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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. | |
18 | It can be used for debugging or analyzing latencies and performance | |
19 | issues that take place outside of user-space. | |
20 | ||
21 | Although ftrace is the function tracer, it also includes an | |
22 | infrastructure that allows for other types of tracing. Some of the | |
f2d9c740 | 23 | tracers that are currently in ftrace include a tracer to trace |
eb6d42ea SR |
24 | context switches, the time it takes for a high priority task to |
25 | run after it was woken up, the time interrupts are disabled, and | |
f2d9c740 SR |
26 | more (ftrace allows for tracer plugins, which means that the list of |
27 | tracers can always grow). | |
eb6d42ea SR |
28 | |
29 | ||
30 | The File System | |
31 | --------------- | |
32 | ||
33 | Ftrace uses the debugfs file system to hold the control files as well | |
34 | as the files to display output. | |
35 | ||
36 | To mount the debugfs system: | |
37 | ||
38 | # mkdir /debug | |
39 | # mount -t debugfs nodev /debug | |
40 | ||
f2d9c740 SR |
41 | (Note: it is more common to mount at /sys/kernel/debug, but for simplicity |
42 | 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 | ||
9b803c0f | 53 | current_tracer: This is used to set or display the current tracer |
eb6d42ea SR |
54 | that is configured. |
55 | ||
9b803c0f | 56 | available_tracers: This holds the different types of tracers that |
a41eebab | 57 | have been compiled into the kernel. The tracers |
f2d9c740 SR |
58 | listed here can be configured by echoing their name |
59 | into current_tracer. | |
eb6d42ea | 60 | |
9b803c0f | 61 | tracing_enabled: This sets or displays whether the current_tracer |
eb6d42ea | 62 | is activated and tracing or not. Echo 0 into this |
f2d9c740 | 63 | file to disable the tracer or 1 to enable it. |
eb6d42ea | 64 | |
9b803c0f | 65 | trace: This file holds the output of the trace in a human readable |
f2d9c740 | 66 | format (described below). |
eb6d42ea | 67 | |
9b803c0f | 68 | latency_trace: This file shows the same trace but the information |
eb6d42ea | 69 | is organized more to display possible latencies |
f2d9c740 | 70 | in the system (described below). |
eb6d42ea | 71 | |
9b803c0f | 72 | trace_pipe: The output is the same as the "trace" file but this |
eb6d42ea SR |
73 | file is meant to be streamed with live tracing. |
74 | Reads from this file will block until new data | |
75 | is retrieved. Unlike the "trace" and "latency_trace" | |
76 | files, this file is a consumer. This means reading | |
77 | from this file causes sequential reads to display | |
78 | more current data. Once data is read from this | |
79 | file, it is consumed, and will not be read | |
80 | again with a sequential read. The "trace" and | |
81 | "latency_trace" files are static, and if the | |
f2d9c740 | 82 | tracer is not adding more data, they will display |
eb6d42ea SR |
83 | the same information every time they are read. |
84 | ||
9b803c0f | 85 | iter_ctrl: This file lets the user control the amount of data |
eb6d42ea SR |
86 | that is displayed in one of the above output |
87 | files. | |
88 | ||
9b803c0f | 89 | trace_max_latency: Some of the tracers record the max latency. |
eb6d42ea SR |
90 | For example, the time interrupts are disabled. |
91 | This time is saved in this file. The max trace | |
92 | will also be stored, and displayed by either | |
93 | "trace" or "latency_trace". A new max trace will | |
94 | only be recorded if the latency is greater than | |
95 | the value in this file. (in microseconds) | |
96 | ||
9b803c0f SR |
97 | trace_entries: This sets or displays the number of bytes each CPU |
98 | buffer can hold. The tracer buffers are the same size | |
99 | for each CPU. The displayed number is the size of the | |
100 | CPU buffer and not total size of all buffers. The | |
f2d9c740 SR |
101 | trace buffers are allocated in pages (blocks of memory |
102 | that the kernel uses for allocation, usually 4 KB in size). | |
9b803c0f SR |
103 | If the last page allocated has room for more bytes |
104 | than requested, the rest of the page will be used, | |
105 | making the actual allocation bigger than requested. | |
106 | (Note, the size may not be a multiple of the page size due | |
107 | to buffer managment overhead.) | |
eb6d42ea SR |
108 | |
109 | This can only be updated when the current_tracer | |
9b803c0f | 110 | is set to "nop". |
eb6d42ea | 111 | |
9b803c0f | 112 | tracing_cpumask: This is a mask that lets the user only trace |
eb6d42ea SR |
113 | on specified CPUS. The format is a hex string |
114 | representing the CPUS. | |
115 | ||
9b803c0f | 116 | set_ftrace_filter: When dynamic ftrace is configured in (see the |
f2d9c740 SR |
117 | section below "dynamic ftrace"), the code is dynamically |
118 | modified (code text rewrite) to disable calling of the | |
119 | function profiler (mcount). This lets tracing be configured | |
120 | in with practically no overhead in performance. This also | |
121 | has a side effect of enabling or disabling specific functions | |
122 | to be traced. Echoing names of functions into this file | |
123 | will limit the trace to only those functions. | |
124 | ||
125 | set_ftrace_notrace: This has an effect opposite to that of | |
126 | set_ftrace_filter. Any function that is added here will not | |
127 | be traced. If a function exists in both set_ftrace_filter | |
128 | and set_ftrace_notrace, the function will _not_ be traced. | |
eb6d42ea | 129 | |
9b803c0f SR |
130 | available_filter_functions: This lists the functions that ftrace |
131 | has processed and can trace. These are the function | |
132 | names that you can pass to "set_ftrace_filter" or | |
133 | "set_ftrace_notrace". (See the section "dynamic ftrace" | |
134 | below for more details.) | |
eb6d42ea SR |
135 | |
136 | ||
137 | The Tracers | |
138 | ----------- | |
139 | ||
f2d9c740 | 140 | Here is the list of current tracers that may be configured. |
eb6d42ea | 141 | |
9b803c0f | 142 | function - function tracer that uses mcount to trace all functions. |
eb6d42ea SR |
143 | |
144 | sched_switch - traces the context switches between tasks. | |
145 | ||
f2d9c740 | 146 | irqsoff - traces the areas that disable interrupts and saves |
eb6d42ea SR |
147 | the trace with the longest max latency. |
148 | See tracing_max_latency. When a new max is recorded, | |
149 | it replaces the old trace. It is best to view this | |
f2d9c740 | 150 | trace via the latency_trace file. |
eb6d42ea | 151 | |
f2d9c740 SR |
152 | preemptoff - Similar to irqsoff but traces and records the amount of |
153 | time for which preemption is disabled. | |
eb6d42ea SR |
154 | |
155 | preemptirqsoff - Similar to irqsoff and preemptoff, but traces and | |
f2d9c740 SR |
156 | records the largest time for which irqs and/or preemption |
157 | is disabled. | |
eb6d42ea SR |
158 | |
159 | wakeup - Traces and records the max latency that it takes for | |
160 | the highest priority task to get scheduled after | |
161 | it has been woken up. | |
162 | ||
9b803c0f SR |
163 | nop - This is not a tracer. To remove all tracers from tracing |
164 | simply echo "nop" into current_tracer. | |
eb6d42ea SR |
165 | |
166 | ||
167 | Examples of using the tracer | |
168 | ---------------------------- | |
169 | ||
f2d9c740 SR |
170 | Here are typical examples of using the tracers when controlling them only |
171 | with the debugfs interface (without using any user-land utilities). | |
eb6d42ea SR |
172 | |
173 | Output format: | |
174 | -------------- | |
175 | ||
f2d9c740 | 176 | Here is an example of the output format of the file "trace" |
eb6d42ea SR |
177 | |
178 | -------- | |
9b803c0f | 179 | # tracer: function |
eb6d42ea SR |
180 | # |
181 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
182 | # | | | | | | |
183 | bash-4251 [01] 10152.583854: path_put <-path_walk | |
