1 ========================
2 ftrace - Function Tracer
3 ========================
5 Copyright 2008 Red Hat Inc.
7 :Author: Steven Rostedt <srostedt@redhat.com>
8 :License: The GNU Free Documentation License, Version 1.2
9 (dual licensed under the GPL v2)
10 :Original Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton,
11 John Kacur, and David Teigland.
13 - Written for: 2.6.28-rc2
15 - Updated for: 4.13 - Copyright 2017 VMware Inc. Steven Rostedt
16 - Converted to rst format - Changbin Du <changbin.du@intel.com>
21 Ftrace is an internal tracer designed to help out developers and
22 designers of systems to find what is going on inside the kernel.
23 It can be used for debugging or analyzing latencies and
24 performance issues that take place outside of user-space.
26 Although ftrace is typically considered the function tracer, it
27 is really a framework of several assorted tracing utilities.
28 There's latency tracing to examine what occurs between interrupts
29 disabled and enabled, as well as for preemption and from a time
30 a task is woken to the task is actually scheduled in.
32 One of the most common uses of ftrace is the event tracing.
33 Throughout the kernel is hundreds of static event points that
34 can be enabled via the tracefs file system to see what is
35 going on in certain parts of the kernel.
37 See events.rst for more information.
40 Implementation Details
41 ----------------------
43 See Documentation/trace/ftrace-design.rst for details for arch porters and such.
49 Ftrace uses the tracefs file system to hold the control files as
50 well as the files to display output.
52 When tracefs is configured into the kernel (which selecting any ftrace
53 option will do) the directory /sys/kernel/tracing will be created. To mount
54 this directory, you can add to your /etc/fstab file::
56 tracefs /sys/kernel/tracing tracefs defaults 0 0
58 Or you can mount it at run time with::
60 mount -t tracefs nodev /sys/kernel/tracing
62 For quicker access to that directory you may want to make a soft link to
65 ln -s /sys/kernel/tracing /tracing
69 Before 4.1, all ftrace tracing control files were within the debugfs
70 file system, which is typically located at /sys/kernel/debug/tracing.
71 For backward compatibility, when mounting the debugfs file system,
72 the tracefs file system will be automatically mounted at:
74 /sys/kernel/debug/tracing
76 All files located in the tracefs file system will be located in that
77 debugfs file system directory as well.
81 Any selected ftrace option will also create the tracefs file system.
82 The rest of the document will assume that you are in the ftrace directory
83 (cd /sys/kernel/tracing) and will only concentrate on the files within that
84 directory and not distract from the content with the extended
85 "/sys/kernel/tracing" path name.
87 That's it! (assuming that you have ftrace configured into your kernel)
89 After mounting tracefs you will have access to the control and output files
90 of ftrace. Here is a list of some of the key files:
93 Note: all time values are in microseconds.
97 This is used to set or display the current tracer
98 that is configured. Changing the current tracer clears
99 the ring buffer content as well as the "snapshot" buffer.
103 This holds the different types of tracers that
104 have been compiled into the kernel. The
105 tracers listed here can be configured by
106 echoing their name into current_tracer.
110 This sets or displays whether writing to the trace
111 ring buffer is enabled. Echo 0 into this file to disable
112 the tracer or 1 to enable it. Note, this only disables
113 writing to the ring buffer, the tracing overhead may
116 The kernel function tracing_off() can be used within the
117 kernel to disable writing to the ring buffer, which will
118 set this file to "0". User space can re-enable tracing by
119 echoing "1" into the file.
121 Note, the function and event trigger "traceoff" will also
122 set this file to zero and stop tracing. Which can also
123 be re-enabled by user space using this file.
127 This file holds the output of the trace in a human
128 readable format (described below). Opening this file for
129 writing with the O_TRUNC flag clears the ring buffer content.
130 Note, this file is not a consumer. If tracing is off
131 (no tracer running, or tracing_on is zero), it will produce
132 the same output each time it is read. When tracing is on,
133 it may produce inconsistent results as it tries to read
134 the entire buffer without consuming it.
138 The output is the same as the "trace" file but this
139 file is meant to be streamed with live tracing.
140 Reads from this file will block until new data is
141 retrieved. Unlike the "trace" file, this file is a
142 consumer. This means reading from this file causes
143 sequential reads to display more current data. Once
144 data is read from this file, it is consumed, and
145 will not be read again with a sequential read. The
146 "trace" file is static, and if the tracer is not
147 adding more data, it will display the same
148 information every time it is read.
152 This file lets the user control the amount of data
153 that is displayed in one of the above output
154 files. Options also exist to modify how a tracer
155 or events work (stack traces, timestamps, etc).
159 This is a directory that has a file for every available
160 trace option (also in trace_options). Options may also be set
161 or cleared by writing a "1" or "0" respectively into the
162 corresponding file with the option name.
