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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 (dual licensed under the GPL v2)
8 Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton,
9 John Kacur, and David Teigland.
11 Written for: 2.6.27-rc1
13 Introduction
14 ------------
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.
21 Although ftrace is the function tracer, it also includes an
22 infrastructure that allows for other types of tracing. Some of the
23 tracers that are currently in ftrace include a tracer to trace
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
26 more (ftrace allows for tracer plugins, which means that the list of
27 tracers can always grow).
30 The File System
31 ---------------
33 Ftrace uses the debugfs file system to hold the control files as well
34 as the files to display output.
36 To mount the debugfs system:
38 # mkdir /debug
39 # mount -t debugfs nodev /debug
41 (Note: it is more common to mount at /sys/kernel/debug, but for simplicity
42 this document will use /debug)
44 That's it! (assuming that you have ftrace configured into your kernel)
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:
51 Note: all time values are in microseconds.
53 current_tracer : This is used to set or display the current tracer
54 that is configured.
56 available_tracers : This holds the different types of tracers that
57 have been compiled into the kernel. The tracers
58 listed here can be configured by echoing their name
59 into current_tracer.
61 tracing_enabled : This sets or displays whether the current_tracer
62 is activated and tracing or not. Echo 0 into this
63 file to disable the tracer or 1 to enable it.
65 trace : This file holds the output of the trace in a human readable
66 format (described below).
68 latency_trace : This file shows the same trace but the information
69 is organized more to display possible latencies
70 in the system (described below).
72 trace_pipe : The output is the same as the "trace" file but this
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
82 tracer is not adding more data, they will display
83 the same information every time they are read.
85 iter_ctrl : This file lets the user control the amount of data
86 that is displayed in one of the above output
87 files.
89 trace_max_latency : Some of the tracers record the max latency.
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)
97 trace_entries : This sets or displays the number of trace
98 entries each CPU buffer can hold. The tracer buffers
99 are the same size for each CPU. The displayed number
100 is the size of the CPU buffer and not total size. The
101 trace buffers are allocated in pages (blocks of memory
102 that the kernel uses for allocation, usually 4 KB in size).
103 Since each entry is smaller than a page, if the last
104 allocated page has room for more entries than were
105 requested, the rest of the page is used to allocate
106 entries.
108 This can only be updated when the current_tracer
109 is set to "none".
111 NOTE: It is planned on changing the allocated buffers
112 from being the number of possible CPUS to
113 the number of online CPUS.
115 tracing_cpumask : This is a mask that lets the user only trace
116 on specified CPUS. The format is a hex string
117 representing the CPUS.
119 set_ftrace_filter : When dynamic ftrace is configured in (see the
120 section below "dynamic ftrace"), the code is dynamically
121 modified (code text rewrite) to disable calling of the
122 function profiler (mcount). This lets tracing be configured
123 in with practically no overhead in performance. This also
124 has a side effect of enabling or disabling specific functions
125 to be traced. Echoing names of functions into this file
126 will limit the trace to only those functions.
128 set_ftrace_notrace: This has an effect opposite to that of
129 set_ftrace_filter. Any function that is added here will not
130 be traced. If a function exists in both set_ftrace_filter
131 and set_ftrace_notrace, the function will _not_ be traced.
133 available_filter_functions : When a function is encountered the first
134 time by the dynamic tracer, it is recorded and
135 later the call is converted into a nop. This file
136 lists the functions that have been recorded
137 by the dynamic tracer and these functions can
138 be used to set the ftrace filter by the above
139 "set_ftrace_filter" file. (See the section "dynamic ftrace"
140 below for more details).
143 The Tracers
144 -----------
146 Here is the list of current tracers that may be configured.
148 ftrace - function tracer that uses mcount to trace all functions.
150 sched_switch - traces the context switches between tasks.
152 irqsoff - traces the areas that disable interrupts and saves
153 the trace with the longest max latency.
154 See tracing_max_latency. When a new max is recorded,
155 it replaces the old trace. It is best to view this
156 trace via the latency_trace file.
158 preemptoff - Similar to irqsoff but traces and records the amount of
159 time for which preemption is disabled.
161 preemptirqsoff - Similar to irqsoff and preemptoff, but traces and
162 records the largest time for which irqs and/or preemption
163 is disabled.
165 wakeup - Traces and records the max latency that it takes for
166 the highest priority task to get scheduled after
167 it has been woken up.
169 none - This is not a tracer. To remove all tracers from tracing
170 simply echo "none" into current_tracer.
173 Examples of using the tracer
174 ----------------------------
176 Here are typical examples of using the tracers when controlling them only
177 with the debugfs interface (without using any user-land utilities).
179 Output format:
180 --------------
182 Here is an example of the output format of the file "trace"
184 --------
185 # tracer: ftrace
186 #
187 # TASK-PID CPU# TIMESTAMP FUNCTION
188 # | | | | |
189 bash-4251 [01] 10152.583854: path_put <-path_walk
190 bash-4251 [01] 10152.583855: dput <-path_put
191 bash-4251 [01] 10152.583855: _atomic_dec_and_lock <-dput
192 --------
194 A header is printed with the tracer name that is represented by the trace.
195 In this case the tracer is "ftrace". Then a header showing the format. Task
196 name "bash", the task PID "4251", the CPU that it was running on
197 "01", the timestamp in <secs>.<usecs> format, the function name that was
198 traced "path_put" and the parent function that called this function
199 "path_walk". The timestamp is the time at which the function was
200 entered.
202 The sched_switch tracer also includes tracing of task wakeups and
203 context switches.
205 ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 2916:115:S
206 ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 10:115:S
207 ksoftirqd/1-7 [01] 1453.070013: 7:115:R ==> 10:115:R
208 events/1-10 [01] 1453.070013: 10:115:S ==> 2916:115:R
209 kondemand/1-2916 [01] 1453.070013: 2916:115:S ==> 7:115:R
210 ksoftirqd/1-7 [01] 1453.070013: 7:115:S ==> 0:140:R
212 Wake ups are represented by a "+" and the context switches are shown as
213 "==>". The format is:
215 Context switches:
217 Previous task Next Task
219 <pid>:<prio>:<state> ==> <pid>:<prio>:<state>
221 Wake ups:
223 Current task Task waking up
225 <pid>:<prio>:<state> + <pid>:<prio>:<state>
227 The prio is the internal kernel priority, which is the inverse of the
228 priority that is usually displayed by user-space tools. Zero represents
229 the highest priority (99). Prio 100 starts the "nice" priorities with
230 100 being equal to nice -20 and 139 being nice 19. The prio "140" is
231 reserved for the idle task which is the lowest priority thread (pid 0).
234 Latency trace format
235 --------------------
237 For traces that display latency times, the latency_trace file gives
238 somewhat more information to see why a latency happened. Here is a typical
239 trace.