184 | bash-4251 [01] 10152.583855: dput <-path_put | |
185 | bash-4251 [01] 10152.583855: _atomic_dec_and_lock <-dput | |
186 | -------- | |
187 | ||
f2d9c740 | 188 | A header is printed with the tracer name that is represented by the trace. |
9b803c0f | 189 | In this case the tracer is "function". Then a header showing the format. Task |
f2d9c740 | 190 | name "bash", the task PID "4251", the CPU that it was running on |
eb6d42ea SR |
191 | "01", the timestamp in <secs>.<usecs> format, the function name that was |
192 | traced "path_put" and the parent function that called this function | |
f2d9c740 SR |
193 | "path_walk". The timestamp is the time at which the function was |
194 | entered. | |
eb6d42ea | 195 | |
f2d9c740 | 196 | The sched_switch tracer also includes tracing of task wakeups and |
eb6d42ea SR |
197 | context switches. |
198 | ||
199 | ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 2916:115:S | |
200 | ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 10:115:S | |
201 | ksoftirqd/1-7 [01] 1453.070013: 7:115:R ==> 10:115:R | |
202 | events/1-10 [01] 1453.070013: 10:115:S ==> 2916:115:R | |
203 | kondemand/1-2916 [01] 1453.070013: 2916:115:S ==> 7:115:R | |
204 | ksoftirqd/1-7 [01] 1453.070013: 7:115:S ==> 0:140:R | |
205 | ||
f2d9c740 | 206 | Wake ups are represented by a "+" and the context switches are shown as |
eb6d42ea SR |
207 | "==>". The format is: |
208 | ||
209 | Context switches: | |
210 | ||
211 | Previous task Next Task | |
212 | ||
213 | <pid>:<prio>:<state> ==> <pid>:<prio>:<state> | |
214 | ||
215 | Wake ups: | |
216 | ||
217 | Current task Task waking up | |
218 | ||
219 | <pid>:<prio>:<state> + <pid>:<prio>:<state> | |
220 | ||
f2d9c740 | 221 | The prio is the internal kernel priority, which is the inverse of the |
eb6d42ea SR |
222 | priority that is usually displayed by user-space tools. Zero represents |
223 | the highest priority (99). Prio 100 starts the "nice" priorities with | |
224 | 100 being equal to nice -20 and 139 being nice 19. The prio "140" is | |
225 | reserved for the idle task which is the lowest priority thread (pid 0). | |
226 | ||
227 | ||
228 | Latency trace format | |
229 | -------------------- | |
230 | ||
231 | For traces that display latency times, the latency_trace file gives | |
f2d9c740 | 232 | somewhat more information to see why a latency happened. Here is a typical |
eb6d42ea SR |
233 | trace. |
234 | ||
235 | # tracer: irqsoff | |
236 | # | |
237 | irqsoff latency trace v1.1.5 on 2.6.26-rc8 | |
238 | -------------------------------------------------------------------- | |
239 | latency: 97 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
240 | ----------------- | |
241 | | task: swapper-0 (uid:0 nice:0 policy:0 rt_prio:0) | |
242 | ----------------- | |
243 | => started at: apic_timer_interrupt | |
244 | => ended at: do_softirq | |
245 | ||
246 | # _------=> CPU# | |
247 | # / _-----=> irqs-off | |
248 | # | / _----=> need-resched | |
249 | # || / _---=> hardirq/softirq | |
250 | # ||| / _--=> preempt-depth | |
251 | # |||| / | |
252 | # ||||| delay | |
253 | # cmd pid ||||| time | caller | |
254 | # \ / ||||| \ | / | |
255 | <idle>-0 0d..1 0us+: trace_hardirqs_off_thunk (apic_timer_interrupt) | |
256 | <idle>-0 0d.s. 97us : __do_softirq (do_softirq) | |
257 | <idle>-0 0d.s1 98us : trace_hardirqs_on (do_softirq) | |
258 | ||
259 | ||
eb6d42ea | 260 | |
f2d9c740 SR |
261 | This shows that the current tracer is "irqsoff" tracing the time for which |
262 | interrupts were disabled. It gives the trace version and the version | |
263 | of the kernel upon which this was executed on (2.6.26-rc8). Then it displays | |
264 | the max latency in microsecs (97 us). The number of trace entries displayed | |
265 | and the total number recorded (both are three: #3/3). The type of | |
eb6d42ea | 266 | preemption that was used (PREEMPT). VP, KP, SP, and HP are always zero |
f2d9c740 | 267 | and are reserved for later use. #P is the number of online CPUS (#P:2). |
eb6d42ea | 268 | |
f2d9c740 | 269 | The task is the process that was running when the latency occurred. |
eb6d42ea SR |
270 | (swapper pid: 0). |
271 | ||
f2d9c740 SR |
272 | The start and stop (the functions in which the interrupts were disabled and |
273 | enabled respectively) that caused the latencies: | |
eb6d42ea SR |
274 | |
275 | apic_timer_interrupt is where the interrupts were disabled. | |
276 | do_softirq is where they were enabled again. | |
277 | ||
278 | The next lines after the header are the trace itself. The header | |
279 | explains which is which. | |
280 | ||
281 | cmd: The name of the process in the trace. | |
282 | ||
283 | pid: The PID of that process. | |
284 | ||
f2d9c740 | 285 | CPU#: The CPU which the process was running on. |
eb6d42ea SR |
286 | |
287 | irqs-off: 'd' interrupts are disabled. '.' otherwise. | |
9244489a SR |
288 | Note: If the architecture does not support a way to |
289 | read the irq flags variable, an 'X' will always | |
290 | be printed here. | |
eb6d42ea SR |
291 | |
292 | need-resched: 'N' task need_resched is set, '.' otherwise. | |
293 | ||
294 | hardirq/softirq: | |
f2d9c740 | 295 | 'H' - hard irq occurred inside a softirq. |
eb6d42ea SR |
296 | 'h' - hard irq is running |
297 | 's' - soft irq is running | |
298 | '.' - normal context. | |
299 | ||
300 | preempt-depth: The level of preempt_disabled | |
301 | ||
302 | The above is mostly meaningful for kernel developers. | |
303 | ||
a41eebab | 304 | time: This differs from the trace file output. The trace file output |
f2d9c740 | 305 | includes an absolute timestamp. The timestamp used by the |
a41eebab | 306 | latency_trace file is relative to the start of the trace. |
eb6d42ea SR |
307 | |
308 | delay: This is just to help catch your eye a bit better. And | |
309 | needs to be fixed to be only relative to the same CPU. | |
a41eebab | 310 | The marks are determined by the difference between this |
eb6d42ea SR |
311 | current trace and the next trace. |
312 | '!' - greater than preempt_mark_thresh (default 100) | |
313 | '+' - greater than 1 microsecond | |
314 | ' ' - less than or equal to 1 microsecond. | |
315 | ||
316 | The rest is the same as the 'trace' file. | |
317 | ||
318 | ||
319 | iter_ctrl | |
320 | --------- | |
321 | ||
322 | The iter_ctrl file is used to control what gets printed in the trace | |
323 | output. To see what is available, simply cat the file: | |
324 | ||
325 | cat /debug/tracing/iter_ctrl | |
326 | print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \ | |
327 | noblock nostacktrace nosched-tree | |
328 | ||
a41eebab | 329 | To disable one of the options, echo in the option prepended with "no". |
eb6d42ea SR |
330 | |
331 | echo noprint-parent > /debug/tracing/iter_ctrl | |
332 | ||
333 | To enable an option, leave off the "no". | |
334 | ||
a41eebab | 335 | echo sym-offset > /debug/tracing/iter_ctrl |
eb6d42ea SR |
336 | |
337 | Here are the available options: | |
338 | ||
339 | print-parent - On function traces, display the calling function | |
340 | as well as the function being traced. | |
341 | ||
342 | print-parent: | |
343 | bash-4000 [01] 1477.606694: simple_strtoul <-strict_strtoul | |
344 | ||
345 | noprint-parent: | |
346 | bash-4000 [01] 1477.606694: simple_strtoul | |
347 | ||
348 | ||
349 | sym-offset - Display not only the function name, but also the offset | |
350 | in the function. For example, instead of seeing just | |
a41eebab | 351 | "ktime_get", you will see "ktime_get+0xb/0x20". |
eb6d42ea SR |
352 | |
353 | sym-offset: | |
354 | bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0 | |
355 | ||
356 | sym-addr - this will also display the function address as well as | |
357 | the function name. | |
358 | ||
359 | sym-addr: | |
360 | bash-4000 [01] 1477.606694: simple_strtoul <c0339346> | |
361 | ||
362 | verbose - This deals with the latency_trace file. | |