166 Some of the tracers record the max latency.
167 For example, the maximum time that interrupts are disabled.
168 The maximum time is saved in this file. The max trace will also be
169 stored, and displayed by "trace". A new max trace will only be
170 recorded if the latency is greater than the value in this file
173 By echoing in a time into this file, no latency will be recorded
174 unless it is greater than the time in this file.
178 Some latency tracers will record a trace whenever the
179 latency is greater than the number in this file.
180 Only active when the file contains a number greater than 0.
185 This sets or displays the number of kilobytes each CPU
186 buffer holds. By default, the trace buffers are the same size
187 for each CPU. The displayed number is the size of the
188 CPU buffer and not total size of all buffers. The
189 trace buffers are allocated in pages (blocks of memory
190 that the kernel uses for allocation, usually 4 KB in size).
191 A few extra pages may be allocated to accommodate buffer management
192 meta-data. If the last page allocated has room for more bytes
193 than requested, the rest of the page will be used,
194 making the actual allocation bigger than requested or shown.
195 ( Note, the size may not be a multiple of the page size
196 due to buffer management meta-data. )
198 Buffer sizes for individual CPUs may vary
199 (see "per_cpu/cpu0/buffer_size_kb" below), and if they do
200 this file will show "X".
202 buffer_total_size_kb:
204 This displays the total combined size of all the trace buffers.
208 If a process is performing tracing, and the ring buffer should be
209 shrunk "freed" when the process is finished, even if it were to be
210 killed by a signal, this file can be used for that purpose. On close
211 of this file, the ring buffer will be resized to its minimum size.
212 Having a process that is tracing also open this file, when the process
213 exits its file descriptor for this file will be closed, and in doing so,
214 the ring buffer will be "freed".
216 It may also stop tracing if disable_on_free option is set.
220 This is a mask that lets the user only trace on specified CPUs.
221 The format is a hex string representing the CPUs.
225 When dynamic ftrace is configured in (see the
226 section below "dynamic ftrace"), the code is dynamically
227 modified (code text rewrite) to disable calling of the
228 function profiler (mcount). This lets tracing be configured
229 in with practically no overhead in performance. This also
230 has a side effect of enabling or disabling specific functions
231 to be traced. Echoing names of functions into this file
232 will limit the trace to only those functions.
233 This influences the tracers "function" and "function_graph"
234 and thus also function profiling (see "function_profile_enabled").
236 The functions listed in "available_filter_functions" are what
237 can be written into this file.
239 This interface also allows for commands to be used. See the
240 "Filter commands" section for more details.
242 As a speed up, since processing strings can be quite expensive
243 and requires a check of all functions registered to tracing, instead
244 an index can be written into this file. A number (starting with "1")
245 written will instead select the same corresponding at the line position
246 of the "available_filter_functions" file.
250 This has an effect opposite to that of
251 set_ftrace_filter. Any function that is added here will not
252 be traced. If a function exists in both set_ftrace_filter
253 and set_ftrace_notrace, the function will _not_ be traced.
257 Have the function tracer only trace the threads whose PID are
260 If the "function-fork" option is set, then when a task whose
261 PID is listed in this file forks, the child's PID will
262 automatically be added to this file, and the child will be
263 traced by the function tracer as well. This option will also
264 cause PIDs of tasks that exit to be removed from the file.
266 set_ftrace_notrace_pid:
268 Have the function tracer ignore threads whose PID are listed in
271 If the "function-fork" option is set, then when a task whose
272 PID is listed in this file forks, the child's PID will
273 automatically be added to this file, and the child will not be
274 traced by the function tracer as well. This option will also
275 cause PIDs of tasks that exit to be removed from the file.
277 If a PID is in both this file and "set_ftrace_pid", then this
278 file takes precedence, and the thread will not be traced.
282 Have the events only trace a task with a PID listed in this file.
283 Note, sched_switch and sched_wake_up will also trace events
286 To have the PIDs of children of tasks with their PID in this file
287 added on fork, enable the "event-fork" option. That option will also
288 cause the PIDs of tasks to be removed from this file when the task
291 set_event_notrace_pid:
293 Have the events not trace a task with a PID listed in this file.
294 Note, sched_switch and sched_wakeup will trace threads not listed
295 in this file, even if a thread's PID is in the file if the
296 sched_switch or sched_wakeup events also trace a thread that should
299 To have the PIDs of children of tasks with their PID in this file
300 added on fork, enable the "event-fork" option. That option will also
301 cause the PIDs of tasks to be removed from this file when the task
306 Functions listed in this file will cause the function graph
307 tracer to only trace these functions and the functions that
308 they call. (See the section "dynamic ftrace" for more details).
309 Note, set_ftrace_filter and set_ftrace_notrace still affects
310 what functions are being traced.
314 Similar to set_graph_function, but will disable function graph
315 tracing when the function is hit until it exits the function.
316 This makes it possible to ignore tracing functions that are called
317 by a specific function.