241 # tracer: irqsoff
242 #
243 irqsoff latency trace v1.1.5 on 2.6.26-rc8
244 --------------------------------------------------------------------
245 latency: 97 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
246 -----------------
247 | task: swapper-0 (uid:0 nice:0 policy:0 rt_prio:0)
248 -----------------
249 => started at: apic_timer_interrupt
250 => ended at: do_softirq
252 # _------=> CPU#
253 # / _-----=> irqs-off
254 # | / _----=> need-resched
255 # || / _---=> hardirq/softirq
256 # ||| / _--=> preempt-depth
257 # |||| /
258 # ||||| delay
259 # cmd pid ||||| time | caller
260 # \ / ||||| \ | /
261 <idle>-0 0d..1 0us+: trace_hardirqs_off_thunk (apic_timer_interrupt)
262 <idle>-0 0d.s. 97us : __do_softirq (do_softirq)
263 <idle>-0 0d.s1 98us : trace_hardirqs_on (do_softirq)
267 This shows that the current tracer is "irqsoff" tracing the time for which
268 interrupts were disabled. It gives the trace version and the version
269 of the kernel upon which this was executed on (2.6.26-rc8). Then it displays
270 the max latency in microsecs (97 us). The number of trace entries displayed
271 and the total number recorded (both are three: #3/3). The type of
272 preemption that was used (PREEMPT). VP, KP, SP, and HP are always zero
273 and are reserved for later use. #P is the number of online CPUS (#P:2).
275 The task is the process that was running when the latency occurred.
276 (swapper pid: 0).
278 The start and stop (the functions in which the interrupts were disabled and
279 enabled respectively) that caused the latencies:
281 apic_timer_interrupt is where the interrupts were disabled.
282 do_softirq is where they were enabled again.
284 The next lines after the header are the trace itself. The header
285 explains which is which.
287 cmd: The name of the process in the trace.
289 pid: The PID of that process.
291 CPU#: The CPU which the process was running on.
293 irqs-off: 'd' interrupts are disabled. '.' otherwise.
294 Note: If the architecture does not support a way to
295 read the irq flags variable, an 'X' will always
296 be printed here.
298 need-resched: 'N' task need_resched is set, '.' otherwise.
300 hardirq/softirq:
301 'H' - hard irq occurred inside a softirq.
302 'h' - hard irq is running
303 's' - soft irq is running
304 '.' - normal context.
306 preempt-depth: The level of preempt_disabled
308 The above is mostly meaningful for kernel developers.
310 time: This differs from the trace file output. The trace file output
311 includes an absolute timestamp. The timestamp used by the
312 latency_trace file is relative to the start of the trace.
314 delay: This is just to help catch your eye a bit better. And
315 needs to be fixed to be only relative to the same CPU.
316 The marks are determined by the difference between this
317 current trace and the next trace.
318 '!' - greater than preempt_mark_thresh (default 100)
319 '+' - greater than 1 microsecond
320 ' ' - less than or equal to 1 microsecond.
322 The rest is the same as the 'trace' file.
325 iter_ctrl
326 ---------
328 The iter_ctrl file is used to control what gets printed in the trace
329 output. To see what is available, simply cat the file:
331 cat /debug/tracing/iter_ctrl
332 print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \
333 noblock nostacktrace nosched-tree
335 To disable one of the options, echo in the option prepended with "no".
337 echo noprint-parent > /debug/tracing/iter_ctrl
339 To enable an option, leave off the "no".
341 echo sym-offset > /debug/tracing/iter_ctrl
343 Here are the available options:
345 print-parent - On function traces, display the calling function
346 as well as the function being traced.
348 print-parent:
349 bash-4000 [01] 1477.606694: simple_strtoul <-strict_strtoul
351 noprint-parent:
352 bash-4000 [01] 1477.606694: simple_strtoul
355 sym-offset - Display not only the function name, but also the offset
356 in the function. For example, instead of seeing just
357 "ktime_get", you will see "ktime_get+0xb/0x20".
359 sym-offset:
360 bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0
362 sym-addr - this will also display the function address as well as
363 the function name.
365 sym-addr:
366 bash-4000 [01] 1477.606694: simple_strtoul <c0339346>
368 verbose - This deals with the latency_trace file.
370 bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
371 (+0.000ms): simple_strtoul (strict_strtoul)
373 raw - This will display raw numbers. This option is best for use with
374 user applications that can translate the raw numbers better than
375 having it done in the kernel.
377 hex - Similar to raw, but the numbers will be in a hexadecimal format.
379 bin - This will print out the formats in raw binary.
381 block - TBD (needs update)
383 stacktrace - This is one of the options that changes the trace itself.
384 When a trace is recorded, so is the stack of functions.
385 This allows for back traces of trace sites.
387 sched-tree - TBD (any users??)
390 sched_switch
391 ------------
393 This tracer simply records schedule switches. Here is an example
394 of how to use it.
396 # echo sched_switch > /debug/tracing/current_tracer
397 # echo 1 > /debug/tracing/tracing_enabled
398 # sleep 1
399 # echo 0 > /debug/tracing/tracing_enabled
400 # cat /debug/tracing/trace
402 # tracer: sched_switch
403 #
404 # TASK-PID CPU# TIMESTAMP FUNCTION
405 # | | | | |
406 bash-3997 [01] 240.132281: 3997:120:R + 4055:120:R
407 bash-3997 [01] 240.132284: 3997:120:R ==> 4055:120:R
408 sleep-4055 [01] 240.132371: 4055:120:S ==> 3997:120:R
409 bash-3997 [01] 240.132454: 3997:120:R + 4055:120:S
410 bash-3997 [01] 240.132457: 3997:120:R ==> 4055:120:R
411 sleep-4055 [01] 240.132460: 4055:120:D ==> 3997:120:R
412 bash-3997 [01] 240.132463: 3997:120:R + 4055:120:D
413 bash-3997 [01] 240.132465: 3997:120:R ==> 4055:120:R
414 <idle>-0 [00] 240.132589: 0:140:R + 4:115:S
415 <idle>-0 [00] 240.132591: 0:140:R ==> 4:115:R
416 ksoftirqd/0-4 [00] 240.132595: 4:115:S ==> 0:140:R
417 <idle>-0 [00] 240.132598: 0:140:R + 4:115:S
418 <idle>-0 [00] 240.132599: 0:140:R ==> 4:115:R
419 ksoftirqd/0-4 [00] 240.132603: 4:115:S ==> 0:140:R
420 sleep-4055 [01] 240.133058: 4055:120:S ==> 3997:120:R
421 [...]
424 As we have discussed previously about this format, the header shows
425 the name of the trace and points to the options. The "FUNCTION"
426 is a misnomer since here it represents the wake ups and context
427 switches.
429 The sched_switch file only lists the wake ups (represented with '+')
430 and context switches ('==>') with the previous task or current task
431 first followed by the next task or task waking up. The format for both
432 of these is PID:KERNEL-PRIO:TASK-STATE. Remember that the KERNEL-PRIO
433 is the inverse of the actual priority with zero (0) being the highest
434 priority and the nice values starting at 100 (nice -20). Below is
435 a quick chart to map the kernel priority to user land priorities.