363 | ||
364 | bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \ | |
365 | (+0.000ms): simple_strtoul (strict_strtoul) | |
366 | ||
367 | raw - This will display raw numbers. This option is best for use with | |
368 | user applications that can translate the raw numbers better than | |
369 | having it done in the kernel. | |
370 | ||
a41eebab | 371 | hex - Similar to raw, but the numbers will be in a hexadecimal format. |
eb6d42ea SR |
372 | |
373 | bin - This will print out the formats in raw binary. | |
374 | ||
375 | block - TBD (needs update) | |
376 | ||
377 | stacktrace - This is one of the options that changes the trace itself. | |
378 | When a trace is recorded, so is the stack of functions. | |
379 | This allows for back traces of trace sites. | |
380 | ||
381 | sched-tree - TBD (any users??) | |
382 | ||
383 | ||
384 | sched_switch | |
385 | ------------ | |
386 | ||
f2d9c740 | 387 | This tracer simply records schedule switches. Here is an example |
a41eebab | 388 | of how to use it. |
eb6d42ea SR |
389 | |
390 | # echo sched_switch > /debug/tracing/current_tracer | |
391 | # echo 1 > /debug/tracing/tracing_enabled | |
392 | # sleep 1 | |
393 | # echo 0 > /debug/tracing/tracing_enabled | |
394 | # cat /debug/tracing/trace | |
395 | ||
396 | # tracer: sched_switch | |
397 | # | |
398 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
399 | # | | | | | | |
400 | bash-3997 [01] 240.132281: 3997:120:R + 4055:120:R | |
401 | bash-3997 [01] 240.132284: 3997:120:R ==> 4055:120:R | |
402 | sleep-4055 [01] 240.132371: 4055:120:S ==> 3997:120:R | |
403 | bash-3997 [01] 240.132454: 3997:120:R + 4055:120:S | |
404 | bash-3997 [01] 240.132457: 3997:120:R ==> 4055:120:R | |
405 | sleep-4055 [01] 240.132460: 4055:120:D ==> 3997:120:R | |
406 | bash-3997 [01] 240.132463: 3997:120:R + 4055:120:D | |
407 | bash-3997 [01] 240.132465: 3997:120:R ==> 4055:120:R | |
408 | <idle>-0 [00] 240.132589: 0:140:R + 4:115:S | |
409 | <idle>-0 [00] 240.132591: 0:140:R ==> 4:115:R | |
410 | ksoftirqd/0-4 [00] 240.132595: 4:115:S ==> 0:140:R | |
411 | <idle>-0 [00] 240.132598: 0:140:R + 4:115:S | |
412 | <idle>-0 [00] 240.132599: 0:140:R ==> 4:115:R | |
413 | ksoftirqd/0-4 [00] 240.132603: 4:115:S ==> 0:140:R | |
414 | sleep-4055 [01] 240.133058: 4055:120:S ==> 3997:120:R | |
415 | [...] | |
416 | ||
417 | ||
418 | As we have discussed previously about this format, the header shows | |
419 | the name of the trace and points to the options. The "FUNCTION" | |
420 | is a misnomer since here it represents the wake ups and context | |
421 | switches. | |
422 | ||
f2d9c740 SR |
423 | The sched_switch file only lists the wake ups (represented with '+') |
424 | and context switches ('==>') with the previous task or current task | |
eb6d42ea SR |
425 | first followed by the next task or task waking up. The format for both |
426 | of these is PID:KERNEL-PRIO:TASK-STATE. Remember that the KERNEL-PRIO | |
427 | is the inverse of the actual priority with zero (0) being the highest | |
428 | priority and the nice values starting at 100 (nice -20). Below is | |
429 | a quick chart to map the kernel priority to user land priorities. | |
430 | ||
431 | Kernel priority: 0 to 99 ==> user RT priority 99 to 0 | |
432 | Kernel priority: 100 to 139 ==> user nice -20 to 19 | |
433 | Kernel priority: 140 ==> idle task priority | |
434 | ||
435 | The task states are: | |
436 | ||
437 | R - running : wants to run, may not actually be running | |
438 | S - sleep : process is waiting to be woken up (handles signals) | |
f2d9c740 SR |
439 | D - disk sleep (uninterruptible sleep) : process must be woken up |
440 | (ignores signals) | |
eb6d42ea SR |
441 | T - stopped : process suspended |
442 | t - traced : process is being traced (with something like gdb) | |
443 | Z - zombie : process waiting to be cleaned up | |
444 | X - unknown | |
445 | ||
446 | ||
447 | ftrace_enabled | |
448 | -------------- | |
449 | ||
f2d9c740 SR |
450 | The following tracers (listed below) give different output depending |
451 | on whether or not the sysctl ftrace_enabled is set. To set ftrace_enabled, | |
eb6d42ea SR |
452 | one can either use the sysctl function or set it via the proc |
453 | file system interface. | |
454 | ||
455 | sysctl kernel.ftrace_enabled=1 | |
456 | ||
457 | or | |
458 | ||
459 | echo 1 > /proc/sys/kernel/ftrace_enabled | |
460 | ||
461 | To disable ftrace_enabled simply replace the '1' with '0' in | |
462 | the above commands. | |
463 | ||
464 | When ftrace_enabled is set the tracers will also record the functions | |
465 | that are within the trace. The descriptions of the tracers | |
466 | will also show an example with ftrace enabled. | |
467 | ||
468 | ||
469 | irqsoff | |
470 | ------- | |
471 | ||
472 | When interrupts are disabled, the CPU can not react to any other | |
473 | external event (besides NMIs and SMIs). This prevents the timer | |
474 | interrupt from triggering or the mouse interrupt from letting the | |
475 | kernel know of a new mouse event. The result is a latency with the | |
476 | reaction time. | |
477 | ||
f2d9c740 SR |
478 | The irqsoff tracer tracks the time for which interrupts are disabled. |
479 | When a new maximum latency is hit, the tracer saves the trace leading up | |
480 | to that latency point so that every time a new maximum is reached, the old | |
481 | saved trace is discarded and the new trace is saved. | |
eb6d42ea | 482 | |
f2d9c740 | 483 | To reset the maximum, echo 0 into tracing_max_latency. Here is an |
eb6d42ea SR |
484 | example: |
485 | ||
486 | # echo irqsoff > /debug/tracing/current_tracer | |
487 | # echo 0 > /debug/tracing/tracing_max_latency | |
488 | # echo 1 > /debug/tracing/tracing_enabled | |
489 | # ls -ltr | |
490 | [...] | |
491 | # echo 0 > /debug/tracing/tracing_enabled | |
492 | # cat /debug/tracing/latency_trace | |
493 | # tracer: irqsoff | |
494 | # | |
f2d9c740 | 495 | irqsoff latency trace v1.1.5 on 2.6.26 |
eb6d42ea | 496 | -------------------------------------------------------------------- |
f2d9c740 | 497 | latency: 12 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) |
eb6d42ea | 498 | ----------------- |
f2d9c740 | 499 | | task: bash-3730 (uid:0 nice:0 policy:0 rt_prio:0) |
eb6d42ea | 500 | ----------------- |
f2d9c740 SR |
501 | => started at: sys_setpgid |
502 | => ended at: sys_setpgid | |
eb6d42ea SR |
503 | |
504 | # _------=> CPU# | |
505 | # / _-----=> irqs-off | |
506 | # | / _----=> need-resched | |
507 | # || / _---=> hardirq/softirq | |
508 | # ||| / _--=> preempt-depth | |
509 | # |||| / | |
510 | # ||||| delay | |
511 | # cmd pid ||||| time | caller | |
512 | # \ / ||||| \ | / | |
f2d9c740 SR |
513 | bash-3730 1d... 0us : _write_lock_irq (sys_setpgid) |
514 | bash-3730 1d..1 1us+: _write_unlock_irq (sys_setpgid) | |
515 | bash-3730 1d..2 14us : trace_hardirqs_on (sys_setpgid) | |
eb6d42ea | 516 | |
eb6d42ea | 517 | |
f2d9c740 SR |
518 | Here we see that that we had a latency of 12 microsecs (which is |
519 | very good). The _write_lock_irq in sys_setpgid disabled interrupts. | |
520 | The difference between the 12 and the displayed timestamp 14us occurred | |
521 | because the clock was incremented between the time of recording the max | |
522 | latency and the time of recording the function that had that latency. | |
eb6d42ea | 523 | |
f2d9c740 SR |
524 | Note the above example had ftrace_enabled not set. If we set the |
525 | ftrace_enabled, we get a much larger output: | |
eb6d42ea SR |
526 | |
527 | # tracer: irqsoff | |
528 | # | |
529 | irqsoff latency trace v1.1.5 on 2.6.26-rc8 | |
530 | -------------------------------------------------------------------- | |
531 | latency: 50 us, #101/101, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
532 | ----------------- | |
533 | | task: ls-4339 (uid:0 nice:0 policy:0 rt_prio:0) | |
534 | ----------------- | |
535 | => started at: __alloc_pages_internal | |