319 available_filter_functions:
321 This lists the functions that ftrace has processed and can trace.
322 These are the function names that you can pass to
323 "set_ftrace_filter", "set_ftrace_notrace",
324 "set_graph_function", or "set_graph_notrace".
325 (See the section "dynamic ftrace" below for more details.)
327 dyn_ftrace_total_info:
329 This file is for debugging purposes. The number of functions that
330 have been converted to nops and are available to be traced.
334 This file is more for debugging ftrace, but can also be useful
335 in seeing if any function has a callback attached to it.
336 Not only does the trace infrastructure use ftrace function
337 trace utility, but other subsystems might too. This file
338 displays all functions that have a callback attached to them
339 as well as the number of callbacks that have been attached.
340 Note, a callback may also call multiple functions which will
341 not be listed in this count.
343 If the callback registered to be traced by a function with
344 the "save regs" attribute (thus even more overhead), a 'R'
345 will be displayed on the same line as the function that
346 is returning registers.
348 If the callback registered to be traced by a function with
349 the "ip modify" attribute (thus the regs->ip can be changed),
350 an 'I' will be displayed on the same line as the function that
353 If a non ftrace trampoline is attached (BPF) a 'D' will be displayed.
354 Note, normal ftrace trampolines can also be attached, but only one
355 "direct" trampoline can be attached to a given function at a time.
357 Some architectures can not call direct trampolines, but instead have
358 the ftrace ops function located above the function entry point. In
359 such cases an 'O' will be displayed.
361 If a function had either the "ip modify" or a "direct" call attached to
362 it in the past, a 'M' will be shown. This flag is never cleared. It is
363 used to know if a function was every modified by the ftrace infrastructure,
364 and can be used for debugging.
366 If the architecture supports it, it will also show what callback
367 is being directly called by the function. If the count is greater
368 than 1 it most likely will be ftrace_ops_list_func().
370 If the callback of a function jumps to a trampoline that is
371 specific to the callback and which is not the standard trampoline,
372 its address will be printed as well as the function that the
377 This file contains all the functions that ever had a function callback
378 to it via the ftrace infrastructure. It has the same format as
379 enabled_functions but shows all functions that have every been
382 To see any function that has every been modified by "ip modify" or a
383 direct trampoline, one can perform the following command:
385 grep ' M ' /sys/kernel/tracing/touched_functions
387 function_profile_enabled:
389 When set it will enable all functions with either the function
390 tracer, or if configured, the function graph tracer. It will
391 keep a histogram of the number of functions that were called
392 and if the function graph tracer was configured, it will also keep
393 track of the time spent in those functions. The histogram
394 content can be displayed in the files:
396 trace_stat/function<cpu> ( function0, function1, etc).
400 A directory that holds different tracing stats.
404 Enable dynamic trace points. See kprobetrace.rst.
408 Dynamic trace points stats. See kprobetrace.rst.
412 Used with the function graph tracer. This is the max depth
413 it will trace into a function. Setting this to a value of
414 one will show only the first kernel function that is called
419 This is for tools that read the raw format files. If an event in
420 the ring buffer references a string, only a pointer to the string
421 is recorded into the buffer and not the string itself. This prevents
422 tools from knowing what that string was. This file displays the string
423 and address for the string allowing tools to map the pointers to what
428 Only the pid of the task is recorded in a trace event unless
429 the event specifically saves the task comm as well. Ftrace
430 makes a cache of pid mappings to comms to try to display
431 comms for events. If a pid for a comm is not listed, then
432 "<...>" is displayed in the output.
434 If the option "record-cmd" is set to "0", then comms of tasks
435 will not be saved during recording. By default, it is enabled.
439 By default, 128 comms are saved (see "saved_cmdlines" above). To
440 increase or decrease the amount of comms that are cached, echo
441 the number of comms to cache into this file.
445 If the option "record-tgid" is set, on each scheduling context switch
446 the Task Group ID of a task is saved in a table mapping the PID of
447 the thread to its TGID. By default, the "record-tgid" option is
452 This displays the "snapshot" buffer and also lets the user
453 take a snapshot of the current running trace.
454 See the "Snapshot" section below for more details.
458 When the stack tracer is activated, this will display the
459 maximum stack size it has encountered.
460 See the "Stack Trace" section below.
464 This displays the stack back trace of the largest stack
465 that was encountered when the stack tracer is activated.
466 See the "Stack Trace" section below.
470 This is similar to "set_ftrace_filter" but it limits what
471 functions the stack tracer will check.
475 Whenever an event is recorded into the ring buffer, a
476 "timestamp" is added. This stamp comes from a specified
477 clock. By default, ftrace uses the "local" clock. This
478 clock is very fast and strictly per cpu, but on some
479 systems it may not be monotonic with respect to other
480 CPUs. In other words, the local clocks may not be in sync
481 with local clocks on other CPUs.