437 Kernel priority: 0 to 99 ==> user RT priority 99 to 0
438 Kernel priority: 100 to 139 ==> user nice -20 to 19
439 Kernel priority: 140 ==> idle task priority
441 The task states are:
443 R - running : wants to run, may not actually be running
444 S - sleep : process is waiting to be woken up (handles signals)
445 D - disk sleep (uninterruptible sleep) : process must be woken up
446 (ignores signals)
447 T - stopped : process suspended
448 t - traced : process is being traced (with something like gdb)
449 Z - zombie : process waiting to be cleaned up
450 X - unknown
453 ftrace_enabled
454 --------------
456 The following tracers (listed below) give different output depending
457 on whether or not the sysctl ftrace_enabled is set. To set ftrace_enabled,
458 one can either use the sysctl function or set it via the proc
459 file system interface.
461 sysctl kernel.ftrace_enabled=1
463 or
465 echo 1 > /proc/sys/kernel/ftrace_enabled
467 To disable ftrace_enabled simply replace the '1' with '0' in
468 the above commands.
470 When ftrace_enabled is set the tracers will also record the functions
471 that are within the trace. The descriptions of the tracers
472 will also show an example with ftrace enabled.
475 irqsoff
476 -------
478 When interrupts are disabled, the CPU can not react to any other
479 external event (besides NMIs and SMIs). This prevents the timer
480 interrupt from triggering or the mouse interrupt from letting the
481 kernel know of a new mouse event. The result is a latency with the
482 reaction time.
484 The irqsoff tracer tracks the time for which interrupts are disabled.
485 When a new maximum latency is hit, the tracer saves the trace leading up
486 to that latency point so that every time a new maximum is reached, the old
487 saved trace is discarded and the new trace is saved.
489 To reset the maximum, echo 0 into tracing_max_latency. Here is an
490 example:
492 # echo irqsoff > /debug/tracing/current_tracer
493 # echo 0 > /debug/tracing/tracing_max_latency
494 # echo 1 > /debug/tracing/tracing_enabled
495 # ls -ltr
496 [...]
497 # echo 0 > /debug/tracing/tracing_enabled
498 # cat /debug/tracing/latency_trace
499 # tracer: irqsoff
500 #
501 irqsoff latency trace v1.1.5 on 2.6.26
502 --------------------------------------------------------------------
503 latency: 12 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
504 -----------------
505 | task: bash-3730 (uid:0 nice:0 policy:0 rt_prio:0)
506 -----------------
507 => started at: sys_setpgid
508 => ended at: sys_setpgid
510 # _------=> CPU#
511 # / _-----=> irqs-off
512 # | / _----=> need-resched
513 # || / _---=> hardirq/softirq
514 # ||| / _--=> preempt-depth
515 # |||| /
516 # ||||| delay
517 # cmd pid ||||| time | caller
518 # \ / ||||| \ | /
519 bash-3730 1d... 0us : _write_lock_irq (sys_setpgid)
520 bash-3730 1d..1 1us+: _write_unlock_irq (sys_setpgid)
521 bash-3730 1d..2 14us : trace_hardirqs_on (sys_setpgid)
524 Here we see that that we had a latency of 12 microsecs (which is
525 very good). The _write_lock_irq in sys_setpgid disabled interrupts.
526 The difference between the 12 and the displayed timestamp 14us occurred
527 because the clock was incremented between the time of recording the max
528 latency and the time of recording the function that had that latency.
530 Note the above example had ftrace_enabled not set. If we set the
531 ftrace_enabled, we get a much larger output:
533 # tracer: irqsoff
534 #
535 irqsoff latency trace v1.1.5 on 2.6.26-rc8
536 --------------------------------------------------------------------
537 latency: 50 us, #101/101, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
538 -----------------
539 | task: ls-4339 (uid:0 nice:0 policy:0 rt_prio:0)
540 -----------------
541 => started at: __alloc_pages_internal
542 => ended at: __alloc_pages_internal
544 # _------=> CPU#
545 # / _-----=> irqs-off
546 # | / _----=> need-resched
547 # || / _---=> hardirq/softirq
548 # ||| / _--=> preempt-depth
549 # |||| /
550 # ||||| delay
551 # cmd pid ||||| time | caller
552 # \ / ||||| \ | /
553 ls-4339 0...1 0us+: get_page_from_freelist (__alloc_pages_internal)
554 ls-4339 0d..1 3us : rmqueue_bulk (get_page_from_freelist)
555 ls-4339 0d..1 3us : _spin_lock (rmqueue_bulk)
556 ls-4339 0d..1 4us : add_preempt_count (_spin_lock)
557 ls-4339 0d..2 4us : __rmqueue (rmqueue_bulk)
558 ls-4339 0d..2 5us : __rmqueue_smallest (__rmqueue)
559 ls-4339 0d..2 5us : __mod_zone_page_state (__rmqueue_smallest)
560 ls-4339 0d..2 6us : __rmqueue (rmqueue_bulk)
561 ls-4339 0d..2 6us : __rmqueue_smallest (__rmqueue)
562 ls-4339 0d..2 7us : __mod_zone_page_state (__rmqueue_smallest)
563 ls-4339 0d..2 7us : __rmqueue (rmqueue_bulk)
564 ls-4339 0d..2 8us : __rmqueue_smallest (__rmqueue)
565 [...]
566 ls-4339 0d..2 46us : __rmqueue_smallest (__rmqueue)
567 ls-4339 0d..2 47us : __mod_zone_page_state (__rmqueue_smallest)
568 ls-4339 0d..2 47us : __rmqueue (rmqueue_bulk)
569 ls-4339 0d..2 48us : __rmqueue_smallest (__rmqueue)
570 ls-4339 0d..2 48us : __mod_zone_page_state (__rmqueue_smallest)
571 ls-4339 0d..2 49us : _spin_unlock (rmqueue_bulk)
572 ls-4339 0d..2 49us : sub_preempt_count (_spin_unlock)
573 ls-4339 0d..1 50us : get_page_from_freelist (__alloc_pages_internal)
574 ls-4339 0d..2 51us : trace_hardirqs_on (__alloc_pages_internal)
578 Here we traced a 50 microsecond latency. But we also see all the
579 functions that were called during that time. Note that by enabling
580 function tracing, we incur an added overhead. This overhead may
581 extend the latency times. But nevertheless, this trace has provided
582 some very helpful debugging information.
585 preemptoff
586 ----------
588 When preemption is disabled, we may be able to receive interrupts but
589 the task cannot be preempted and a higher priority task must wait
590 for preemption to be enabled again before it can preempt a lower
591 priority task.
593 The preemptoff tracer traces the places that disable preemption.
594 Like the irqsoff tracer, it records the maximum latency for which preemption
595 was disabled. The control of preemptoff tracer is much like the irqsoff
596 tracer.
598 # echo preemptoff > /debug/tracing/current_tracer
599 # echo 0 > /debug/tracing/tracing_max_latency
600 # echo 1 > /debug/tracing/tracing_enabled
601 # ls -ltr
602 [...]