536 | => ended at: __alloc_pages_internal | |
537 | ||
538 | # _------=> CPU# | |
539 | # / _-----=> irqs-off | |
540 | # | / _----=> need-resched | |
541 | # || / _---=> hardirq/softirq | |
542 | # ||| / _--=> preempt-depth | |
543 | # |||| / | |
544 | # ||||| delay | |
545 | # cmd pid ||||| time | caller | |
546 | # \ / ||||| \ | / | |
547 | ls-4339 0...1 0us+: get_page_from_freelist (__alloc_pages_internal) | |
548 | ls-4339 0d..1 3us : rmqueue_bulk (get_page_from_freelist) | |
549 | ls-4339 0d..1 3us : _spin_lock (rmqueue_bulk) | |
550 | ls-4339 0d..1 4us : add_preempt_count (_spin_lock) | |
551 | ls-4339 0d..2 4us : __rmqueue (rmqueue_bulk) | |
552 | ls-4339 0d..2 5us : __rmqueue_smallest (__rmqueue) | |
553 | ls-4339 0d..2 5us : __mod_zone_page_state (__rmqueue_smallest) | |
554 | ls-4339 0d..2 6us : __rmqueue (rmqueue_bulk) | |
555 | ls-4339 0d..2 6us : __rmqueue_smallest (__rmqueue) | |
556 | ls-4339 0d..2 7us : __mod_zone_page_state (__rmqueue_smallest) | |
557 | ls-4339 0d..2 7us : __rmqueue (rmqueue_bulk) | |
558 | ls-4339 0d..2 8us : __rmqueue_smallest (__rmqueue) | |
559 | [...] | |
560 | ls-4339 0d..2 46us : __rmqueue_smallest (__rmqueue) | |
561 | ls-4339 0d..2 47us : __mod_zone_page_state (__rmqueue_smallest) | |
562 | ls-4339 0d..2 47us : __rmqueue (rmqueue_bulk) | |
563 | ls-4339 0d..2 48us : __rmqueue_smallest (__rmqueue) | |
564 | ls-4339 0d..2 48us : __mod_zone_page_state (__rmqueue_smallest) | |
565 | ls-4339 0d..2 49us : _spin_unlock (rmqueue_bulk) | |
566 | ls-4339 0d..2 49us : sub_preempt_count (_spin_unlock) | |
567 | ls-4339 0d..1 50us : get_page_from_freelist (__alloc_pages_internal) | |
568 | ls-4339 0d..2 51us : trace_hardirqs_on (__alloc_pages_internal) | |
569 | ||
570 | ||
eb6d42ea SR |
571 | |
572 | Here we traced a 50 microsecond latency. But we also see all the | |
a41eebab | 573 | functions that were called during that time. Note that by enabling |
f2d9c740 | 574 | function tracing, we incur an added overhead. This overhead may |
a41eebab SR |
575 | extend the latency times. But nevertheless, this trace has provided |
576 | some very helpful debugging information. | |
eb6d42ea SR |
577 | |
578 | ||
579 | preemptoff | |
580 | ---------- | |
581 | ||
a41eebab SR |
582 | When preemption is disabled, we may be able to receive interrupts but |
583 | the task cannot be preempted and a higher priority task must wait | |
eb6d42ea SR |
584 | for preemption to be enabled again before it can preempt a lower |
585 | priority task. | |
586 | ||
a41eebab | 587 | The preemptoff tracer traces the places that disable preemption. |
f2d9c740 SR |
588 | Like the irqsoff tracer, it records the maximum latency for which preemption |
589 | was disabled. The control of preemptoff tracer is much like the irqsoff | |
590 | tracer. | |
eb6d42ea SR |
591 | |
592 | # echo preemptoff > /debug/tracing/current_tracer | |
593 | # echo 0 > /debug/tracing/tracing_max_latency | |
594 | # echo 1 > /debug/tracing/tracing_enabled | |
595 | # ls -ltr | |
596 | [...] | |
597 | # echo 0 > /debug/tracing/tracing_enabled | |
598 | # cat /debug/tracing/latency_trace | |
599 | # tracer: preemptoff | |
600 | # | |
601 | preemptoff latency trace v1.1.5 on 2.6.26-rc8 | |
602 | -------------------------------------------------------------------- | |
603 | latency: 29 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
604 | ----------------- | |
605 | | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) | |
606 | ----------------- | |
607 | => started at: do_IRQ | |
608 | => ended at: __do_softirq | |
609 | ||
610 | # _------=> CPU# | |
611 | # / _-----=> irqs-off | |
612 | # | / _----=> need-resched | |
613 | # || / _---=> hardirq/softirq | |
614 | # ||| / _--=> preempt-depth | |
615 | # |||| / | |
616 | # ||||| delay | |
617 | # cmd pid ||||| time | caller | |
618 | # \ / ||||| \ | / | |
619 | sshd-4261 0d.h. 0us+: irq_enter (do_IRQ) | |
620 | sshd-4261 0d.s. 29us : _local_bh_enable (__do_softirq) | |
621 | sshd-4261 0d.s1 30us : trace_preempt_on (__do_softirq) | |
622 | ||
623 | ||
eb6d42ea SR |
624 | This has some more changes. Preemption was disabled when an interrupt |
625 | came in (notice the 'h'), and was enabled while doing a softirq. | |
626 | (notice the 's'). But we also see that interrupts have been disabled | |
627 | when entering the preempt off section and leaving it (the 'd'). | |
628 | We do not know if interrupts were enabled in the mean time. | |
629 | ||
630 | # tracer: preemptoff | |
631 | # | |
632 | preemptoff latency trace v1.1.5 on 2.6.26-rc8 | |
633 | -------------------------------------------------------------------- | |
634 | latency: 63 us, #87/87, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
635 | ----------------- | |
636 | | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) | |
637 | ----------------- | |
638 | => started at: remove_wait_queue | |
639 | => ended at: __do_softirq | |
640 | ||
641 | # _------=> CPU# | |
642 | # / _-----=> irqs-off | |
643 | # | / _----=> need-resched | |
644 | # || / _---=> hardirq/softirq | |
645 | # ||| / _--=> preempt-depth | |
646 | # |||| / | |
647 | # ||||| delay | |
648 | # cmd pid ||||| time | caller | |
649 | # \ / ||||| \ | / | |
650 | sshd-4261 0d..1 0us : _spin_lock_irqsave (remove_wait_queue) | |
651 | sshd-4261 0d..1 1us : _spin_unlock_irqrestore (remove_wait_queue) | |
652 | sshd-4261 0d..1 2us : do_IRQ (common_interrupt) | |
653 | sshd-4261 0d..1 2us : irq_enter (do_IRQ) | |
654 | sshd-4261 0d..1 2us : idle_cpu (irq_enter) | |
655 | sshd-4261 0d..1 3us : add_preempt_count (irq_enter) | |
656 | sshd-4261 0d.h1 3us : idle_cpu (irq_enter) | |
657 | sshd-4261 0d.h. 4us : handle_fasteoi_irq (do_IRQ) | |
658 | [...] | |
659 | sshd-4261 0d.h. 12us : add_preempt_count (_spin_lock) | |
660 | sshd-4261 0d.h1 12us : ack_ioapic_quirk_irq (handle_fasteoi_irq) | |
661 | sshd-4261 0d.h1 13us : move_native_irq (ack_ioapic_quirk_irq) | |
662 | sshd-4261 0d.h1 13us : _spin_unlock (handle_fasteoi_irq) | |
663 | sshd-4261 0d.h1 14us : sub_preempt_count (_spin_unlock) | |
664 | sshd-4261 0d.h1 14us : irq_exit (do_IRQ) | |
665 | sshd-4261 0d.h1 15us : sub_preempt_count (irq_exit) | |
666 | sshd-4261 0d..2 15us : do_softirq (irq_exit) | |
667 | sshd-4261 0d... 15us : __do_softirq (do_softirq) | |
668 | sshd-4261 0d... 16us : __local_bh_disable (__do_softirq) | |
669 | sshd-4261 0d... 16us+: add_preempt_count (__local_bh_disable) | |
670 | sshd-4261 0d.s4 20us : add_preempt_count (__local_bh_disable) | |
671 | sshd-4261 0d.s4 21us : sub_preempt_count (local_bh_enable) | |
672 | sshd-4261 0d.s5 21us : sub_preempt_count (local_bh_enable) | |
673 | [...] | |
674 | sshd-4261 0d.s6 41us : add_preempt_count (__local_bh_disable) | |
675 | sshd-4261 0d.s6 42us : sub_preempt_count (local_bh_enable) | |
676 | sshd-4261 0d.s7 42us : sub_preempt_count (local_bh_enable) | |
677 | sshd-4261 0d.s5 43us : add_preempt_count (__local_bh_disable) | |
678 | sshd-4261 0d.s5 43us : sub_preempt_count (local_bh_enable_ip) | |
679 | sshd-4261 0d.s6 44us : sub_preempt_count (local_bh_enable_ip) | |
680 | sshd-4261 0d.s5 44us : add_preempt_count (__local_bh_disable) | |
681 | sshd-4261 0d.s5 45us : sub_preempt_count (local_bh_enable) | |
682 | [...] | |
683 | sshd-4261 0d.s. 63us : _local_bh_enable (__do_softirq) | |
684 | sshd-4261 0d.s1 64us : trace_preempt_on (__do_softirq) | |
685 | ||
686 | ||
687 | The above is an example of the preemptoff trace with ftrace_enabled | |
688 | set. Here we see that interrupts were disabled the entire time. | |
689 | The irq_enter code lets us know that we entered an interrupt 'h'. | |
690 | Before that, the functions being traced still show that it is not | |
f2d9c740 | 691 | in an interrupt, but we can see from the functions themselves that |
eb6d42ea SR |
692 | this is not the case. |
693 | ||
f2d9c740 SR |
694 | Notice that __do_softirq when called does not have a preempt_count. |
