # echo 0 > /debug/tracing/tracing_enabled | |
1016 | # cat /debug/tracing/trace | |
1017 | # tracer: ftrace | |
1018 | # | |
1019 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1020 | # | | | | | | |
1021 | bash-4003 [00] 123.638713: finish_task_switch <-schedule | |
1022 | bash-4003 [00] 123.638714: _spin_unlock_irq <-finish_task_switch | |
1023 | bash-4003 [00] 123.638714: sub_preempt_count <-_spin_unlock_irq | |
1024 | bash-4003 [00] 123.638715: hrtick_set <-schedule | |
1025 | bash-4003 [00] 123.638715: _spin_lock_irqsave <-hrtick_set | |
1026 | bash-4003 [00] 123.638716: add_preempt_count <-_spin_lock_irqsave | |
1027 | bash-4003 [00] 123.638716: _spin_unlock_irqrestore <-hrtick_set | |
1028 | bash-4003 [00] 123.638717: sub_preempt_count <-_spin_unlock_irqrestore | |
1029 | bash-4003 [00] 123.638717: hrtick_clear <-hrtick_set | |
1030 | bash-4003 [00] 123.638718: sub_preempt_count <-schedule | |
1031 | bash-4003 [00] 123.638718: sub_preempt_count <-preempt_schedule | |
1032 | bash-4003 [00] 123.638719: wait_for_completion <-__stop_machine_run | |
1033 | bash-4003 [00] 123.638719: wait_for_common <-wait_for_completion | |
1034 | bash-4003 [00] 123.638720: _spin_lock_irq <-wait_for_common | |
1035 | bash-4003 [00] 123.638720: add_preempt_count <-_spin_lock_irq | |
1036 | [...] | |
1037 | ||
1038 | ||
f2d9c740 SR |
1039 | Note: ftrace uses ring buffers to store the above entries. The newest data |
1040 | may overwrite the oldest data. Sometimes using echo to stop the trace | |
1041 | is not sufficient because the tracing could have overwritten the data | |
1042 | that you wanted to record. For this reason, it is sometimes better to | |
1043 | disable tracing directly from a program. This allows you to stop the | |
1044 | tracing at the point that you hit the part that you are interested in. | |
1045 | To disable the tracing directly from a C program, something like following | |
1046 | code snippet can be used: | |
eb6d42ea SR |
1047 | |
1048 | int trace_fd; | |
1049 | [...] | |
1050 | int main(int argc, char *argv[]) { | |
1051 | [...] | |
1052 | trace_fd = open("/debug/tracing/tracing_enabled", O_WRONLY); | |
1053 | [...] | |
1054 | if (condition_hit()) { | |
f2d9c740 | 1055 | write(trace_fd, "0", 1); |
eb6d42ea SR |
1056 | } |
1057 | [...] | |
1058 | } | |
1059 | ||
f2d9c740 SR |
1060 | Note: Here we hard coded the path name. The debugfs mount is not |
1061 | guaranteed to be at /debug (and is more commonly at /sys/kernel/debug). | |
1062 | For simple one time traces, the above is sufficent. For anything else, | |
1063 | a search through /proc/mounts may be needed to find where the debugfs | |
1064 | file-system is mounted. | |
eb6d42ea SR |
1065 | |
1066 | dynamic ftrace | |
1067 | -------------- | |
1068 | ||
f2d9c740 | 1069 | If CONFIG_DYNAMIC_FTRACE is set, the system will run with |
eb6d42ea SR |
1070 | virtually no overhead when function tracing is disabled. The way |
1071 | this works is the mcount function call (placed at the start of | |
1072 | every kernel function, produced by the -pg switch in gcc), starts | |
f2d9c740 SR |
1073 | of pointing to a simple return. (Enabling FTRACE will include the |
1074 | -pg switch in the compiling of the kernel.) | |
eb6d42ea | 1075 | |
a41eebab SR |
1076 | When dynamic ftrace is initialized, it calls kstop_machine to make |
1077 | the machine act like a uniprocessor so that it can freely modify code | |
1078 | without worrying about other processors executing that same code. At | |
1079 | initialization, the mcount calls are changed to call a "record_ip" | |
eb6d42ea SR |
1080 | function. After this, the first time a kernel function is called, |
1081 | it has the calling address saved in a hash table. | |
1082 | ||
1083 | Later on the ftraced kernel thread is awoken and will again call | |
1084 | kstop_machine if new functions have been recorded. The ftraced thread | |
1085 | will change all calls to mcount to "nop". Just calling mcount | |