603 # echo 0 > /debug/tracing/tracing_enabled
604 # cat /debug/tracing/latency_trace
605 # tracer: preemptoff
606 #
607 preemptoff latency trace v1.1.5 on 2.6.26-rc8
608 --------------------------------------------------------------------
609 latency: 29 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
610 -----------------
611 | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
612 -----------------
613 => started at: do_IRQ
614 => ended at: __do_softirq
616 # _------=> CPU#
617 # / _-----=> irqs-off
618 # | / _----=> need-resched
619 # || / _---=> hardirq/softirq
620 # ||| / _--=> preempt-depth
621 # |||| /
622 # ||||| delay
623 # cmd pid ||||| time | caller
624 # \ / ||||| \ | /
625 sshd-4261 0d.h. 0us+: irq_enter (do_IRQ)
626 sshd-4261 0d.s. 29us : _local_bh_enable (__do_softirq)
627 sshd-4261 0d.s1 30us : trace_preempt_on (__do_softirq)
630 This has some more changes. Preemption was disabled when an interrupt
631 came in (notice the 'h'), and was enabled while doing a softirq.
632 (notice the 's'). But we also see that interrupts have been disabled
633 when entering the preempt off section and leaving it (the 'd').
634 We do not know if interrupts were enabled in the mean time.
636 # tracer: preemptoff
637 #
638 preemptoff latency trace v1.1.5 on 2.6.26-rc8
639 --------------------------------------------------------------------
640 latency: 63 us, #87/87, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
641 -----------------
642 | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
643 -----------------
644 => started at: remove_wait_queue
645 => ended at: __do_softirq
647 # _------=> CPU#
648 # / _-----=> irqs-off
649 # | / _----=> need-resched
650 # || / _---=> hardirq/softirq
651 # ||| / _--=> preempt-depth
652 # |||| /
653 # ||||| delay
654 # cmd pid ||||| time | caller
655 # \ / ||||| \ | /
656 sshd-4261 0d..1 0us : _spin_lock_irqsave (remove_wait_queue)
657 sshd-4261 0d..1 1us : _spin_unlock_irqrestore (remove_wait_queue)
658 sshd-4261 0d..1 2us : do_IRQ (common_interrupt)
659 sshd-4261 0d..1 2us : irq_enter (do_IRQ)
660 sshd-4261 0d..1 2us : idle_cpu (irq_enter)
661 sshd-4261 0d..1 3us : add_preempt_count (irq_enter)
662 sshd-4261 0d.h1 3us : idle_cpu (irq_enter)
663 sshd-4261 0d.h. 4us : handle_fasteoi_irq (do_IRQ)
664 [...]
665 sshd-4261 0d.h. 12us : add_preempt_count (_spin_lock)
666 sshd-4261 0d.h1 12us : ack_ioapic_quirk_irq (handle_fasteoi_irq)
667 sshd-4261 0d.h1 13us : move_native_irq (ack_ioapic_quirk_irq)
668 sshd-4261 0d.h1 13us : _spin_unlock (handle_fasteoi_irq)
669 sshd-4261 0d.h1 14us : sub_preempt_count (_spin_unlock)
670 sshd-4261 0d.h1 14us : irq_exit (do_IRQ)
671 sshd-4261 0d.h1 15us : sub_preempt_count (irq_exit)
672 sshd-4261 0d..2 15us : do_softirq (irq_exit)
673 sshd-4261 0d... 15us : __do_softirq (do_softirq)
674 sshd-4261 0d... 16us : __local_bh_disable (__do_softirq)
675 sshd-4261 0d... 16us+: add_preempt_count (__local_bh_disable)
676 sshd-4261 0d.s4 20us : add_preempt_count (__local_bh_disable)
677 sshd-4261 0d.s4 21us : sub_preempt_count (local_bh_enable)
678 sshd-4261 0d.s5 21us : sub_preempt_count (local_bh_enable)
679 [...]
680 sshd-4261 0d.s6 41us : add_preempt_count (__local_bh_disable)
681 sshd-4261 0d.s6 42us : sub_preempt_count (local_bh_enable)
682 sshd-4261 0d.s7 42us : sub_preempt_count (local_bh_enable)
683 sshd-4261 0d.s5 43us : add_preempt_count (__local_bh_disable)
684 sshd-4261 0d.s5 43us : sub_preempt_count (local_bh_enable_ip)
685 sshd-4261 0d.s6 44us : sub_preempt_count (local_bh_enable_ip)
686 sshd-4261 0d.s5 44us : add_preempt_count (__local_bh_disable)
687 sshd-4261 0d.s5 45us : sub_preempt_count (local_bh_enable)
688 [...]
689 sshd-4261 0d.s. 63us : _local_bh_enable (__do_softirq)
690 sshd-4261 0d.s1 64us : trace_preempt_on (__do_softirq)
693 The above is an example of the preemptoff trace with ftrace_enabled
694 set. Here we see that interrupts were disabled the entire time.
695 The irq_enter code lets us know that we entered an interrupt 'h'.
696 Before that, the functions being traced still show that it is not
697 in an interrupt, but we can see from the functions themselves that
698 this is not the case.
700 Notice that __do_softirq when called does not have a preempt_count.
701 It may seem that we missed a preempt enabling. What really happened
702 is that the preempt count is held on the thread's stack and we
703 switched to the softirq stack (4K stacks in effect). The code
704 does not copy the preempt count, but because interrupts are disabled,
705 we do not need to worry about it. Having a tracer like this is good
706 for letting people know what really happens inside the kernel.
709 preemptirqsoff
710 --------------
712 Knowing the locations that have interrupts disabled or preemption
713 disabled for the longest times is helpful. But sometimes we would
714 like to know when either preemption and/or interrupts are disabled.
716 Consider the following code:
718 local_irq_disable();
719 call_function_with_irqs_off();
720 preempt_disable();
721 call_function_with_irqs_and_preemption_off();
722 local_irq_enable();
723 call_function_with_preemption_off();
724 preempt_enable();
726 The irqsoff tracer will record the total length of
727 call_function_with_irqs_off() and
728 call_function_with_irqs_and_preemption_off().
730 The preemptoff tracer will record the total length of
731 call_function_with_irqs_and_preemption_off() and
732 call_function_with_preemption_off().
734 But neither will trace the time that interrupts and/or preemption
735 is disabled. This total time is the time that we can not schedule.
736 To record this time, use the preemptirqsoff tracer.
738 Again, using this trace is much like the irqsoff and preemptoff tracers.
740 # echo preemptirqsoff > /debug/tracing/current_tracer
741 # echo 0 > /debug/tracing/tracing_max_latency
742 # echo 1 > /debug/tracing/tracing_enabled
743 # ls -ltr
744 [...]
745 # echo 0 > /debug/tracing/tracing_enabled
746 # cat /debug/tracing/latency_trace
747 # tracer: preemptirqsoff
748 #
749 preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
750 --------------------------------------------------------------------
751 latency: 293 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
752 -----------------
753 | task: ls-4860 (uid:0 nice:0 policy:0 rt_prio:0)
754 -----------------
755 => started at: apic_timer_interrupt
756 => ended at: __do_softirq
758 # _------=> CPU#
759 # / _-----=> irqs-off
760 # | / _----=> need-resched
761 # || / _---=> hardirq/softirq
762 # ||| / _--=> preempt-depth
763 # |||| /
764 # ||||| delay
765 # cmd pid ||||| time | caller
766 # \ / ||||| \ | /
767 ls-4860 0d... 0us!: trace_hardirqs_off_thunk (apic_timer_interrupt)
768 ls-4860 0d.s. 294us : _local_bh_enable (__do_softirq)
769 ls-4860 0d.s1 294us : trace_preempt_on (__do_softirq)
773 The trace_hardirqs_off_thunk is called from assembly on x86 when
774 interrupts are disabled in the assembly code. Without the function
775 tracing, we do not know if interrupts were enabled within the preemption
776 points. We do see that it started with preemption enabled.