695 | It may seem that we missed a preempt enabling. What really happened | |
696 | is that the preempt count is held on the thread's stack and we | |
eb6d42ea | 697 | switched to the softirq stack (4K stacks in effect). The code |
a41eebab | 698 | does not copy the preempt count, but because interrupts are disabled, |
f2d9c740 SR |
699 | we do not need to worry about it. Having a tracer like this is good |
700 | for letting people know what really happens inside the kernel. | |
eb6d42ea SR |
701 | |
702 | ||
703 | preemptirqsoff | |
704 | -------------- | |
705 | ||
706 | Knowing the locations that have interrupts disabled or preemption | |
707 | disabled for the longest times is helpful. But sometimes we would | |
708 | like to know when either preemption and/or interrupts are disabled. | |
709 | ||
f2d9c740 | 710 | Consider the following code: |
eb6d42ea SR |
711 | |
712 | local_irq_disable(); | |
713 | call_function_with_irqs_off(); | |
714 | preempt_disable(); | |
715 | call_function_with_irqs_and_preemption_off(); | |
716 | local_irq_enable(); | |
717 | call_function_with_preemption_off(); | |
718 | preempt_enable(); | |
719 | ||
720 | The irqsoff tracer will record the total length of | |
721 | call_function_with_irqs_off() and | |
722 | call_function_with_irqs_and_preemption_off(). | |
723 | ||
724 | The preemptoff tracer will record the total length of | |
725 | call_function_with_irqs_and_preemption_off() and | |
726 | call_function_with_preemption_off(). | |
727 | ||
728 | But neither will trace the time that interrupts and/or preemption | |
729 | is disabled. This total time is the time that we can not schedule. | |
730 | To record this time, use the preemptirqsoff tracer. | |
731 | ||
732 | Again, using this trace is much like the irqsoff and preemptoff tracers. | |
733 | ||
a41eebab | 734 | # echo preemptirqsoff > /debug/tracing/current_tracer |
eb6d42ea SR |
735 | # echo 0 > /debug/tracing/tracing_max_latency |
736 | # echo 1 > /debug/tracing/tracing_enabled | |
737 | # ls -ltr | |
738 | [...] | |
739 | # echo 0 > /debug/tracing/tracing_enabled | |
740 | # cat /debug/tracing/latency_trace | |
741 | # tracer: preemptirqsoff | |
742 | # | |
743 | preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8 | |
744 | -------------------------------------------------------------------- | |
745 | latency: 293 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
746 | ----------------- | |
747 | | task: ls-4860 (uid:0 nice:0 policy:0 rt_prio:0) | |
748 | ----------------- | |
749 | => started at: apic_timer_interrupt | |
750 | => ended at: __do_softirq | |
751 | ||
752 | # _------=> CPU# | |
753 | # / _-----=> irqs-off | |
754 | # | / _----=> need-resched | |
755 | # || / _---=> hardirq/softirq | |
756 | # ||| / _--=> preempt-depth | |
757 | # |||| / | |
758 | # ||||| delay | |
759 | # cmd pid ||||| time | caller | |
760 | # \ / ||||| \ | / | |
761 | ls-4860 0d... 0us!: trace_hardirqs_off_thunk (apic_timer_interrupt) | |
762 | ls-4860 0d.s. 294us : _local_bh_enable (__do_softirq) | |
763 | ls-4860 0d.s1 294us : trace_preempt_on (__do_softirq) | |
764 | ||
765 | ||
eb6d42ea SR |
766 | |
767 | The trace_hardirqs_off_thunk is called from assembly on x86 when | |
768 | interrupts are disabled in the assembly code. Without the function | |
f2d9c740 | 769 | tracing, we do not know if interrupts were enabled within the preemption |
eb6d42ea SR |
770 | points. We do see that it started with preemption enabled. |
771 | ||
772 | Here is a trace with ftrace_enabled set: | |
773 | ||
774 | ||
775 | # tracer: preemptirqsoff | |
776 | # | |
777 | preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8 | |
778 | -------------------------------------------------------------------- | |
779 | latency: 105 us, #183/183, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
780 | ----------------- | |
781 | | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) | |
782 | ----------------- | |
783 | => started at: write_chan | |
784 | => ended at: __do_softirq | |
785 | ||
786 | # _------=> CPU# | |
787 | # / _-----=> irqs-off | |
788 | # | / _----=> need-resched | |
789 | # || / _---=> hardirq/softirq | |
790 | # ||| / _--=> preempt-depth | |
791 | # |||| / | |
792 | # ||||| delay | |
793 | # cmd pid ||||| time | caller | |
794 | # \ / ||||| \ | / | |
795 | ls-4473 0.N.. 0us : preempt_schedule (write_chan) | |
796 | ls-4473 0dN.1 1us : _spin_lock (schedule) | |
797 | ls-4473 0dN.1 2us : add_preempt_count (_spin_lock) | |
798 | ls-4473 0d..2 2us : put_prev_task_fair (schedule) | |
799 | [...] | |
800 | ls-4473 0d..2 13us : set_normalized_timespec (ktime_get_ts) | |
801 | ls-4473 0d..2 13us : __switch_to (schedule) | |
802 | sshd-4261 0d..2 14us : finish_task_switch (schedule) | |
803 | sshd-4261 0d..2 14us : _spin_unlock_irq (finish_task_switch) | |
804 | sshd-4261 0d..1 15us : add_preempt_count (_spin_lock_irqsave) | |
805 | sshd-4261 0d..2 16us : _spin_unlock_irqrestore (hrtick_set) | |
806 | sshd-4261 0d..2 16us : do_IRQ (common_interrupt) | |
807 | sshd-4261 0d..2 17us : irq_enter (do_IRQ) | |
808 | sshd-4261 0d..2 17us : idle_cpu (irq_enter) | |
809 | sshd-4261 0d..2 18us : add_preempt_count (irq_enter) | |
810 | sshd-4261 0d.h2 18us : idle_cpu (irq_enter) | |
811 | sshd-4261 0d.h. 18us : handle_fasteoi_irq (do_IRQ) | |
812 | sshd-4261 0d.h. 19us : _spin_lock (handle_fasteoi_irq) | |
813 | sshd-4261 0d.h. 19us : add_preempt_count (_spin_lock) | |
814 | sshd-4261 0d.h1 20us : _spin_unlock (handle_fasteoi_irq) | |
815 | sshd-4261 0d.h1 20us : sub_preempt_count (_spin_unlock) | |
816 | [...] | |
817 | sshd-4261 0d.h1 28us : _spin_unlock (handle_fasteoi_irq) | |
818 | sshd-4261 0d.h1 29us : sub_preempt_count (_spin_unlock) | |
819 | sshd-4261 0d.h2 29us : irq_exit (do_IRQ) | |
820 | sshd-4261 0d.h2 29us : sub_preempt_count (irq_exit) | |
821 | sshd-4261 0d..3 30us : do_softirq (irq_exit) | |
822 | sshd-4261 0d... 30us : __do_softirq (do_softirq) | |
823 | sshd-4261 0d... 31us : __local_bh_disable (__do_softirq) | |
824 | sshd-4261 0d... 31us+: add_preempt_count (__local_bh_disable) | |
825 | sshd-4261 0d.s4 34us : add_preempt_count (__local_bh_disable) | |
826 | [...] | |
827 | sshd-4261 0d.s3 43us : sub_preempt_count (local_bh_enable_ip) | |
828 | sshd-4261 0d.s4 44us : sub_preempt_count (local_bh_enable_ip) | |
829 | sshd-4261 0d.s3 44us : smp_apic_timer_interrupt (apic_timer_interrupt) | |
830 | sshd-4261 0d.s3 45us : irq_enter (smp_apic_timer_interrupt) | |
831 | sshd-4261 0d.s3 45us : idle_cpu (irq_enter) | |
832 | sshd-4261 0d.s3 46us : add_preempt_count (irq_enter) | |
833 | sshd-4261 0d.H3 46us : idle_cpu (irq_enter) | |
834 | sshd-4261 0d.H3 47us : hrtimer_interrupt (smp_apic_timer_interrupt) | |
835 | sshd-4261 0d.H3 47us : ktime_get (hrtimer_interrupt) | |
836 | [...] | |
837 | sshd-4261 0d.H3 81us : tick_program_event (hrtimer_interrupt) | |
838 | sshd-4261 0d.H3 82us : ktime_get (tick_program_event) | |
839 | sshd-4261 0d.H3 82us : ktime_get_ts (ktime_get) | |
840 | sshd-4261 0d.H3 83us : getnstimeofday (ktime_get_ts) | |
841 | sshd-4261 0d.H3 83us : set_normalized_timespec (ktime_get_ts) | |
842 | sshd-4261 0d.H3 84us : clockevents_program_event (tick_program_event) | |
843 | sshd-4261 0d.H3 84us : lapic_next_event (clockevents_program_event) | |
844 | sshd-4261 0d.H3 85us : irq_exit (smp_apic_timer_interrupt) | |
845 | sshd-4261 0d.H3 85us : sub_preempt_count (irq_exit) | |
846 | sshd-4261 0d.s4 86us : sub_preempt_count (irq_exit) | |
847 | sshd-4261 0d.s3 86us : add_preempt_count (__local_bh_disable) | |
848 | [...] | |
849 | sshd-4261 0d.s1 98us : sub_preempt_count (net_rx_action) | |
850 | sshd-4261 0d.s. 99us : add_preempt_count (_spin_lock_irq) | |
851 | sshd-4261 0d.s1 99us+: _spin_unlock_irq (run_timer_softirq) | |
852 | sshd-4261 0d.s. 104us : _local_bh_enable (__do_softirq) | |
853 | sshd-4261 0d.s. 104us : sub_preempt_count (_local_bh_enable) | |