1086 | and having mcount return has shown a 10% overhead. By converting | |
f2d9c740 | 1087 | it to a nop, there is no measurable overhead to the system. |
eb6d42ea SR |
1088 | |
1089 | One special side-effect to the recording of the functions being | |
f2d9c740 SR |
1090 | traced is that we can now selectively choose which functions we |
1091 | wish to trace and which ones we want the mcount calls to remain as | |
eb6d42ea SR |
1092 | nops. |
1093 | ||
a41eebab | 1094 | Two files are used, one for enabling and one for disabling the tracing |
f2d9c740 | 1095 | of specified functions. They are: |
eb6d42ea SR |
1096 | |
1097 | set_ftrace_filter | |
1098 | ||
1099 | and | |
1100 | ||
1101 | set_ftrace_notrace | |
1102 | ||
a41eebab | 1103 | A list of available functions that you can add to these files is listed |
eb6d42ea SR |
1104 | in: |
1105 | ||
1106 | available_filter_functions | |
1107 | ||
1108 | # cat /debug/tracing/available_filter_functions | |
1109 | put_prev_task_idle | |
1110 | kmem_cache_create | |
1111 | pick_next_task_rt | |
1112 | get_online_cpus | |
1113 | pick_next_task_fair | |
1114 | mutex_lock | |
1115 | [...] | |
1116 | ||
f2d9c740 | 1117 | If I am only interested in sys_nanosleep and hrtimer_interrupt: |
eb6d42ea SR |
1118 | |
1119 | # echo sys_nanosleep hrtimer_interrupt \ | |
1120 | > /debug/tracing/set_ftrace_filter | |
1121 | # echo ftrace > /debug/tracing/current_tracer | |
1122 | # echo 1 > /debug/tracing/tracing_enabled | |
1123 | # usleep 1 | |
1124 | # echo 0 > /debug/tracing/tracing_enabled | |
1125 | # cat /debug/tracing/trace | |
1126 | # tracer: ftrace | |
1127 | # | |
1128 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1129 | # | | | | | | |
1130 | usleep-4134 [00] 1317.070017: hrtimer_interrupt <-smp_apic_timer_interrupt | |
1131 | usleep-4134 [00] 1317.070111: sys_nanosleep <-syscall_call | |
1132 | <idle>-0 [00] 1317.070115: hrtimer_interrupt <-smp_apic_timer_interrupt | |
1133 | ||
f2d9c740 | 1134 | To see which functions are being traced, you can cat the file: |
eb6d42ea SR |
1135 | |
1136 | # cat /debug/tracing/set_ftrace_filter | |
1137 | hrtimer_interrupt | |
1138 | sys_nanosleep | |
1139 | ||
1140 | ||
f2d9c740 | 1141 | Perhaps this is not enough. The filters also allow simple wild cards. |
a41eebab | 1142 | Only the following are currently available |
eb6d42ea | 1143 | |
a41eebab | 1144 | <match>* - will match functions that begin with <match> |
eb6d42ea SR |
1145 | *<match> - will match functions that end with <match> |
1146 | *<match>* - will match functions that have <match> in it | |
1147 | ||
f2d9c740 | 1148 | These are the only wild cards which are supported. |
eb6d42ea SR |
1149 | |
1150 | <match>*<match> will not work. | |
1151 | ||
1152 | # echo hrtimer_* > /debug/tracing/set_ftrace_filter | |
1153 | ||
1154 | Produces: | |
1155 | ||
1156 | # tracer: ftrace | |
1157 | # | |
1158 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1159 | # | | | | | | |
1160 | bash-4003 [00] 1480.611794: hrtimer_init <-copy_process | |
1161 | bash-4003 [00] 1480.611941: hrtimer_start <-hrtick_set | |
1162 | bash-4003 [00] 1480.611956: hrtimer_cancel <-hrtick_clear | |
1163 | bash-4003 [00] 1480.611956: hrtimer_try_to_cancel <-hrtimer_cancel | |
1164 | <idle>-0 [00] 1480.612019: hrtimer_get_next_event <-get_next_timer_interrupt | |
1165 | <idle>-0 [00] 1480.612025: hrtimer_get_next_event <-get_next_timer_interrupt | |
1166 | <idle>-0 [00] 1480.612032: hrtimer_get_next_event <-get_next_timer_interrupt | |
1167 | <idle>-0 [00] 1480.612037: hrtimer_get_next_event <-get_next_timer_interrupt | |
1168 | <idle>-0 [00] 1480.612382: hrtimer_get_next_event <-get_next_timer_interrupt | |
1169 | ||
1170 | ||
1171 | Notice that we lost the sys_nanosleep. | |
1172 | ||
1173 | # cat /debug/tracing/set_ftrace_filter | |
1174 | hrtimer_run_queues | |
1175 | hrtimer_run_pending | |
1176 | hrtimer_init | |
1177 | hrtimer_cancel | |
1178 | hrtimer_try_to_cancel | |
1179 | hrtimer_forward | |
