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1 ftrace - Function Tracer
2 ========================
4 Copyright 2008 Red Hat Inc.
5 Author: Steven Rostedt <srostedt@redhat.com>
6 License: The GNU Free Documentation License, Version 1.2
7 Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton,
8 John Kacur, and David Teigland.
10 Written for: 2.6.27-rc1
12 Introduction
13 ------------
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.
20 Although ftrace is the function tracer, it also includes an
21 infrastructure that allows for other types of tracing. Some of the
22 tracers that are currently in ftrace include a tracer to trace
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
25 more (ftrace allows for tracer plugins, which means that the list of
26 tracers can always grow).
29 The File System
30 ---------------
32 Ftrace uses the debugfs file system to hold the control files as well
33 as the files to display output.
35 To mount the debugfs system:
37 # mkdir /debug
38 # mount -t debugfs nodev /debug
40 (Note: it is more common to mount at /sys/kernel/debug, but for simplicity
41 this document will use /debug)
43 That's it! (assuming that you have ftrace configured into your kernel)
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:
50 Note: all time values are in microseconds.
52 current_tracer : This is used to set or display the current tracer
53 that is configured.
55 available_tracers : This holds the different types of tracers that
56 have been compiled into the kernel. The tracers
57 listed here can be configured by echoing their name
58 into current_tracer.
60 tracing_enabled : This sets or displays whether the current_tracer
61 is activated and tracing or not. Echo 0 into this
62 file to disable the tracer or 1 to enable it.
64 trace : This file holds the output of the trace in a human readable
65 format (described below).
67 latency_trace : This file shows the same trace but the information
68 is organized more to display possible latencies
69 in the system (described below).
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
81 tracer is not adding more data, they will display
82 the same information every time they are read.
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.
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)
96 trace_entries : This sets or displays the number of trace
97 entries each CPU buffer can hold. The tracer buffers
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).
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.
107 This can only be updated when the current_tracer
108 is set to "none".
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.
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.
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.
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.
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
138 "set_ftrace_filter" file. (See the section "dynamic ftrace"
139 below for more details).
142 The Tracers
143 -----------
145 Here is the list of current tracers that may be configured.
147 ftrace - function tracer that uses mcount to trace all functions.
149 sched_switch - traces the context switches between tasks.
151 irqsoff - traces the areas that disable interrupts and saves
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
155 trace via the latency_trace file.
157 preemptoff - Similar to irqsoff but traces and records the amount of
158 time for which preemption is disabled.
160 preemptirqsoff - Similar to irqsoff and preemptoff, but traces and
161 records the largest time for which irqs and/or preemption
162 is disabled.
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.
168 none - This is not a tracer. To remove all tracers from tracing
169 simply echo "none" into current_tracer.
172 Examples of using the tracer
173 ----------------------------
175 Here are typical examples of using the tracers when controlling them only
176 with the debugfs interface (without using any user-land utilities).
178 Output format:
179 --------------
181 Here is an example of the output format of the file "trace"
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 --------
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
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
198 "path_walk". The timestamp is the time at which the function was
199 entered.
201 The sched_switch tracer also includes tracing of task wakeups and
202 context switches.
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
211 Wake ups are represented by a "+" and the context switches are shown as
212 "==>". The format is:
214 Context switches:
216 Previous task Next Task
218 <pid>:<prio>:<state> ==> <pid>:<prio>:<state>
220 Wake ups:
222 Current task Task waking up
224 <pid>:<prio>:<state> + <pid>:<prio>:<state>
226 The prio is the internal kernel priority, which is the inverse of the
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).
233 Latency trace format
234 --------------------
236 For traces that display latency times, the latency_trace file gives
237 somewhat more information to see why a latency happened. Here is a typical
238 trace.
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
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)
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
271 preemption that was used (PREEMPT). VP, KP, SP, and HP are always zero
272 and are reserved for later use. #P is the number of online CPUS (#P:2).
274 The task is the process that was running when the latency occurred.
275 (swapper pid: 0).
277 The start and stop (the functions in which the interrupts were disabled and
278 enabled respectively) that caused the latencies:
280 apic_timer_interrupt is where the interrupts were disabled.
281 do_softirq is where they were enabled again.
283 The next lines after the header are the trace itself. The header
284 explains which is which.
286 cmd: The name of the process in the trace.
288 pid: The PID of that process.
290 CPU#: The CPU which the process was running on.
292 irqs-off: 'd' interrupts are disabled. '.' otherwise.
294 need-resched: 'N' task need_resched is set, '.' otherwise.
296 hardirq/softirq:
297 'H' - hard irq occurred inside a softirq.
298 'h' - hard irq is running
299 's' - soft irq is running
300 '.' - normal context.
302 preempt-depth: The level of preempt_disabled
304 The above is mostly meaningful for kernel developers.
306 time: This differs from the trace file output. The trace file output
307 includes an absolute timestamp. The timestamp used by the
308 latency_trace file is relative to the start of the trace.
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.
312 The marks are determined by the difference between this
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.