854 | sshd-4261 0d.s. 105us : _local_bh_enable (__do_softirq) | |
855 | sshd-4261 0d.s1 105us : trace_preempt_on (__do_softirq) | |
856 | ||
857 | ||
858 | This is a very interesting trace. It started with the preemption of | |
859 | the ls task. We see that the task had the "need_resched" bit set | |
f2d9c740 SR |
860 | via the 'N' in the trace. Interrupts were disabled before the spin_lock |
861 | at the beginning of the trace. We see that a schedule took place to run | |
a41eebab SR |
862 | sshd. When the interrupts were enabled, we took an interrupt. |
863 | On return from the interrupt handler, the softirq ran. We took another | |
f2d9c740 | 864 | interrupt while running the softirq as we see from the capital 'H'. |
eb6d42ea SR |
865 | |
866 | ||
867 | wakeup | |
868 | ------ | |
869 | ||
f2d9c740 SR |
870 | In a Real-Time environment it is very important to know the wakeup |
871 | time it takes for the highest priority task that is woken up to the | |
872 | time that it executes. This is also known as "schedule latency". | |
eb6d42ea SR |
873 | I stress the point that this is about RT tasks. It is also important |
874 | to know the scheduling latency of non-RT tasks, but the average | |
875 | schedule latency is better for non-RT tasks. Tools like | |
a41eebab | 876 | LatencyTop are more appropriate for such measurements. |
eb6d42ea | 877 | |
a41eebab | 878 | Real-Time environments are interested in the worst case latency. |
eb6d42ea SR |
879 | That is the longest latency it takes for something to happen, and |
880 | not the average. We can have a very fast scheduler that may only | |
881 | have a large latency once in a while, but that would not work well | |
882 | with Real-Time tasks. The wakeup tracer was designed to record | |
883 | the worst case wakeups of RT tasks. Non-RT tasks are not recorded | |
884 | because the tracer only records one worst case and tracing non-RT | |
885 | tasks that are unpredictable will overwrite the worst case latency | |
886 | of RT tasks. | |
887 | ||
888 | Since this tracer only deals with RT tasks, we will run this slightly | |
a41eebab SR |
889 | differently than we did with the previous tracers. Instead of performing |
890 | an 'ls', we will run 'sleep 1' under 'chrt' which changes the | |
eb6d42ea SR |
891 | priority of the task. |
892 | ||
893 | # echo wakeup > /debug/tracing/current_tracer | |
894 | # echo 0 > /debug/tracing/tracing_max_latency | |
895 | # echo 1 > /debug/tracing/tracing_enabled | |
896 | # chrt -f 5 sleep 1 | |
897 | # echo 0 > /debug/tracing/tracing_enabled | |
898 | # cat /debug/tracing/latency_trace | |
899 | # tracer: wakeup | |
900 | # | |
901 | wakeup latency trace v1.1.5 on 2.6.26-rc8 | |
902 | -------------------------------------------------------------------- | |
903 | latency: 4 us, #2/2, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
904 | ----------------- | |
905 | | task: sleep-4901 (uid:0 nice:0 policy:1 rt_prio:5) | |
906 | ----------------- | |
907 | ||
908 | # _------=> CPU# | |
909 | # / _-----=> irqs-off | |
910 | # | / _----=> need-resched | |
911 | # || / _---=> hardirq/softirq | |
912 | # ||| / _--=> preempt-depth | |
913 | # |||| / | |
914 | # ||||| delay | |
915 | # cmd pid ||||| time | caller | |
916 | # \ / ||||| \ | / | |
917 | <idle>-0 1d.h4 0us+: try_to_wake_up (wake_up_process) | |
918 | <idle>-0 1d..4 4us : schedule (cpu_idle) | |
919 | ||
920 | ||
eb6d42ea | 921 | |
a41eebab | 922 | Running this on an idle system, we see that it only took 4 microseconds |
eb6d42ea | 923 | to perform the task switch. Note, since the trace marker in the |
a41eebab | 924 | schedule is before the actual "switch", we stop the tracing when |
eb6d42ea SR |
925 | the recorded task is about to schedule in. This may change if |
926 | we add a new marker at the end of the scheduler. | |
927 | ||
928 | Notice that the recorded task is 'sleep' with the PID of 4901 and it | |
929 | has an rt_prio of 5. This priority is user-space priority and not | |
930 | the internal kernel priority. The policy is 1 for SCHED_FIFO and 2 | |
931 | for SCHED_RR. | |
932 | ||
933 | Doing the same with chrt -r 5 and ftrace_enabled set. | |
934 | ||
935 | # tracer: wakeup | |
936 | # | |
937 | wakeup latency trace v1.1.5 on 2.6.26-rc8 | |
938 | -------------------------------------------------------------------- | |
939 | latency: 50 us, #60/60, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) | |
940 | ----------------- | |
941 | | task: sleep-4068 (uid:0 nice:0 policy:2 rt_prio:5) | |
942 | ----------------- | |
943 | ||
944 | # _------=> CPU# | |
945 | # / _-----=> irqs-off | |
946 | # | / _----=> need-resched | |
947 | # || / _---=> hardirq/softirq | |
948 | # ||| / _--=> preempt-depth | |
949 | # |||| / | |
950 | # ||||| delay | |
951 | # cmd pid ||||| time | caller | |
952 | # \ / ||||| \ | / | |
953 | ksoftirq-7 1d.H3 0us : try_to_wake_up (wake_up_process) | |
954 | ksoftirq-7 1d.H4 1us : sub_preempt_count (marker_probe_cb) | |
955 | ksoftirq-7 1d.H3 2us : check_preempt_wakeup (try_to_wake_up) | |
956 | ksoftirq-7 1d.H3 3us : update_curr (check_preempt_wakeup) | |
957 | ksoftirq-7 1d.H3 4us : calc_delta_mine (update_curr) | |
958 | ksoftirq-7 1d.H3 5us : __resched_task (check_preempt_wakeup) | |
959 | ksoftirq-7 1d.H3 6us : task_wake_up_rt (try_to_wake_up) | |
960 | ksoftirq-7 1d.H3 7us : _spin_unlock_irqrestore (try_to_wake_up) | |
961 | [...] | |
962 | ksoftirq-7 1d.H2 17us : irq_exit (smp_apic_timer_interrupt) | |
963 | ksoftirq-7 1d.H2 18us : sub_preempt_count (irq_exit) | |
964 | ksoftirq-7 1d.s3 19us : sub_preempt_count (irq_exit) | |
965 | ksoftirq-7 1..s2 20us : rcu_process_callbacks (__do_softirq) | |
966 | [...] | |
967 | ksoftirq-7 1..s2 26us : __rcu_process_callbacks (rcu_process_callbacks) | |
968 | ksoftirq-7 1d.s2 27us : _local_bh_enable (__do_softirq) | |
969 | ksoftirq-7 1d.s2 28us : sub_preempt_count (_local_bh_enable) | |
970 | ksoftirq-7 1.N.3 29us : sub_preempt_count (ksoftirqd) | |
971 | ksoftirq-7 1.N.2 30us : _cond_resched (ksoftirqd) | |
972 | ksoftirq-7 1.N.2 31us : __cond_resched (_cond_resched) | |
973 | ksoftirq-7 1.N.2 32us : add_preempt_count (__cond_resched) | |
974 | ksoftirq-7 1.N.2 33us : schedule (__cond_resched) | |
975 | ksoftirq-7 1.N.2 33us : add_preempt_count (schedule) | |
976 | ksoftirq-7 1.N.3 34us : hrtick_clear (schedule) | |
977 | ksoftirq-7 1dN.3 35us : _spin_lock (schedule) | |
978 | ksoftirq-7 1dN.3 36us : add_preempt_count (_spin_lock) | |
979 | ksoftirq-7 1d..4 37us : put_prev_task_fair (schedule) | |
980 | ksoftirq-7 1d..4 38us : update_curr (put_prev_task_fair) | |
981 | [...] | |
982 | ksoftirq-7 1d..5 47us : _spin_trylock (tracing_record_cmdline) | |
983 | ksoftirq-7 1d..5 48us : add_preempt_count (_spin_trylock) | |
984 | ksoftirq-7 1d..6 49us : _spin_unlock (tracing_record_cmdline) | |
985 | ksoftirq-7 1d..6 49us : sub_preempt_count (_spin_unlock) | |
986 | ksoftirq-7 1d..4 50us : schedule (__cond_resched) | |
987 | ||
988 | The interrupt went off while running ksoftirqd. This task runs at | |
f2d9c740 | 989 | SCHED_OTHER. Why did not we see the 'N' set early? This may be |
a41eebab | 990 | a harmless bug with x86_32 and 4K stacks. On x86_32 with 4K stacks |
f2d9c740 | 991 | configured, the interrupt and softirq run with their own stack. |
a41eebab SR |
992 | Some information is held on the top of the task's stack (need_resched |
993 | and preempt_count are both stored there). The setting of the NEED_RESCHED | |
994 | bit is done directly to the task's stack, but the reading of the | |
995 | NEED_RESCHED is done by looking at the current stack, which in this case | |
996 | is the stack for the hard interrupt. This hides the fact that NEED_RESCHED | |
f2d9c740 | 997 | has been set. We do not see the 'N' until we switch back to the task's |
a41eebab | 998 | assigned stack. |
eb6d42ea | 999 | |