483 Usual clocks for tracing::
486 [local] global counter x86-tsc
488 The clock with the square brackets around it is the one in effect.
491 Default clock, but may not be in sync across CPUs
494 This clock is in sync with all CPUs but may
495 be a bit slower than the local clock.
498 This is not a clock at all, but literally an atomic
499 counter. It counts up one by one, but is in sync
500 with all CPUs. This is useful when you need to
501 know exactly the order events occurred with respect to
502 each other on different CPUs.
505 This uses the jiffies counter and the time stamp
506 is relative to the time since boot up.
509 This makes ftrace use the same clock that perf uses.
510 Eventually perf will be able to read ftrace buffers
511 and this will help out in interleaving the data.
514 Architectures may define their own clocks. For
515 example, x86 uses its own TSC cycle clock here.
518 This uses the powerpc timebase register value.
519 This is in sync across CPUs and can also be used
520 to correlate events across hypervisor/guest if
524 This uses the fast monotonic clock (CLOCK_MONOTONIC)
525 which is monotonic and is subject to NTP rate adjustments.
528 This is the raw monotonic clock (CLOCK_MONOTONIC_RAW)
529 which is monotonic but is not subject to any rate adjustments
530 and ticks at the same rate as the hardware clocksource.
533 This is the boot clock (CLOCK_BOOTTIME) and is based on the
534 fast monotonic clock, but also accounts for time spent in
535 suspend. Since the clock access is designed for use in
536 tracing in the suspend path, some side effects are possible
537 if clock is accessed after the suspend time is accounted before
538 the fast mono clock is updated. In this case, the clock update
539 appears to happen slightly sooner than it normally would have.
540 Also on 32-bit systems, it's possible that the 64-bit boot offset
541 sees a partial update. These effects are rare and post
542 processing should be able to handle them. See comments in the
543 ktime_get_boot_fast_ns() function for more information.
546 This is the tai clock (CLOCK_TAI) and is derived from the wall-
547 clock time. However, this clock does not experience
548 discontinuities and backwards jumps caused by NTP inserting leap
549 seconds. Since the clock access is designed for use in tracing,
550 side effects are possible. The clock access may yield wrong
551 readouts in case the internal TAI offset is updated e.g., caused
552 by setting the system time or using adjtimex() with an offset.
553 These effects are rare and post processing should be able to
554 handle them. See comments in the ktime_get_tai_fast_ns()
555 function for more information.
557 To set a clock, simply echo the clock name into this file::
559 # echo global > trace_clock
561 Setting a clock clears the ring buffer content as well as the
566 This is a very useful file for synchronizing user space
567 with events happening in the kernel. Writing strings into
568 this file will be written into the ftrace buffer.
570 It is useful in applications to open this file at the start
571 of the application and just reference the file descriptor
574 void trace_write(const char *fmt, ...)
584 n = vsnprintf(buf, 256, fmt, ap);
587 write(trace_fd, buf, n);
592 trace_fd = open("trace_marker", O_WRONLY);
594 Note: Writing into the trace_marker file can also initiate triggers
595 that are written into /sys/kernel/tracing/events/ftrace/print/trigger
596 See "Event triggers" in Documentation/trace/events.rst and an
597 example in Documentation/trace/histogram.rst (Section 3.)
601 This is similar to trace_marker above, but is meant for binary data
602 to be written to it, where a tool can be used to parse the data
607 Add dynamic tracepoints in programs.
612 Uprobe statistics. See uprobetrace.txt
616 This is a way to make multiple trace buffers where different
617 events can be recorded in different buffers.
618 See "Instances" section below.
622 This is the trace event directory. It holds event tracepoints
623 (also known as static tracepoints) that have been compiled
624 into the kernel. It shows what event tracepoints exist
625 and how they are grouped by system. There are "enable"
626 files at various levels that can enable the tracepoints
627 when a "1" is written to them.
629 See events.rst for more information.
633 By echoing in the event into this file, will enable that event.
635 See events.rst for more information.
639 A list of events that can be enabled in tracing.
641 See events.rst for more information.
645 Certain tracers may change the timestamp mode used when
646 logging trace events into the event buffer. Events with
647 different modes can coexist within a buffer but the mode in
648 effect when an event is logged determines which timestamp mode
649 is used for that event. The default timestamp mode is
652 Usual timestamp modes for tracing:
657 The timestamp mode with the square brackets around it is the
660 delta: Default timestamp mode - timestamp is a delta against
661 a per-buffer timestamp.
663 absolute: The timestamp is a full timestamp, not a delta
664 against some other value. As such it takes up more
665 space and is less efficient.
669 Directory for the Hardware Latency Detector.
670 See "Hardware Latency Detector" section below.
674 This is a directory that contains the trace per_cpu information.
676 per_cpu/cpu0/buffer_size_kb:
678 The ftrace buffer is defined per_cpu. That is, there's a separate
679 buffer for each CPU to allow writes to be done atomically,
680 and free from cache bouncing. These buffers may have different
681 size buffers. This file is similar to the buffer_size_kb
682 file, but it only displays or sets the buffer size for the
683 specific CPU. (here cpu0).