1180 | hrtimer_start | |
1181 | hrtimer_reprogram | |
1182 | hrtimer_force_reprogram | |
1183 | hrtimer_get_next_event | |
1184 | hrtimer_interrupt | |
1185 | hrtimer_nanosleep | |
1186 | hrtimer_wakeup | |
1187 | hrtimer_get_remaining | |
1188 | hrtimer_get_res | |
1189 | hrtimer_init_sleeper | |
1190 | ||
1191 | ||
1192 | This is because the '>' and '>>' act just like they do in bash. | |
1193 | To rewrite the filters, use '>' | |
1194 | To append to the filters, use '>>' | |
1195 | ||
a41eebab | 1196 | To clear out a filter so that all functions will be recorded again: |
eb6d42ea SR |
1197 | |
1198 | # echo > /debug/tracing/set_ftrace_filter | |
1199 | # cat /debug/tracing/set_ftrace_filter | |
1200 | # | |
1201 | ||
1202 | Again, now we want to append. | |
1203 | ||
1204 | # echo sys_nanosleep > /debug/tracing/set_ftrace_filter | |
1205 | # cat /debug/tracing/set_ftrace_filter | |
1206 | sys_nanosleep | |
1207 | # echo hrtimer_* >> /debug/tracing/set_ftrace_filter | |
1208 | # cat /debug/tracing/set_ftrace_filter | |
1209 | hrtimer_run_queues | |
1210 | hrtimer_run_pending | |
1211 | hrtimer_init | |
1212 | hrtimer_cancel | |
1213 | hrtimer_try_to_cancel | |
1214 | hrtimer_forward | |
1215 | hrtimer_start | |
1216 | hrtimer_reprogram | |
1217 | hrtimer_force_reprogram | |
1218 | hrtimer_get_next_event | |
1219 | hrtimer_interrupt | |
1220 | sys_nanosleep | |
1221 | hrtimer_nanosleep | |
1222 | hrtimer_wakeup | |
1223 | hrtimer_get_remaining | |
1224 | hrtimer_get_res | |
1225 | hrtimer_init_sleeper | |
1226 | ||
1227 | ||
1228 | The set_ftrace_notrace prevents those functions from being traced. | |
1229 | ||
1230 | # echo '*preempt*' '*lock*' > /debug/tracing/set_ftrace_notrace | |
1231 | ||
1232 | Produces: | |
1233 | ||
1234 | # tracer: ftrace | |
1235 | # | |
1236 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1237 | # | | | | | | |
1238 | bash-4043 [01] 115.281644: finish_task_switch <-schedule | |
1239 | bash-4043 [01] 115.281645: hrtick_set <-schedule | |
1240 | bash-4043 [01] 115.281645: hrtick_clear <-hrtick_set | |
1241 | bash-4043 [01] 115.281646: wait_for_completion <-__stop_machine_run | |
1242 | bash-4043 [01] 115.281647: wait_for_common <-wait_for_completion | |
1243 | bash-4043 [01] 115.281647: kthread_stop <-stop_machine_run | |
1244 | bash-4043 [01] 115.281648: init_waitqueue_head <-kthread_stop | |
1245 | bash-4043 [01] 115.281648: wake_up_process <-kthread_stop | |
1246 | bash-4043 [01] 115.281649: try_to_wake_up <-wake_up_process | |
1247 | ||
1248 | We can see that there's no more lock or preempt tracing. | |
1249 | ||
1250 | ftraced | |
1251 | ------- | |
1252 | ||
1253 | As mentioned above, when dynamic ftrace is configured in, a kernel | |
1254 | thread wakes up once a second and checks to see if there are mcount | |
a41eebab SR |
1255 | calls that need to be converted into nops. If there are not any, then |
1256 | it simply goes back to sleep. But if there are some, it will call | |
eb6d42ea SR |
1257 | kstop_machine to convert the calls to nops. |
1258 | ||
f2d9c740 | 1259 | There may be a case in which you do not want this added latency. |
eb6d42ea SR |
1260 | Perhaps you are doing some audio recording and this activity might |
1261 | cause skips in the playback. There is an interface to disable | |
f2d9c740 | 1262 | and enable the "ftraced" kernel thread. |
eb6d42ea SR |
1263 | |
1264 | # echo 0 > /debug/tracing/ftraced_enabled | |
1265 | ||
f2d9c740 SR |
1266 | This will disable the calling of kstop_machine to update the |
1267 | mcount calls to nops. Remember that there is a large overhead | |
eb6d42ea SR |
1268 | to calling mcount. Without this kernel thread, that overhead will |
1269 | exist. | |
1270 | ||
a41eebab SR |
1271 | If there are recorded calls to mcount, any write to the ftraced_enabled |
1272 | file will cause the kstop_machine to run. This means that a | |
eb6d42ea SR |
1273 | user can manually perform the updates when they want to by simply |