318 The rest is the same as the 'trace' file.
321 iter_ctrl
322 ---------
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:
327 cat /debug/tracing/iter_ctrl
328 print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \
329 noblock nostacktrace nosched-tree
331 To disable one of the options, echo in the option prepended with "no".
333 echo noprint-parent > /debug/tracing/iter_ctrl
335 To enable an option, leave off the "no".
337 echo sym-offset > /debug/tracing/iter_ctrl
339 Here are the available options:
341 print-parent - On function traces, display the calling function
342 as well as the function being traced.
344 print-parent:
345 bash-4000 [01] 1477.606694: simple_strtoul <-strict_strtoul
347 noprint-parent:
348 bash-4000 [01] 1477.606694: simple_strtoul
351 sym-offset - Display not only the function name, but also the offset
352 in the function. For example, instead of seeing just
353 "ktime_get", you will see "ktime_get+0xb/0x20".
355 sym-offset:
356 bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0
358 sym-addr - this will also display the function address as well as
359 the function name.
361 sym-addr:
362 bash-4000 [01] 1477.606694: simple_strtoul <c0339346>
364 verbose - This deals with the latency_trace file.
366 bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
367 (+0.000ms): simple_strtoul (strict_strtoul)
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.
373 hex - Similar to raw, but the numbers will be in a hexadecimal format.
375 bin - This will print out the formats in raw binary.
377 block - TBD (needs update)
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.
383 sched-tree - TBD (any users??)
386 sched_switch
387 ------------
389 This tracer simply records schedule switches. Here is an example
390 of how to use it.
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
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 [...]
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.
425 The sched_switch file only lists the wake ups (represented with '+')
426 and context switches ('==>') with the previous task or current task
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.
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
437 The task states are:
439 R - running : wants to run, may not actually be running
440 S - sleep : process is waiting to be woken up (handles signals)
441 D - disk sleep (uninterruptible sleep) : process must be woken up
442 (ignores signals)
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
449 ftrace_enabled
450 --------------
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,
454 one can either use the sysctl function or set it via the proc
455 file system interface.
457 sysctl kernel.ftrace_enabled=1
459 or
461 echo 1 > /proc/sys/kernel/ftrace_enabled
463 To disable ftrace_enabled simply replace the '1' with '0' in
464 the above commands.
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.
471 irqsoff
472 -------
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.
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.
485 To reset the maximum, echo 0 into tracing_max_latency. Here is an
486 example:
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 #
497 irqsoff latency trace v1.1.5 on 2.6.26
498 --------------------------------------------------------------------
499 latency: 12 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
500 -----------------
501 | task: bash-3730 (uid:0 nice:0 policy:0 rt_prio:0)
502 -----------------
503 => started at: sys_setpgid
504 => ended at: sys_setpgid
506 # _------=> CPU#
507 # / _-----=> irqs-off
508 # | / _----=> need-resched
509 # || / _---=> hardirq/softirq
510 # ||| / _--=> preempt-depth
511 # |||| /
512 # ||||| delay
513 # cmd pid ||||| time | caller
514 # \ / ||||| \ | /
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)
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.
526 Note the above example had ftrace_enabled not set. If we set the
527 ftrace_enabled, we get a much larger output:
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
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)
574 Here we traced a 50 microsecond latency. But we also see all the
575 functions that were called during that time. Note that by enabling
576 function tracing, we incur an added overhead. This overhead may
577 extend the latency times. But nevertheless, this trace has provided
578 some very helpful debugging information.
581 preemptoff
582 ----------
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
586 for preemption to be enabled again before it can preempt a lower
587 priority task.
589 The preemptoff tracer traces the places that disable preemption.
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.
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
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)
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.
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
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)
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
693 in an interrupt, but we can see from the functions themselves that
694 this is not the case.
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
699 switched to the softirq stack (4K stacks in effect). The code
700 does not copy the preempt count, but because interrupts are disabled,
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.
705 preemptirqsoff
706 --------------
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.
712 Consider the following code:
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();
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().
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().
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.
734 Again, using this trace is much like the irqsoff and preemptoff tracers.
736 # echo preemptirqsoff > /debug/tracing/current_tracer
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
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)
769 The trace_hardirqs_off_thunk is called from assembly on x86 when
770 interrupts are disabled in the assembly code. Without the function
771 tracing, we do not know if interrupts were enabled within the preemption
772 points. We do see that it started with preemption enabled.
774 Here is a trace with ftrace_enabled set:
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
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)
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
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
864 sshd. When the interrupts were enabled, we took an interrupt.
865 On return from the interrupt handler, the softirq ran. We took another
866 interrupt while running the softirq as we see from the capital 'H'.
869 wakeup
870 ------
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".
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
878 LatencyTop are more appropriate for such measurements.
880 Real-Time environments are interested in the worst case latency.
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.
890 Since this tracer only deals with RT tasks, we will run this slightly
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
893 priority of the task.
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 -----------------
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)
924 Running this on an idle system, we see that it only took 4 microseconds
925 to perform the task switch. Note, since the trace marker in the
926 schedule is before the actual "switch", we stop the tracing when
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.