687 This is similar to the "trace" file, but it will only display
688 the data specific for the CPU. If written to, it only clears
689 the specific CPU buffer.
691 per_cpu/cpu0/trace_pipe
693 This is similar to the "trace_pipe" file, and is a consuming
694 read, but it will only display (and consume) the data specific
697 per_cpu/cpu0/trace_pipe_raw
699 For tools that can parse the ftrace ring buffer binary format,
700 the trace_pipe_raw file can be used to extract the data
701 from the ring buffer directly. With the use of the splice()
702 system call, the buffer data can be quickly transferred to
703 a file or to the network where a server is collecting the
706 Like trace_pipe, this is a consuming reader, where multiple
707 reads will always produce different data.
709 per_cpu/cpu0/snapshot:
711 This is similar to the main "snapshot" file, but will only
712 snapshot the current CPU (if supported). It only displays
713 the content of the snapshot for a given CPU, and if
714 written to, only clears this CPU buffer.
716 per_cpu/cpu0/snapshot_raw:
718 Similar to the trace_pipe_raw, but will read the binary format
719 from the snapshot buffer for the given CPU.
723 This displays certain stats about the ring buffer:
726 The number of events that are still in the buffer.
729 The number of lost events due to overwriting when
733 Should always be zero.
734 This gets set if so many events happened within a nested
735 event (ring buffer is re-entrant), that it fills the
736 buffer and starts dropping events.
739 Bytes actually read (not overwritten).
742 The oldest timestamp in the buffer
745 The current timestamp
748 Events lost due to overwrite option being off.
751 The number of events read.
756 Here is the list of current tracers that may be configured.
760 Function call tracer to trace all kernel functions.
764 Similar to the function tracer except that the
765 function tracer probes the functions on their entry
766 whereas the function graph tracer traces on both entry
767 and exit of the functions. It then provides the ability
768 to draw a graph of function calls similar to C code
773 The block tracer. The tracer used by the blktrace user
778 The Hardware Latency tracer is used to detect if the hardware
779 produces any latency. See "Hardware Latency Detector" section
784 Traces the areas that disable interrupts and saves
785 the trace with the longest max latency.
786 See tracing_max_latency. When a new max is recorded,
787 it replaces the old trace. It is best to view this
788 trace with the latency-format option enabled, which
789 happens automatically when the tracer is selected.
793 Similar to irqsoff but traces and records the amount of
794 time for which preemption is disabled.
798 Similar to irqsoff and preemptoff, but traces and
799 records the largest time for which irqs and/or preemption
804 Traces and records the max latency that it takes for
805 the highest priority task to get scheduled after
806 it has been woken up.
807 Traces all tasks as an average developer would expect.
811 Traces and records the max latency that it takes for just
812 RT tasks (as the current "wakeup" does). This is useful
813 for those interested in wake up timings of RT tasks.
817 Traces and records the max latency that it takes for
818 a SCHED_DEADLINE task to be woken (as the "wakeup" and
823 A special tracer that is used to trace binary module.
824 It will trace all the calls that a module makes to the
825 hardware. Everything it writes and reads from the I/O
830 This tracer can be configured when tracing likely/unlikely
831 calls within the kernel. It will trace when a likely and
832 unlikely branch is hit and if it was correct in its prediction
837 This is the "trace nothing" tracer. To remove all
838 tracers from tracing simply echo "nop" into
844 For most ftrace commands, failure modes are obvious and communicated
845 using standard return codes.
847 For other more involved commands, extended error information may be
848 available via the tracing/error_log file. For the commands that
849 support it, reading the tracing/error_log file after an error will
850 display more detailed information about what went wrong, if
851 information is available. The tracing/error_log file is a circular
852 error log displaying a small number (currently, 8) of ftrace errors
853 for the last (8) failed commands.
855 The extended error information and usage takes the form shown in
858 # echo xxx > /sys/kernel/tracing/events/sched/sched_wakeup/trigger
859 echo: write error: Invalid argument
861 # cat /sys/kernel/tracing/error_log
862 [ 5348.887237] location: error: Couldn't yyy: zzz
865 [ 7517.023364] location: error: Bad rrr: sss
869 To clear the error log, echo the empty string into it::
871 # echo > /sys/kernel/tracing/error_log
873 Examples of using the tracer
874 ----------------------------
876 Here are typical examples of using the tracers when controlling
877 them only with the tracefs interface (without using any
878 user-land utilities).