9b803c0f SR |
1000 | function |
1001 | -------- | |
eb6d42ea | 1002 | |
9b803c0f SR |
1003 | This tracer is the function tracer. Enabling the function tracer |
1004 | can be done from the debug file system. Make sure the ftrace_enabled is | |
1005 | set; otherwise this tracer is a nop. | |
eb6d42ea SR |
1006 | |
1007 | # sysctl kernel.ftrace_enabled=1 | |
9b803c0f | 1008 | # echo function > /debug/tracing/current_tracer |
eb6d42ea SR |
1009 | # echo 1 > /debug/tracing/tracing_enabled |
1010 | # usleep 1 | |
1011 | # echo 0 > /debug/tracing/tracing_enabled | |
1012 | # cat /debug/tracing/trace | |
9b803c0f | 1013 | # tracer: function |
eb6d42ea SR |
1014 | # |
1015 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1016 | # | | | | | | |
1017 | bash-4003 [00] 123.638713: finish_task_switch <-schedule | |
1018 | bash-4003 [00] 123.638714: _spin_unlock_irq <-finish_task_switch | |
1019 | bash-4003 [00] 123.638714: sub_preempt_count <-_spin_unlock_irq | |
1020 | bash-4003 [00] 123.638715: hrtick_set <-schedule | |
1021 | bash-4003 [00] 123.638715: _spin_lock_irqsave <-hrtick_set | |
1022 | bash-4003 [00] 123.638716: add_preempt_count <-_spin_lock_irqsave | |
1023 | bash-4003 [00] 123.638716: _spin_unlock_irqrestore <-hrtick_set | |
1024 | bash-4003 [00] 123.638717: sub_preempt_count <-_spin_unlock_irqrestore | |
1025 | bash-4003 [00] 123.638717: hrtick_clear <-hrtick_set | |
1026 | bash-4003 [00] 123.638718: sub_preempt_count <-schedule | |
1027 | bash-4003 [00] 123.638718: sub_preempt_count <-preempt_schedule | |
1028 | bash-4003 [00] 123.638719: wait_for_completion <-__stop_machine_run | |
1029 | bash-4003 [00] 123.638719: wait_for_common <-wait_for_completion | |
1030 | bash-4003 [00] 123.638720: _spin_lock_irq <-wait_for_common | |
1031 | bash-4003 [00] 123.638720: add_preempt_count <-_spin_lock_irq | |
1032 | [...] | |
1033 | ||
1034 | ||
9b803c0f SR |
1035 | Note: function tracer uses ring buffers to store the above entries. |
1036 | The newest data may overwrite the oldest data. Sometimes using echo to | |
1037 | stop the trace is not sufficient because the tracing could have overwritten | |
1038 | the data that you wanted to record. For this reason, it is sometimes better to | |
f2d9c740 SR |
1039 | disable tracing directly from a program. This allows you to stop the |
1040 | tracing at the point that you hit the part that you are interested in. | |
1041 | To disable the tracing directly from a C program, something like following | |
1042 | code snippet can be used: | |
eb6d42ea SR |
1043 | |
1044 | int trace_fd; | |
1045 | [...] | |
1046 | int main(int argc, char *argv[]) { | |
1047 | [...] | |
1048 | trace_fd = open("/debug/tracing/tracing_enabled", O_WRONLY); | |
1049 | [...] | |
1050 | if (condition_hit()) { | |
f2d9c740 | 1051 | write(trace_fd, "0", 1); |
eb6d42ea SR |
1052 | } |
1053 | [...] | |
1054 | } | |
1055 | ||
f2d9c740 SR |
1056 | Note: Here we hard coded the path name. The debugfs mount is not |
1057 | guaranteed to be at /debug (and is more commonly at /sys/kernel/debug). | |
1058 | For simple one time traces, the above is sufficent. For anything else, | |
1059 | a search through /proc/mounts may be needed to find where the debugfs | |
1060 | file-system is mounted. | |
eb6d42ea SR |
1061 | |
1062 | dynamic ftrace | |
1063 | -------------- | |
1064 | ||
f2d9c740 | 1065 | If CONFIG_DYNAMIC_FTRACE is set, the system will run with |
eb6d42ea SR |
1066 | virtually no overhead when function tracing is disabled. The way |
1067 | this works is the mcount function call (placed at the start of | |
1068 | every kernel function, produced by the -pg switch in gcc), starts | |
f2d9c740 SR |
1069 | of pointing to a simple return. (Enabling FTRACE will include the |
1070 | -pg switch in the compiling of the kernel.) | |
eb6d42ea | 1071 | |
9b803c0f SR |
1072 | At compile time every C file object is run through the |
1073 | recordmcount.pl script (located in the scripts directory). This | |
1074 | script will process the C object using objdump to find all the | |
1075 | locations in the .text section that call mcount. (Note, only | |
1076 | the .text section is processed, since processing other sections | |
1077 | like .init.text may cause races due to those sections being freed). | |
1078 | ||
1079 | A new section called "__mcount_loc" is created that holds references | |
1080 | to all the mcount call sites in the .text section. This section is | |
1081 | compiled back into the original object. The final linker will add | |
1082 | all these references into a single table. | |
1083 | ||
1084 | On boot up, before SMP is initialized, the dynamic ftrace code | |
1085 | scans this table and updates all the locations into nops. It also | |
1086 | records the locations, which are added to the available_filter_functions | |
1087 | list. Modules are processed as they are loaded and before they are | |
1088 | executed. When a module is unloaded, it also removes its functions from | |
1089 | the ftrace function list. This is automatic in the module unload | |
1090 | code, and the module author does not need to worry about it. | |
1091 | ||
1092 | When tracing is enabled, kstop_machine is called to prevent races | |
1093 | with the CPUS executing code being modified (which can cause the | |
1094 | CPU to do undesireable things), and the nops are patched back | |
1095 | to calls. But this time, they do not call mcount (which is just | |
1096 | a function stub). They now call into the ftrace infrastructure. | |
eb6d42ea SR |
1097 | |
1098 | One special side-effect to the recording of the functions being | |
f2d9c740 SR |
1099 | traced is that we can now selectively choose which functions we |
1100 | wish to trace and which ones we want the mcount calls to remain as | |
eb6d42ea SR |
1101 | nops. |
1102 | ||
a41eebab | 1103 | Two files are used, one for enabling and one for disabling the tracing |
f2d9c740 | 1104 | of specified functions. They are: |
eb6d42ea SR |
1105 | |
1106 | set_ftrace_filter | |
1107 | ||
1108 | and | |
1109 | ||
1110 | set_ftrace_notrace | |
1111 | ||
a41eebab | 1112 | A list of available functions that you can add to these files is listed |
eb6d42ea SR |
1113 | in: |
1114 | ||
1115 | available_filter_functions | |
1116 | ||
1117 | # cat /debug/tracing/available_filter_functions | |
1118 | put_prev_task_idle | |
1119 | kmem_cache_create | |
1120 | pick_next_task_rt | |
1121 | get_online_cpus | |
1122 | pick_next_task_fair | |
1123 | mutex_lock | |
1124 | [...] | |
1125 | ||
f2d9c740 | 1126 | If I am only interested in sys_nanosleep and hrtimer_interrupt: |
eb6d42ea SR |
1127 | |
1128 | # echo sys_nanosleep hrtimer_interrupt \ | |
1129 | > /debug/tracing/set_ftrace_filter | |
1130 | # echo ftrace > /debug/tracing/current_tracer | |
1131 | # echo 1 > /debug/tracing/tracing_enabled | |
1132 | # usleep 1 | |
1133 | # echo 0 > /debug/tracing/tracing_enabled | |
1134 | # cat /debug/tracing/trace | |
1135 | # tracer: ftrace | |
1136 | # | |
1137 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1138 | # | | | | | | |
1139 | usleep-4134 [00] 1317.070017: hrtimer_interrupt <-smp_apic_timer_interrupt | |
1140 | usleep-4134 [00] 1317.070111: sys_nanosleep <-syscall_call | |
1141 | <idle>-0 [00] 1317.070115: hrtimer_interrupt <-smp_apic_timer_interrupt | |
1142 | ||
f2d9c740 | 1143 | To see which functions are being traced, you can cat the file: |
eb6d42ea SR |
1144 | |
1145 | # cat /debug/tracing/set_ftrace_filter | |
1146 | hrtimer_interrupt | |
1147 | sys_nanosleep | |
1148 | ||
1149 | ||
f2d9c740 | 1150 | Perhaps this is not enough. The filters also allow simple wild cards. |
a41eebab | 1151 | Only the following are currently available |
eb6d42ea | 1152 | |
a41eebab | 1153 | <match>* - will match functions that begin with <match> |
eb6d42ea SR |
1154 | *<match> - will match functions that end with <match> |
1155 | *<match>* - will match functions that have <match> in it | |