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.
935 Doing the same with chrt -r 5 and ftrace_enabled set.
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 -----------------
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)
990 The interrupt went off while running ksoftirqd. This task runs at
991 SCHED_OTHER. Why did not we see the 'N' set early? This may be
992 a harmless bug with x86_32 and 4K stacks. On x86_32 with 4K stacks
993 configured, the interrupt and softirq run with their own stack.
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
999 has been set. We do not see the 'N' until we switch back to the task's
1000 assigned stack.
1002 ftrace
1003 ------
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.
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 [...]
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:
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()) {
1055 write(trace_fd, "0", 1);
1056 }
1057 [...]
1058 }
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.
1066 dynamic ftrace
1067 --------------
1069 If CONFIG_DYNAMIC_FTRACE is set, the system will run with
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
1073 of pointing to a simple return. (Enabling FTRACE will include the
1074 -pg switch in the compiling of the kernel.)
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"
1080 function. After this, the first time a kernel function is called,
1081 it has the calling address saved in a hash table.
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
1087 it to a nop, there is no measurable overhead to the system.
1089 One special side-effect to the recording of the functions being
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
1092 nops.
1094 Two files are used, one for enabling and one for disabling the tracing
1095 of specified functions. They are:
1097 set_ftrace_filter
1099 and
1101 set_ftrace_notrace
1103 A list of available functions that you can add to these files is listed
1104 in:
1106 available_filter_functions
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 [...]
1117 If I am only interested in sys_nanosleep and hrtimer_interrupt:
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
1134 To see which functions are being traced, you can cat the file:
1136 # cat /debug/tracing/set_ftrace_filter
1137 hrtimer_interrupt
1138 sys_nanosleep
1141 Perhaps this is not enough. The filters also allow simple wild cards.
1142 Only the following are currently available
1144 <match>* - will match functions that begin with <match>
1145 *<match> - will match functions that end with <match>
1146 *<match>* - will match functions that have <match> in it
1148 These are the only wild cards which are supported.
1150 <match>*<match> will not work.
1152 # echo hrtimer_* > /debug/tracing/set_ftrace_filter
1154 Produces:
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
1171 Notice that we lost the sys_nanosleep.
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
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 '>>'
1196 To clear out a filter so that all functions will be recorded again:
1198 # echo > /debug/tracing/set_ftrace_filter
1199 # cat /debug/tracing/set_ftrace_filter
1200 #
1202 Again, now we want to append.
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
1228 The set_ftrace_notrace prevents those functions from being traced.
1230 # echo '*preempt*' '*lock*' > /debug/tracing/set_ftrace_notrace
1232 Produces:
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
1248 We can see that there's no more lock or preempt tracing.
1250 ftraced
1251 -------
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
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
1257 kstop_machine to convert the calls to nops.
1259 There may be a case in which you do not want this added latency.
1260 Perhaps you are doing some audio recording and this activity might
1261 cause skips in the playback. There is an interface to disable
1262 and enable the "ftraced" kernel thread.
1264 # echo 0 > /debug/tracing/ftraced_enabled
1266 This will disable the calling of kstop_machine to update the
1267 mcount calls to nops. Remember that there is a large overhead
1268 to calling mcount. Without this kernel thread, that overhead will
1269 exist.
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
1273 user can manually perform the updates when they want to by simply
1274 echoing a '0' into the ftraced_enabled file.
1276 The updates are also done at the beginning of enabling a tracer
1277 that uses ftrace function recording.
1280 trace_pipe
1281 ----------
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.
1285 This means that subsequent reads will be different. The trace
1286 is live.
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 # | | | | |
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
1314 Note, reading the trace_pipe file will block until more input is added.
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.
1319 trace entries
1320 -------------
1322 Having too much or not enough data can be troublesome in diagnosing
1323 an issue in the kernel. The file trace_entries is used to modify
1324 the size of the internal trace buffers. The number listed
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.
1329 # cat /debug/tracing/trace_entries
1330 65620
1332 Note, to modify this, you must have tracing completely disabled. To do that,
1333 echo "none" into the current_tracer. If the current_tracer is not set
1334 to "none", an EINVAL error will be returned.
1336 # echo none > /debug/tracing/current_tracer
1337 # echo 100000 > /debug/tracing/trace_entries
1338 # cat /debug/tracing/trace_entries
1339 100045
1342 Notice that we echoed in 100,000 but the size is 100,045. The entries
1343 are held in individual pages. It allocates the number of pages it takes
1344 to fulfill the request. If more entries may fit on the last page
1345 then they will be added.
1347 # echo 1 > /debug/tracing/trace_entries
1348 # cat /debug/tracing/trace_entries
1349 85
1351 This shows us that 85 entries can fit in a single page.
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.
1356 # echo 1000000000000 > /debug/tracing/trace_entries
1357 -bash: echo: write error: Cannot allocate memory
1358 # cat /debug/tracing/trace_entries
1359 85