883 Here is an example of the output format of the file "trace"::
887 # entries-in-buffer/entries-written: 140080/250280 #P:4
890 # / _----=> need-resched
891 # | / _---=> hardirq/softirq
892 # || / _--=> preempt-depth
894 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
896 bash-1977 [000] .... 17284.993652: sys_close <-system_call_fastpath
897 bash-1977 [000] .... 17284.993653: __close_fd <-sys_close
898 bash-1977 [000] .... 17284.993653: _raw_spin_lock <-__close_fd
899 sshd-1974 [003] .... 17284.993653: __srcu_read_unlock <-fsnotify
900 bash-1977 [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock
901 bash-1977 [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd
902 bash-1977 [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock
903 bash-1977 [000] .... 17284.993657: filp_close <-__close_fd
904 bash-1977 [000] .... 17284.993657: dnotify_flush <-filp_close
905 sshd-1974 [003] .... 17284.993658: sys_select <-system_call_fastpath
908 A header is printed with the tracer name that is represented by
909 the trace. In this case the tracer is "function". Then it shows the
910 number of events in the buffer as well as the total number of entries
911 that were written. The difference is the number of entries that were
912 lost due to the buffer filling up (250280 - 140080 = 110200 events
915 The header explains the content of the events. Task name "bash", the task
916 PID "1977", the CPU that it was running on "000", the latency format
917 (explained below), the timestamp in <secs>.<usecs> format, the
918 function name that was traced "sys_close" and the parent function that
919 called this function "system_call_fastpath". The timestamp is the time
920 at which the function was entered.
925 When the latency-format option is enabled or when one of the latency
926 tracers is set, the trace file gives somewhat more information to see
927 why a latency happened. Here is a typical trace::
931 # irqsoff latency trace v1.1.5 on 3.8.0-test+
932 # --------------------------------------------------------------------
933 # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
935 # | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0)
937 # => started at: __lock_task_sighand
938 # => ended at: _raw_spin_unlock_irqrestore
942 # / _-----=> irqs-off
943 # | / _----=> need-resched
944 # || / _---=> hardirq/softirq
945 # ||| / _--=> preempt-depth
947 # cmd pid ||||| time | caller
949 ps-6143 2d... 0us!: trace_hardirqs_off <-__lock_task_sighand
950 ps-6143 2d..1 259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore
951 ps-6143 2d..1 263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore
952 ps-6143 2d..1 306us : <stack trace>
953 => trace_hardirqs_on_caller
955 => _raw_spin_unlock_irqrestore
962 => system_call_fastpath
965 This shows that the current tracer is "irqsoff" tracing the time
966 for which interrupts were disabled. It gives the trace version (which
967 never changes) and the version of the kernel upon which this was executed on
968 (3.8). Then it displays the max latency in microseconds (259 us). The number
969 of trace entries displayed and the total number (both are four: #4/4).
970 VP, KP, SP, and HP are always zero and are reserved for later use.
971 #P is the number of online CPUs (#P:4).
973 The task is the process that was running when the latency
974 occurred. (ps pid: 6143).
976 The start and stop (the functions in which the interrupts were
977 disabled and enabled respectively) that caused the latencies:
979 - __lock_task_sighand is where the interrupts were disabled.
980 - _raw_spin_unlock_irqrestore is where they were enabled again.
982 The next lines after the header are the trace itself. The header
983 explains which is which.
985 cmd: The name of the process in the trace.
987 pid: The PID of that process.
989 CPU#: The CPU which the process was running on.
991 irqs-off: 'd' interrupts are disabled. '.' otherwise.
992 .. caution:: If the architecture does not support a way to
993 read the irq flags variable, an 'X' will always
997 - 'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set,
998 - 'n' only TIF_NEED_RESCHED is set,
999 - 'p' only PREEMPT_NEED_RESCHED is set,
1003 - 'Z' - NMI occurred inside a hardirq
1004 - 'z' - NMI is running
1005 - 'H' - hard irq occurred inside a softirq.
1006 - 'h' - hard irq is running
1007 - 's' - soft irq is running
1008 - '.' - normal context.
1010 preempt-depth: The level of preempt_disabled
1012 The above is mostly meaningful for kernel developers.
1015 When the latency-format option is enabled, the trace file
1016 output includes a timestamp relative to the start of the
1017 trace. This differs from the output when latency-format
1018 is disabled, which includes an absolute timestamp.
1021 This is just to help catch your eye a bit better. And
1022 needs to be fixed to be only relative to the same CPU.
1023 The marks are determined by the difference between this
1024 current trace and the next trace.
1026 - '$' - greater than 1 second
1027 - '@' - greater than 100 millisecond
1028 - '*' - greater than 10 millisecond
1029 - '#' - greater than 1000 microsecond
1030 - '!' - greater than 100 microsecond
1031 - '+' - greater than 10 microsecond
1032 - ' ' - less than or equal to 10 microsecond.
1034 The rest is the same as the 'trace' file.
1036 Note, the latency tracers will usually end with a back trace
1037 to easily find where the latency occurred.
1042 The trace_options file (or the options directory) is used to control
1043 what gets printed in the trace output, or manipulate the tracers.