778 Here is a trace with ftrace_enabled set:
781 # tracer: preemptirqsoff
782 #
783 preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
784 --------------------------------------------------------------------
785 latency: 105 us, #183/183, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
786 -----------------
787 | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
788 -----------------
789 => started at: write_chan
790 => ended at: __do_softirq
792 # _------=> CPU#
793 # / _-----=> irqs-off
794 # | / _----=> need-resched
795 # || / _---=> hardirq/softirq
796 # ||| / _--=> preempt-depth
797 # |||| /
798 # ||||| delay
799 # cmd pid ||||| time | caller
800 # \ / ||||| \ | /
801 ls-4473 0.N.. 0us : preempt_schedule (write_chan)
802 ls-4473 0dN.1 1us : _spin_lock (schedule)
803 ls-4473 0dN.1 2us : add_preempt_count (_spin_lock)
804 ls-4473 0d..2 2us : put_prev_task_fair (schedule)
805 [...]
806 ls-4473 0d..2 13us : set_normalized_timespec (ktime_get_ts)
807 ls-4473 0d..2 13us : __switch_to (schedule)
808 sshd-4261 0d..2 14us : finish_task_switch (schedule)
809 sshd-4261 0d..2 14us : _spin_unlock_irq (finish_task_switch)
810 sshd-4261 0d..1 15us : add_preempt_count (_spin_lock_irqsave)
811 sshd-4261 0d..2 16us : _spin_unlock_irqrestore (hrtick_set)
812 sshd-4261 0d..2 16us : do_IRQ (common_interrupt)
813 sshd-4261 0d..2 17us : irq_enter (do_IRQ)
814 sshd-4261 0d..2 17us : idle_cpu (irq_enter)
815 sshd-4261 0d..2 18us : add_preempt_count (irq_enter)
816 sshd-4261 0d.h2 18us : idle_cpu (irq_enter)
817 sshd-4261 0d.h. 18us : handle_fasteoi_irq (do_IRQ)
818 sshd-4261 0d.h. 19us : _spin_lock (handle_fasteoi_irq)
819 sshd-4261 0d.h. 19us : add_preempt_count (_spin_lock)
820 sshd-4261 0d.h1 20us : _spin_unlock (handle_fasteoi_irq)
821 sshd-4261 0d.h1 20us : sub_preempt_count (_spin_unlock)
822 [...]
823 sshd-4261 0d.h1 28us : _spin_unlock (handle_fasteoi_irq)
824 sshd-4261 0d.h1 29us : sub_preempt_count (_spin_unlock)
825 sshd-4261 0d.h2 29us : irq_exit (do_IRQ)
826 sshd-4261 0d.h2 29us : sub_preempt_count (irq_exit)
827 sshd-4261 0d..3 30us : do_softirq (irq_exit)
828 sshd-4261 0d... 30us : __do_softirq (do_softirq)
829 sshd-4261 0d... 31us : __local_bh_disable (__do_softirq)
830 sshd-4261 0d... 31us+: add_preempt_count (__local_bh_disable)
831 sshd-4261 0d.s4 34us : add_preempt_count (__local_bh_disable)
832 [...]
833 sshd-4261 0d.s3 43us : sub_preempt_count (local_bh_enable_ip)
834 sshd-4261 0d.s4 44us : sub_preempt_count (local_bh_enable_ip)
835 sshd-4261 0d.s3 44us : smp_apic_timer_interrupt (apic_timer_interrupt)
836 sshd-4261 0d.s3 45us : irq_enter (smp_apic_timer_interrupt)
837 sshd-4261 0d.s3 45us : idle_cpu (irq_enter)
838 sshd-4261 0d.s3 46us : add_preempt_count (irq_enter)
839 sshd-4261 0d.H3 46us : idle_cpu (irq_enter)
840 sshd-4261 0d.H3 47us : hrtimer_interrupt (smp_apic_timer_interrupt)
841 sshd-4261 0d.H3 47us : ktime_get (hrtimer_interrupt)
842 [...]
843 sshd-4261 0d.H3 81us : tick_program_event (hrtimer_interrupt)
844 sshd-4261 0d.H3 82us : ktime_get (tick_program_event)
845 sshd-4261 0d.H3 82us : ktime_get_ts (ktime_get)
846 sshd-4261 0d.H3 83us : getnstimeofday (ktime_get_ts)
847 sshd-4261 0d.H3 83us : set_normalized_timespec (ktime_get_ts)
848 sshd-4261 0d.H3 84us : clockevents_program_event (tick_program_event)
849 sshd-4261 0d.H3 84us : lapic_next_event (clockevents_program_event)
850 sshd-4261 0d.H3 85us : irq_exit (smp_apic_timer_interrupt)
851 sshd-4261 0d.H3 85us : sub_preempt_count (irq_exit)
852 sshd-4261 0d.s4 86us : sub_preempt_count (irq_exit)
853 sshd-4261 0d.s3 86us : add_preempt_count (__local_bh_disable)
854 [...]
855 sshd-4261 0d.s1 98us : sub_preempt_count (net_rx_action)
856 sshd-4261 0d.s. 99us : add_preempt_count (_spin_lock_irq)
857 sshd-4261 0d.s1 99us+: _spin_unlock_irq (run_timer_softirq)
858 sshd-4261 0d.s. 104us : _local_bh_enable (__do_softirq)
859 sshd-4261 0d.s. 104us : sub_preempt_count (_local_bh_enable)
860 sshd-4261 0d.s. 105us : _local_bh_enable (__do_softirq)
861 sshd-4261 0d.s1 105us : trace_preempt_on (__do_softirq)
864 This is a very interesting trace. It started with the preemption of
865 the ls task. We see that the task had the "need_resched" bit set
866 via the 'N' in the trace. Interrupts were disabled before the spin_lock
867 at the beginning of the trace. We see that a schedule took place to run
868 sshd. When the interrupts were enabled, we took an interrupt.
869 On return from the interrupt handler, the softirq ran. We took another
870 interrupt while running the softirq as we see from the capital 'H'.
873 wakeup
874 ------
876 In a Real-Time environment it is very important to know the wakeup
877 time it takes for the highest priority task that is woken up to the
878 time that it executes. This is also known as "schedule latency".
879 I stress the point that this is about RT tasks. It is also important
880 to know the scheduling latency of non-RT tasks, but the average
881 schedule latency is better for non-RT tasks. Tools like
882 LatencyTop are more appropriate for such measurements.
884 Real-Time environments are interested in the worst case latency.
885 That is the longest latency it takes for something to happen, and
886 not the average. We can have a very fast scheduler that may only
887 have a large latency once in a while, but that would not work well
888 with Real-Time tasks. The wakeup tracer was designed to record
889 the worst case wakeups of RT tasks. Non-RT tasks are not recorded
890 because the tracer only records one worst case and tracing non-RT
891 tasks that are unpredictable will overwrite the worst case latency
892 of RT tasks.