1156 | ||
f2d9c740 | 1157 | These are the only wild cards which are supported. |
eb6d42ea SR |
1158 | |
1159 | <match>*<match> will not work. | |
1160 | ||
1161 | # echo hrtimer_* > /debug/tracing/set_ftrace_filter | |
1162 | ||
1163 | Produces: | |
1164 | ||
1165 | # tracer: ftrace | |
1166 | # | |
1167 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1168 | # | | | | | | |
1169 | bash-4003 [00] 1480.611794: hrtimer_init <-copy_process | |
1170 | bash-4003 [00] 1480.611941: hrtimer_start <-hrtick_set | |
1171 | bash-4003 [00] 1480.611956: hrtimer_cancel <-hrtick_clear | |
1172 | bash-4003 [00] 1480.611956: hrtimer_try_to_cancel <-hrtimer_cancel | |
1173 | <idle>-0 [00] 1480.612019: hrtimer_get_next_event <-get_next_timer_interrupt | |
1174 | <idle>-0 [00] 1480.612025: hrtimer_get_next_event <-get_next_timer_interrupt | |
1175 | <idle>-0 [00] 1480.612032: hrtimer_get_next_event <-get_next_timer_interrupt | |
1176 | <idle>-0 [00] 1480.612037: hrtimer_get_next_event <-get_next_timer_interrupt | |
1177 | <idle>-0 [00] 1480.612382: hrtimer_get_next_event <-get_next_timer_interrupt | |
1178 | ||
1179 | ||
1180 | Notice that we lost the sys_nanosleep. | |
1181 | ||
1182 | # cat /debug/tracing/set_ftrace_filter | |
1183 | hrtimer_run_queues | |
1184 | hrtimer_run_pending | |
1185 | hrtimer_init | |
1186 | hrtimer_cancel | |
1187 | hrtimer_try_to_cancel | |
1188 | hrtimer_forward | |
1189 | hrtimer_start | |
1190 | hrtimer_reprogram | |
1191 | hrtimer_force_reprogram | |
1192 | hrtimer_get_next_event | |
1193 | hrtimer_interrupt | |
1194 | hrtimer_nanosleep | |
1195 | hrtimer_wakeup | |
1196 | hrtimer_get_remaining | |
1197 | hrtimer_get_res | |
1198 | hrtimer_init_sleeper | |
1199 | ||
1200 | ||
1201 | This is because the '>' and '>>' act just like they do in bash. | |
1202 | To rewrite the filters, use '>' | |
1203 | To append to the filters, use '>>' | |
1204 | ||
a41eebab | 1205 | To clear out a filter so that all functions will be recorded again: |
eb6d42ea SR |
1206 | |
1207 | # echo > /debug/tracing/set_ftrace_filter | |
1208 | # cat /debug/tracing/set_ftrace_filter | |
1209 | # | |
1210 | ||
1211 | Again, now we want to append. | |
1212 | ||
1213 | # echo sys_nanosleep > /debug/tracing/set_ftrace_filter | |
1214 | # cat /debug/tracing/set_ftrace_filter | |
1215 | sys_nanosleep | |
1216 | # echo hrtimer_* >> /debug/tracing/set_ftrace_filter | |
1217 | # cat /debug/tracing/set_ftrace_filter | |
1218 | hrtimer_run_queues | |
1219 | hrtimer_run_pending | |
1220 | hrtimer_init | |
1221 | hrtimer_cancel | |
1222 | hrtimer_try_to_cancel | |
1223 | hrtimer_forward | |
1224 | hrtimer_start | |
1225 | hrtimer_reprogram | |
1226 | hrtimer_force_reprogram | |
1227 | hrtimer_get_next_event | |
1228 | hrtimer_interrupt | |
1229 | sys_nanosleep | |
1230 | hrtimer_nanosleep | |
1231 | hrtimer_wakeup | |
1232 | hrtimer_get_remaining | |
1233 | hrtimer_get_res | |
1234 | hrtimer_init_sleeper | |
1235 | ||
1236 | ||
1237 | The set_ftrace_notrace prevents those functions from being traced. | |
1238 | ||
1239 | # echo '*preempt*' '*lock*' > /debug/tracing/set_ftrace_notrace | |
1240 | ||
1241 | Produces: | |
1242 | ||
1243 | # tracer: ftrace | |
1244 | # | |
1245 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1246 | # | | | | | | |
1247 | bash-4043 [01] 115.281644: finish_task_switch <-schedule | |
1248 | bash-4043 [01] 115.281645: hrtick_set <-schedule | |
1249 | bash-4043 [01] 115.281645: hrtick_clear <-hrtick_set | |
1250 | bash-4043 [01] 115.281646: wait_for_completion <-__stop_machine_run | |
1251 | bash-4043 [01] 115.281647: wait_for_common <-wait_for_completion | |
1252 | bash-4043 [01] 115.281647: kthread_stop <-stop_machine_run | |
1253 | bash-4043 [01] 115.281648: init_waitqueue_head <-kthread_stop | |
1254 | bash-4043 [01] 115.281648: wake_up_process <-kthread_stop | |
1255 | bash-4043 [01] 115.281649: try_to_wake_up <-wake_up_process | |
1256 | ||
1257 | We can see that there's no more lock or preempt tracing. | |
1258 | ||
eb6d42ea SR |
1259 | trace_pipe |
1260 | ---------- | |
1261 | ||
f2d9c740 SR |
1262 | The trace_pipe outputs the same content as the trace file, but the effect |
1263 | on the tracing is different. Every read from trace_pipe is consumed. | |
eb6d42ea SR |
1264 | This means that subsequent reads will be different. The trace |
1265 | is live. | |
1266 | ||
9b803c0f | 1267 | # echo function > /debug/tracing/current_tracer |
eb6d42ea SR |
1268 | # cat /debug/tracing/trace_pipe > /tmp/trace.out & |
1269 | [1] 4153 | |
1270 | # echo 1 > /debug/tracing/tracing_enabled | |
1271 | # usleep 1 | |
1272 | # echo 0 > /debug/tracing/tracing_enabled | |
1273 | # cat /debug/tracing/trace | |
9b803c0f | 1274 | # tracer: function |
eb6d42ea SR |
1275 | # |
1276 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
1277 | # | | | | | | |
1278 | ||
1279 | # | |
1280 | # cat /tmp/trace.out | |
1281 | bash-4043 [00] 41.267106: finish_task_switch <-schedule | |
1282 | bash-4043 [00] 41.267106: hrtick_set <-schedule | |
1283 | bash-4043 [00] 41.267107: hrtick_clear <-hrtick_set | |
1284 | bash-4043 [00] 41.267108: wait_for_completion <-__stop_machine_run | |
1285 | bash-4043 [00] 41.267108: wait_for_common <-wait_for_completion | |
1286 | bash-4043 [00] 41.267109: kthread_stop <-stop_machine_run | |
1287 | bash-4043 [00] 41.267109: init_waitqueue_head <-kthread_stop | |
1288 | bash-4043 [00] 41.267110: wake_up_process <-kthread_stop | |
1289 | bash-4043 [00] 41.267110: try_to_wake_up <-wake_up_process | |
1290 | bash-4043 [00] 41.267111: select_task_rq_rt <-try_to_wake_up | |
1291 | ||
1292 | ||
f2d9c740 | 1293 | Note, reading the trace_pipe file will block until more input is added. |
eb6d42ea | 1294 | By changing the tracer, trace_pipe will issue an EOF. We needed |
9b803c0f | 1295 | to set the function tracer _before_ we "cat" the trace_pipe file. |
eb6d42ea SR |
1296 | |
1297 | ||
1298 | trace entries | |
1299 | ------------- | |
1300 | ||
1301 | Having too much or not enough data can be troublesome in diagnosing | |
f2d9c740 | 1302 | an issue in the kernel. The file trace_entries is used to modify |
a41eebab | 1303 | the size of the internal trace buffers. The number listed |
eb6d42ea SR |
1304 | is the number of entries that can be recorded per CPU. To know |
1305 | the full size, multiply the number of possible CPUS with the | |
1306 | number of entries. | |
1307 | ||
1308 | # cat /debug/tracing/trace_entries | |
1309 | 65620 | |
1310 | ||
a41eebab | 1311 | Note, to modify this, you must have tracing completely disabled. To do that, |
9b803c0f SR |
1312 | echo "nop" into the current_tracer. If the current_tracer is not set |
1313 | to "nop", an EINVAL error will be returned. | |
eb6d42ea | 1314 | |
9b803c0f | 1315 | # echo nop > /debug/tracing/current_tracer |
eb6d42ea SR |
1316 | # echo 100000 > /debug/tracing/trace_entries |
1317 | # cat /debug/tracing/trace_entries | |
1318 | 100045 | |
1319 | ||
1320 | ||
1321 | Notice that we echoed in 100,000 but the size is 100,045. The entries | |
f2d9c740 | 1322 | are held in individual pages. It allocates the number of pages it takes |
eb6d42ea | 1323 | to fulfill the request. If more entries may fit on the last page |
f2d9c740 | 1324 | then they will be added. |
eb6d42ea SR |
1325 | |
1326 | # echo 1 > /debug/tracing/trace_entries | |
1327 | # cat /debug/tracing/trace_entries | |
1328 | 85 | |
1329 | ||
f2d9c740 | 1330 | This shows us that 85 entries can fit in a single page. |
eb6d42ea | 1331 | |
f2d9c740 SR |
1332 | The number of pages which will be allocated is limited to a percentage |
1333 | of available memory. Allocating too much will produce an error. | |
eb6d42ea SR |
1334 | |
1335 | # echo 1000000000000 > /debug/tracing/trace_entries | |
1336 | -bash: echo: write error: Cannot allocate memory | |
1337 | # cat /debug/tracing/trace_entries | |
1338 | 85 | |
1339 |