1044 To see what is available, simply cat the file::
1076 To disable one of the options, echo in the option prepended with
1079 echo noprint-parent > trace_options
1081 To enable an option, leave off the "no"::
1083 echo sym-offset > trace_options
1085 Here are the available options:
1088 On function traces, display the calling (parent)
1089 function as well as the function being traced.
1093 bash-4000 [01] 1477.606694: simple_strtoul <-kstrtoul
1096 bash-4000 [01] 1477.606694: simple_strtoul
1100 Display not only the function name, but also the
1101 offset in the function. For example, instead of
1102 seeing just "ktime_get", you will see
1103 "ktime_get+0xb/0x20".
1107 bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0
1110 This will also display the function address as well
1111 as the function name.
1115 bash-4000 [01] 1477.606694: simple_strtoul <c0339346>
1118 This deals with the trace file when the
1119 latency-format option is enabled.
1122 bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
1123 (+0.000ms): simple_strtoul (kstrtoul)
1126 This will display raw numbers. This option is best for
1127 use with user applications that can translate the raw
1128 numbers better than having it done in the kernel.
1131 Similar to raw, but the numbers will be in a hexadecimal format.
1134 This will print out the formats in raw binary.
1137 When set, reading trace_pipe will not block when polled.
1140 Print the fields as described by their types. This is a better
1141 option than using hex, bin or raw, as it gives a better parsing
1142 of the content of the event.
1145 Can disable trace_printk() from writing into the buffer.
1148 It is sometimes confusing when the CPU buffers are full
1149 and one CPU buffer had a lot of events recently, thus
1150 a shorter time frame, were another CPU may have only had
1151 a few events, which lets it have older events. When
1152 the trace is reported, it shows the oldest events first,
1153 and it may look like only one CPU ran (the one with the
1154 oldest events). When the annotate option is set, it will
1155 display when a new CPU buffer started::
1157 <idle>-0 [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on
1158 <idle>-0 [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on
1159 <idle>-0 [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore
1160 ##### CPU 2 buffer started ####
1161 <idle>-0 [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle
1162 <idle>-0 [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog
1163 <idle>-0 [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock
1166 This option changes the trace. It records a
1167 stacktrace of the current user space thread after
1171 when user stacktrace are enabled, look up which
1172 object the address belongs to, and print a
1173 relative address. This is especially useful when
1174 ASLR is on, otherwise you don't get a chance to
1175 resolve the address to object/file/line after
1176 the app is no longer running
1178 The lookup is performed when you read
1179 trace,trace_pipe. Example::
1181 a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
1182 x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
1186 When set, trace_printk()s will only show the format
1187 and not their parameters (if trace_bprintk() or
1188 trace_bputs() was used to save the trace_printk()).
1191 Show only the event data. Hides the comm, PID,
1192 timestamp, CPU, and other useful data.
1195 This option changes the trace output. When it is enabled,
1196 the trace displays additional information about the
1197 latency, as described in "Latency trace format".
1200 When set, opening the trace file for read, will pause
1201 writing to the ring buffer (as if tracing_on was set to zero).
1202 This simulates the original behavior of the trace file.
1203 When the file is closed, tracing will be enabled again.
1206 When set, "%p" in the event printk format displays the
1207 hashed pointer value instead of real address.
1208 This will be useful if you want to find out which hashed
1209 value is corresponding to the real value in trace log.
1212 When any event or tracer is enabled, a hook is enabled
1213 in the sched_switch trace point to fill comm cache
1214 with mapped pids and comms. But this may cause some
1215 overhead, and if you only care about pids, and not the
1216 name of the task, disabling this option can lower the
1217 impact of tracing. See "saved_cmdlines".
1220 When any event or tracer is enabled, a hook is enabled
1221 in the sched_switch trace point to fill the cache of
1222 mapped Thread Group IDs (TGID) mapping to pids. See
1226 This controls what happens when the trace buffer is
1227 full. If "1" (default), the oldest events are
1228 discarded and overwritten. If "0", then the newest
1229 events are discarded.
1230 (see per_cpu/cpu0/stats for overrun and dropped)
1233 When the free_buffer is closed, tracing will
1234 stop (tracing_on set to 0).
1237 Shows the interrupt, preempt count, need resched data.
1238 When disabled, the trace looks like::
1242 # entries-in-buffer/entries-written: 144405/9452052 #P:4
1244 # TASK-PID CPU# TIMESTAMP FUNCTION
1246 <idle>-0 [002] 23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up
1247 <idle>-0 [002] 23636.756054: activate_task <-ttwu_do_activate.constprop.89
1248 <idle>-0 [002] 23636.756055: enqueue_task <-activate_task
1252 When set, the trace_marker is writable (only by root).
1253 When disabled, the trace_marker will error with EINVAL
1257 When set, tasks with PIDs listed in set_event_pid will have
1258 the PIDs of their children added to set_event_pid when those
1259 tasks fork. Also, when tasks with PIDs in set_event_pid exit,
1260 their PIDs will be removed from the file.