894 Since this tracer only deals with RT tasks, we will run this slightly
895 differently than we did with the previous tracers. Instead of performing
896 an 'ls', we will run 'sleep 1' under 'chrt' which changes the
897 priority of the task.
899 # echo wakeup > /debug/tracing/current_tracer
900 # echo 0 > /debug/tracing/tracing_max_latency
901 # echo 1 > /debug/tracing/tracing_enabled
902 # chrt -f 5 sleep 1
903 # echo 0 > /debug/tracing/tracing_enabled
904 # cat /debug/tracing/latency_trace
905 # tracer: wakeup
906 #
907 wakeup latency trace v1.1.5 on 2.6.26-rc8
908 --------------------------------------------------------------------
909 latency: 4 us, #2/2, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
910 -----------------
911 | task: sleep-4901 (uid:0 nice:0 policy:1 rt_prio:5)
912 -----------------
914 # _------=> CPU#
915 # / _-----=> irqs-off
916 # | / _----=> need-resched
917 # || / _---=> hardirq/softirq
918 # ||| / _--=> preempt-depth
919 # |||| /
920 # ||||| delay
921 # cmd pid ||||| time | caller
922 # \ / ||||| \ | /
923 <idle>-0 1d.h4 0us+: try_to_wake_up (wake_up_process)
924 <idle>-0 1d..4 4us : schedule (cpu_idle)
928 Running this on an idle system, we see that it only took 4 microseconds
929 to perform the task switch. Note, since the trace marker in the
930 schedule is before the actual "switch", we stop the tracing when
931 the recorded task is about to schedule in. This may change if
932 we add a new marker at the end of the scheduler.
934 Notice that the recorded task is 'sleep' with the PID of 4901 and it
935 has an rt_prio of 5. This priority is user-space priority and not
936 the internal kernel priority. The policy is 1 for SCHED_FIFO and 2
937 for SCHED_RR.
939 Doing the same with chrt -r 5 and ftrace_enabled set.
941 # tracer: wakeup
942 #
943 wakeup latency trace v1.1.5 on 2.6.26-rc8
944 --------------------------------------------------------------------
945 latency: 50 us, #60/60, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
946 -----------------
947 | task: sleep-4068 (uid:0 nice:0 policy:2 rt_prio:5)
948 -----------------
950 # _------=> CPU#
951 # / _-----=> irqs-off
952 # | / _----=> need-resched
953 # || / _---=> hardirq/softirq
954 # ||| / _--=> preempt-depth
955 # |||| /
956 # ||||| delay
957 # cmd pid ||||| time | caller
958 # \ / ||||| \ | /
959 ksoftirq-7 1d.H3 0us : try_to_wake_up (wake_up_process)
960 ksoftirq-7 1d.H4 1us : sub_preempt_count (marker_probe_cb)
961 ksoftirq-7 1d.H3 2us : check_preempt_wakeup (try_to_wake_up)
962 ksoftirq-7 1d.H3 3us : update_curr (check_preempt_wakeup)
963 ksoftirq-7 1d.H3 4us : calc_delta_mine (update_curr)
964 ksoftirq-7 1d.H3 5us : __resched_task (check_preempt_wakeup)
965 ksoftirq-7 1d.H3 6us : task_wake_up_rt (try_to_wake_up)
966 ksoftirq-7 1d.H3 7us : _spin_unlock_irqrestore (try_to_wake_up)
967 [...]
968 ksoftirq-7 1d.H2 17us : irq_exit (smp_apic_timer_interrupt)
969 ksoftirq-7 1d.H2 18us : sub_preempt_count (irq_exit)
970 ksoftirq-7 1d.s3 19us : sub_preempt_count (irq_exit)
971 ksoftirq-7 1..s2 20us : rcu_process_callbacks (__do_softirq)
972 [...]
973 ksoftirq-7 1..s2 26us : __rcu_process_callbacks (rcu_process_callbacks)
974 ksoftirq-7 1d.s2 27us : _local_bh_enable (__do_softirq)
975 ksoftirq-7 1d.s2 28us : sub_preempt_count (_local_bh_enable)
976 ksoftirq-7 1.N.3 29us : sub_preempt_count (ksoftirqd)
977 ksoftirq-7 1.N.2 30us : _cond_resched (ksoftirqd)
978 ksoftirq-7 1.N.2 31us : __cond_resched (_cond_resched)
979 ksoftirq-7 1.N.2 32us : add_preempt_count (__cond_resched)
980 ksoftirq-7 1.N.2 33us : schedule (__cond_resched)
981 ksoftirq-7 1.N.2 33us : add_preempt_count (schedule)
982 ksoftirq-7 1.N.3 34us : hrtick_clear (schedule)
983 ksoftirq-7 1dN.3 35us : _spin_lock (schedule)
984 ksoftirq-7 1dN.3 36us : add_preempt_count (_spin_lock)
985 ksoftirq-7 1d..4 37us : put_prev_task_fair (schedule)
986 ksoftirq-7 1d..4 38us : update_curr (put_prev_task_fair)
987 [...]
988 ksoftirq-7 1d..5 47us : _spin_trylock (tracing_record_cmdline)
989 ksoftirq-7 1d..5 48us : add_preempt_count (_spin_trylock)
990 ksoftirq-7 1d..6 49us : _spin_unlock (tracing_record_cmdline)
991 ksoftirq-7 1d..6 49us : sub_preempt_count (_spin_unlock)
992 ksoftirq-7 1d..4 50us : schedule (__cond_resched)
994 The interrupt went off while running ksoftirqd. This task runs at
995 SCHED_OTHER. Why did not we see the 'N' set early? This may be
996 a harmless bug with x86_32 and 4K stacks. On x86_32 with 4K stacks
997 configured, the interrupt and softirq run with their own stack.
998 Some information is held on the top of the task's stack (need_resched
999 and preempt_count are both stored there). The setting of the NEED_RESCHED
1000 bit is done directly to the task's stack, but the reading of the
1001 NEED_RESCHED is done by looking at the current stack, which in this case
1002 is the stack for the hard interrupt. This hides the fact that NEED_RESCHED
1003 has been set. We do not see the 'N' until we switch back to the task's
1004 assigned stack.
1006 ftrace
1007 ------
1009 ftrace is not only the name of the tracing infrastructure, but it
1010 is also a name of one of the tracers. The tracer is the function
1011 tracer. Enabling the function tracer can be done from the
1012 debug file system. Make sure the ftrace_enabled is set otherwise
1013 this tracer is a nop.