1274 | echoing a '0' into the ftraced_enabled file. | |
1275 | ||
1276 | The updates are also done at the beginning of enabling a tracer | |
1277 | that uses ftrace function recording. | |
1278 | ||
1279 | ||
1280 | trace_pipe | |
1281 | ---------- | |
1282 | ||
f2d9c740 SR |
1283 | The trace_pipe outputs the same content as the trace file, but the effect |
1284 | on the tracing is different. Every read from trace_pipe is consumed. | |
eb6d42ea SR |
1285 | This means that subsequent reads will be different. The trace |
1286 | is live. | |
1287 | ||
1288 | # echo ftrace > /debug/tracing/current_tracer | |
1289 | # cat /debug/tracing/trace_pipe > /tmp/trace.out & | |
1290 | [1] 4153 | |
1291 | # echo 1 > /debug/tracing/tracing_enabled | |
1292 | # usleep 1 | |
1293 | # echo 0 > /debug/tracing/tracing_enabled | |
1294 | # cat /debug/tracing/trace | |
1295 | # tracer: ftrace | |
1296 | # | |
1297 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1298 | # | | | | | | |
1299 | ||
1300 | # | |
1301 | # cat /tmp/trace.out | |
1302 | bash-4043 [00] 41.267106: finish_task_switch <-schedule | |
1303 | bash-4043 [00] 41.267106: hrtick_set <-schedule | |
1304 | bash-4043 [00] 41.267107: hrtick_clear <-hrtick_set | |
1305 | bash-4043 [00] 41.267108: wait_for_completion <-__stop_machine_run | |
1306 | bash-4043 [00] 41.267108: wait_for_common <-wait_for_completion | |
1307 | bash-4043 [00] 41.267109: kthread_stop <-stop_machine_run | |
1308 | bash-4043 [00] 41.267109: init_waitqueue_head <-kthread_stop | |
1309 | bash-4043 [00] 41.267110: wake_up_process <-kthread_stop | |
1310 | bash-4043 [00] 41.267110: try_to_wake_up <-wake_up_process | |
1311 | bash-4043 [00] 41.267111: select_task_rq_rt <-try_to_wake_up | |
1312 | ||
1313 | ||
f2d9c740 | 1314 | Note, reading the trace_pipe file will block until more input is added. |
eb6d42ea SR |
1315 | By changing the tracer, trace_pipe will issue an EOF. We needed |
1316 | to set the ftrace tracer _before_ cating the trace_pipe file. | |
1317 | ||
1318 | ||
1319 | trace entries | |
1320 | ------------- | |
1321 | ||
1322 | Having too much or not enough data can be troublesome in diagnosing | |
f2d9c740 | 1323 | an issue in the kernel. The file trace_entries is used to modify |
a41eebab | 1324 | the size of the internal trace buffers. The number listed |
eb6d42ea SR |
1325 | is the number of entries that can be recorded per CPU. To know |
1326 | the full size, multiply the number of possible CPUS with the | |
1327 | number of entries. | |
1328 | ||
1329 | # cat /debug/tracing/trace_entries | |
1330 | 65620 | |
1331 | ||
a41eebab | 1332 | Note, to modify this, you must have tracing completely disabled. To do that, |
f2d9c740 SR |
1333 | echo "none" into the current_tracer. If the current_tracer is not set |
1334 | to "none", an EINVAL error will be returned. | |
eb6d42ea SR |
1335 | |
1336 | # echo none > /debug/tracing/current_tracer | |
1337 | # echo 100000 > /debug/tracing/trace_entries | |
1338 | # cat /debug/tracing/trace_entries | |
1339 | 100045 | |
1340 | ||
1341 | ||
1342 | Notice that we echoed in 100,000 but the size is 100,045. The entries | |
f2d9c740 | 1343 | are held in individual pages. It allocates the number of pages it takes |
eb6d42ea | 1344 | to fulfill the request. If more entries may fit on the last page |
f2d9c740 | 1345 | then they will be added. |
eb6d42ea SR |
1346 | |
1347 | # echo 1 > /debug/tracing/trace_entries | |
1348 | # cat /debug/tracing/trace_entries | |
1349 | 85 | |
1350 | ||
f2d9c740 | 1351 | This shows us that 85 entries can fit in a single page. |
eb6d42ea | 1352 | |
f2d9c740 SR |
1353 | The number of pages which will be allocated is limited to a percentage |
1354 | of available memory. Allocating too much will produce an error. | |
eb6d42ea SR |
1355 | |
1356 | # echo 1000000000000 > /debug/tracing/trace_entries | |
1357 | -bash: echo: write error: Cannot allocate memory | |
1358 | # cat /debug/tracing/trace_entries | |
1359 | 85 | |
1360 |