1015 # sysctl kernel.ftrace_enabled=1
1016 # echo ftrace > /debug/tracing/current_tracer
1017 # echo 1 > /debug/tracing/tracing_enabled
1018 # usleep 1
1019 # echo 0 > /debug/tracing/tracing_enabled
1020 # cat /debug/tracing/trace
1021 # tracer: ftrace
1022 #
1023 # TASK-PID CPU# TIMESTAMP FUNCTION
1024 # | | | | |
1025 bash-4003 [00] 123.638713: finish_task_switch <-schedule
1026 bash-4003 [00] 123.638714: _spin_unlock_irq <-finish_task_switch
1027 bash-4003 [00] 123.638714: sub_preempt_count <-_spin_unlock_irq
1028 bash-4003 [00] 123.638715: hrtick_set <-schedule
1029 bash-4003 [00] 123.638715: _spin_lock_irqsave <-hrtick_set
1030 bash-4003 [00] 123.638716: add_preempt_count <-_spin_lock_irqsave
1031 bash-4003 [00] 123.638716: _spin_unlock_irqrestore <-hrtick_set
1032 bash-4003 [00] 123.638717: sub_preempt_count <-_spin_unlock_irqrestore
1033 bash-4003 [00] 123.638717: hrtick_clear <-hrtick_set
1034 bash-4003 [00] 123.638718: sub_preempt_count <-schedule
1035 bash-4003 [00] 123.638718: sub_preempt_count <-preempt_schedule
1036 bash-4003 [00] 123.638719: wait_for_completion <-__stop_machine_run
1037 bash-4003 [00] 123.638719: wait_for_common <-wait_for_completion
1038 bash-4003 [00] 123.638720: _spin_lock_irq <-wait_for_common
1039 bash-4003 [00] 123.638720: add_preempt_count <-_spin_lock_irq
1040 [...]
1043 Note: ftrace uses ring buffers to store the above entries. The newest data
1044 may overwrite the oldest data. Sometimes using echo to stop the trace
1045 is not sufficient because the tracing could have overwritten the data
1046 that you wanted to record. For this reason, it is sometimes better to
1047 disable tracing directly from a program. This allows you to stop the
1048 tracing at the point that you hit the part that you are interested in.
1049 To disable the tracing directly from a C program, something like following
1050 code snippet can be used:
1052 int trace_fd;
1053 [...]
1054 int main(int argc, char *argv[]) {
1055 [...]
1056 trace_fd = open("/debug/tracing/tracing_enabled", O_WRONLY);
1057 [...]
1058 if (condition_hit()) {
1059 write(trace_fd, "0", 1);
1060 }
1061 [...]
1062 }
1064 Note: Here we hard coded the path name. The debugfs mount is not
1065 guaranteed to be at /debug (and is more commonly at /sys/kernel/debug).
1066 For simple one time traces, the above is sufficent. For anything else,
1067 a search through /proc/mounts may be needed to find where the debugfs
1068 file-system is mounted.
1070 dynamic ftrace
1071 --------------
1073 If CONFIG_DYNAMIC_FTRACE is set, the system will run with
1074 virtually no overhead when function tracing is disabled. The way
1075 this works is the mcount function call (placed at the start of
1076 every kernel function, produced by the -pg switch in gcc), starts
1077 of pointing to a simple return. (Enabling FTRACE will include the
1078 -pg switch in the compiling of the kernel.)
1080 When dynamic ftrace is initialized, it calls kstop_machine to make
1081 the machine act like a uniprocessor so that it can freely modify code
1082 without worrying about other processors executing that same code. At
1083 initialization, the mcount calls are changed to call a "record_ip"
1084 function. After this, the first time a kernel function is called,
1085 it has the calling address saved in a hash table.
1087 Later on the ftraced kernel thread is awoken and will again call
1088 kstop_machine if new functions have been recorded. The ftraced thread
1089 will change all calls to mcount to "nop". Just calling mcount
1090 and having mcount return has shown a 10% overhead. By converting
1091 it to a nop, there is no measurable overhead to the system.
1093 One special side-effect to the recording of the functions being
1094 traced is that we can now selectively choose which functions we
1095 wish to trace and which ones we want the mcount calls to remain as
1096 nops.
1098 Two files are used, one for enabling and one for disabling the tracing
1099 of specified functions. They are:
1101 set_ftrace_filter
1103 and
1105 set_ftrace_notrace
1107 A list of available functions that you can add to these files is listed
1108 in:
1110 available_filter_functions
1112 # cat /debug/tracing/available_filter_functions
1113 put_prev_task_idle
1114 kmem_cache_create
1115 pick_next_task_rt
1116 get_online_cpus
1117 pick_next_task_fair
1118 mutex_lock
1119 [...]
1121 If I am only interested in sys_nanosleep and hrtimer_interrupt:
1123 # echo sys_nanosleep hrtimer_interrupt \
1124 > /debug/tracing/set_ftrace_filter
1125 # echo ftrace > /debug/tracing/current_tracer
1126 # echo 1 > /debug/tracing/tracing_enabled
1127 # usleep 1
1128 # echo 0 > /debug/tracing/tracing_enabled
1129 # cat /debug/tracing/trace
1130 # tracer: ftrace
1131 #
1132 # TASK-PID CPU# TIMESTAMP FUNCTION
1133 # | | | | |
1134 usleep-4134 [00] 1317.070017: hrtimer_interrupt <-smp_apic_timer_interrupt
1135 usleep-4134 [00] 1317.070111: sys_nanosleep <-syscall_call
1136 <idle>-0 [00] 1317.070115: hrtimer_interrupt <-smp_apic_timer_interrupt
1138 To see which functions are being traced, you can cat the file:
1140 # cat /debug/tracing/set_ftrace_filter
1141 hrtimer_interrupt
1142 sys_nanosleep
1145 Perhaps this is not enough. The filters also allow simple wild cards.
1146 Only the following are currently available
1148 <match>* - will match functions that begin with <match>
1149 *<match> - will match functions that end with <match>
1150 *<match>* - will match functions that have <match> in it
1152 These are the only wild cards which are supported.
1154 <match>*<match> will not work.
1156 # echo hrtimer_* > /debug/tracing/set_ftrace_filter
1158 Produces:
1160 # tracer: ftrace
1161 #
1162 # TASK-PID CPU# TIMESTAMP FUNCTION
1163 # | | | | |
1164 bash-4003 [00] 1480.611794: hrtimer_init <-copy_process
1165 bash-4003 [00] 1480.611941: hrtimer_start <-hrtick_set
1166 bash-4003 [00] 1480.611956: hrtimer_cancel <-hrtick_clear
1167 bash-4003 [00] 1480.611956: hrtimer_try_to_cancel <-hrtimer_cancel
1168 <idle>-0 [00] 1480.612019: hrtimer_get_next_event <-get_next_timer_interrupt
1169 <idle>-0 [00] 1480.612025: hrtimer_get_next_event <-get_next_timer_interrupt
1170 <idle>-0 [00] 1480.612032: hrtimer_get_next_event <-get_next_timer_interrupt
1171 <idle>-0 [00] 1480.612037: hrtimer_get_next_event <-get_next_timer_interrupt
1172 <idle>-0 [00] 1480.612382: hrtimer_get_next_event <-get_next_timer_interrupt
1175 Notice that we lost the sys_nanosleep.
1177 # cat /debug/tracing/set_ftrace_filter
1178 hrtimer_run_queues
1179 hrtimer_run_pending
1180 hrtimer_init
1181 hrtimer_cancel
1182 hrtimer_try_to_cancel
1183 hrtimer_forward
1184 hrtimer_start
1185 hrtimer_reprogram
1186 hrtimer_force_reprogram
1187 hrtimer_get_next_event
1188 hrtimer_interrupt
1189 hrtimer_nanosleep
1190 hrtimer_wakeup
1191 hrtimer_get_remaining
1192 hrtimer_get_res
1193 hrtimer_init_sleeper
1196 This is because the '>' and '>>' act just like they do in bash.
1197 To rewrite the filters, use '>'
1198 To append to the filters, use '>>'
1200 To clear out a filter so that all functions will be recorded again:
1202 # echo > /debug/tracing/set_ftrace_filter
1203 # cat /debug/tracing/set_ftrace_filter
1204 #
1206 Again, now we want to append.
1208 # echo sys_nanosleep > /debug/tracing/set_ftrace_filter
1209 # cat /debug/tracing/set_ftrace_filter
1210 sys_nanosleep
1211 # echo hrtimer_* >> /debug/tracing/set_ftrace_filter
1212 # cat /debug/tracing/set_ftrace_filter
1213 hrtimer_run_queues
1214 hrtimer_run_pending
1215 hrtimer_init
1216 hrtimer_cancel
1217 hrtimer_try_to_cancel
1218 hrtimer_forward
1219 hrtimer_start
1220 hrtimer_reprogram
1221 hrtimer_force_reprogram
1222 hrtimer_get_next_event
1223 hrtimer_interrupt
1224 sys_nanosleep
1225 hrtimer_nanosleep
1226 hrtimer_wakeup
1227 hrtimer_get_remaining
1228 hrtimer_get_res
1229 hrtimer_init_sleeper
1232 The set_ftrace_notrace prevents those functions from being traced.
1234 # echo '*preempt*' '*lock*' > /debug/tracing/set_ftrace_notrace
1236 Produces:
1238 # tracer: ftrace
1239 #
1240 # TASK-PID CPU# TIMESTAMP FUNCTION
1241 # | | | | |
1242 bash-4043 [01] 115.281644: finish_task_switch <-schedule
1243 bash-4043 [01] 115.281645: hrtick_set <-schedule
1244 bash-4043 [01] 115.281645: hrtick_clear <-hrtick_set
1245 bash-4043 [01] 115.281646: wait_for_completion <-__stop_machine_run
1246 bash-4043 [01] 115.281647: wait_for_common <-wait_for_completion
1247 bash-4043 [01] 115.281647: kthread_stop <-stop_machine_run
1248 bash-4043 [01] 115.281648: init_waitqueue_head <-kthread_stop
1249 bash-4043 [01] 115.281648: wake_up_process <-kthread_stop
1250 bash-4043 [01] 115.281649: try_to_wake_up <-wake_up_process
1252 We can see that there's no more lock or preempt tracing.
1254 ftraced
1255 -------
1257 As mentioned above, when dynamic ftrace is configured in, a kernel
1258 thread wakes up once a second and checks to see if there are mcount
1259 calls that need to be converted into nops. If there are not any, then
1260 it simply goes back to sleep. But if there are some, it will call
1261 kstop_machine to convert the calls to nops.
1263 There may be a case in which you do not want this added latency.
1264 Perhaps you are doing some audio recording and this activity might
1265 cause skips in the playback. There is an interface to disable
1266 and enable the "ftraced" kernel thread.
1268 # echo 0 > /debug/tracing/ftraced_enabled
1270 This will disable the calling of kstop_machine to update the
1271 mcount calls to nops. Remember that there is a large overhead
1272 to calling mcount. Without this kernel thread, that overhead will
1273 exist.
1275 If there are recorded calls to mcount, any write to the ftraced_enabled
1276 file will cause the kstop_machine to run. This means that a
1277 user can manually perform the updates when they want to by simply
1278 echoing a '0' into the ftraced_enabled file.
1280 The updates are also done at the beginning of enabling a tracer
1281 that uses ftrace function recording.
1284 trace_pipe
1285 ----------
1287 The trace_pipe outputs the same content as the trace file, but the effect
1288 on the tracing is different. Every read from trace_pipe is consumed.
1289 This means that subsequent reads will be different. The trace
1290 is live.
1292 # echo ftrace > /debug/tracing/current_tracer
1293 # cat /debug/tracing/trace_pipe > /tmp/trace.out &
1294 [1] 4153
1295 # echo 1 > /debug/tracing/tracing_enabled
1296 # usleep 1
1297 # echo 0 > /debug/tracing/tracing_enabled
1298 # cat /debug/tracing/trace
1299 # tracer: ftrace
1300 #
1301 # TASK-PID CPU# TIMESTAMP FUNCTION
1302 # | | | | |
1304 #
1305 # cat /tmp/trace.out
1306 bash-4043 [00] 41.267106: finish_task_switch <-schedule
1307 bash-4043 [00] 41.267106: hrtick_set <-schedule
1308 bash-4043 [00] 41.267107: hrtick_clear <-hrtick_set
1309 bash-4043 [00] 41.267108: wait_for_completion <-__stop_machine_run
1310 bash-4043 [00] 41.267108: wait_for_common <-wait_for_completion
1311 bash-4043 [00] 41.267109: kthread_stop <-stop_machine_run
1312 bash-4043 [00] 41.267109: init_waitqueue_head <-kthread_stop
1313 bash-4043 [00] 41.267110: wake_up_process <-kthread_stop
1314 bash-4043 [00] 41.267110: try_to_wake_up <-wake_up_process
1315 bash-4043 [00] 41.267111: select_task_rq_rt <-try_to_wake_up
1318 Note, reading the trace_pipe file will block until more input is added.
1319 By changing the tracer, trace_pipe will issue an EOF. We needed
1320 to set the ftrace tracer _before_ cating the trace_pipe file.
1323 trace entries
1324 -------------
1326 Having too much or not enough data can be troublesome in diagnosing
1327 an issue in the kernel. The file trace_entries is used to modify
1328 the size of the internal trace buffers. The number listed
1329 is the number of entries that can be recorded per CPU. To know
1330 the full size, multiply the number of possible CPUS with the
1331 number of entries.
1333 # cat /debug/tracing/trace_entries
1334 65620
1336 Note, to modify this, you must have tracing completely disabled. To do that,
1337 echo "none" into the current_tracer. If the current_tracer is not set
1338 to "none", an EINVAL error will be returned.
1340 # echo none > /debug/tracing/current_tracer
1341 # echo 100000 > /debug/tracing/trace_entries
1342 # cat /debug/tracing/trace_entries
1343 100045
1346 Notice that we echoed in 100,000 but the size is 100,045. The entries
1347 are held in individual pages. It allocates the number of pages it takes
1348 to fulfill the request. If more entries may fit on the last page
1349 then they will be added.
1351 # echo 1 > /debug/tracing/trace_entries
1352 # cat /debug/tracing/trace_entries
1353 85
1355 This shows us that 85 entries can fit in a single page.
1357 The number of pages which will be allocated is limited to a percentage
1358 of available memory. Allocating too much will produce an error.
1360 # echo 1000000000000 > /debug/tracing/trace_entries
1361 -bash: echo: write error: Cannot allocate memory
1362 # cat /debug/tracing/trace_entries
1363 85