1262 This affects PIDs listed in set_event_notrace_pid as well.
1265 The latency tracers will enable function tracing
1266 if this option is enabled (default it is). When
1267 it is disabled, the latency tracers do not trace
1268 functions. This keeps the overhead of the tracer down
1269 when performing latency tests.
1272 When set, tasks with PIDs listed in set_ftrace_pid will
1273 have the PIDs of their children added to set_ftrace_pid
1274 when those tasks fork. Also, when tasks with PIDs in
1275 set_ftrace_pid exit, their PIDs will be removed from the
1278 This affects PIDs in set_ftrace_notrace_pid as well.
1281 When set, the latency tracers (irqsoff, wakeup, etc) will
1282 use function graph tracing instead of function tracing.
1285 When set, a stack trace is recorded after any trace event
1289 Enable branch tracing with the tracer. This enables branch
1290 tracer along with the currently set tracer. Enabling this
1291 with the "nop" tracer is the same as just enabling the
1294 .. tip:: Some tracers have their own options. They only appear in this
1295 file when the tracer is active. They always appear in the
1299 Here are the per tracer options:
1301 Options for function tracer:
1304 When set, a stack trace is recorded after every
1305 function that is recorded. NOTE! Limit the functions
1306 that are recorded before enabling this, with
1307 "set_ftrace_filter" otherwise the system performance
1308 will be critically degraded. Remember to disable
1309 this option before clearing the function filter.
1311 Options for function_graph tracer:
1313 Since the function_graph tracer has a slightly different output
1314 it has its own options to control what is displayed.
1317 When set, the "overrun" of the graph stack is
1318 displayed after each function traced. The
1319 overrun, is when the stack depth of the calls
1320 is greater than what is reserved for each task.
1321 Each task has a fixed array of functions to
1322 trace in the call graph. If the depth of the
1323 calls exceeds that, the function is not traced.
1324 The overrun is the number of functions missed
1325 due to exceeding this array.
1328 When set, the CPU number of the CPU where the trace
1329 occurred is displayed.
1332 When set, if the function takes longer than
1333 A certain amount, then a delay marker is
1334 displayed. See "delay" above, under the
1338 Unlike other tracers, the process' command line
1339 is not displayed by default, but instead only
1340 when a task is traced in and out during a context
1341 switch. Enabling this options has the command
1342 of each process displayed at every line.
1345 At the end of each function (the return)
1346 the duration of the amount of time in the
1347 function is displayed in microseconds.
1350 When set, the timestamp is displayed at each line.
1353 When disabled, functions that happen inside an
1354 interrupt will not be traced.
1357 When set, the return event will include the function
1358 that it represents. By default this is off, and
1359 only a closing curly bracket "}" is displayed for
1360 the return of a function.
1363 When running function graph tracer, to include
1364 the time a task schedules out in its function.
1365 When enabled, it will account time the task has been
1366 scheduled out as part of the function call.
1369 When running function profiler with function graph tracer,
1370 to include the time to call nested functions. When this is
1371 not set, the time reported for the function will only
1372 include the time the function itself executed for, not the
1373 time for functions that it called.
1375 Options for blk tracer:
1378 Shows a more minimalistic output.
1384 When interrupts are disabled, the CPU can not react to any other
1385 external event (besides NMIs and SMIs). This prevents the timer
1386 interrupt from triggering or the mouse interrupt from letting
1387 the kernel know of a new mouse event. The result is a latency
1388 with the reaction time.
1390 The irqsoff tracer tracks the time for which interrupts are
1391 disabled. When a new maximum latency is hit, the tracer saves
1392 the trace leading up to that latency point so that every time a
1393 new maximum is reached, the old saved trace is discarded and the
1396 To reset the maximum, echo 0 into tracing_max_latency. Here is
1399 # echo 0 > options/function-trace
1400 # echo irqsoff > current_tracer
1401 # echo 1 > tracing_on
1402 # echo 0 > tracing_max_latency
1405 # echo 0 > tracing_on
1409 # irqsoff latency trace v1.1.5 on 3.8.0-test+
1410 # --------------------------------------------------------------------
1411 # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1413 # | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0)
1415 # => started at: run_timer_softirq
1416 # => ended at: run_timer_softirq
1420 # / _-----=> irqs-off
1421 # | / _----=> need-resched
1422 # || / _---=> hardirq/softirq
1423 # ||| / _--=> preempt-depth
1425 # cmd pid ||||| time | caller
1427 <idle>-0 0d.s2 0us+: _raw_spin_lock_irq <-run_timer_softirq
1428 <idle>-0 0dNs3 17us : _raw_spin_unlock_irq <-run_timer_softirq
1429 <idle>-0 0dNs3 17us+: trace_hardirqs_on <-run_timer_softirq
1430 <idle>-0 0dNs3 25us : <stack trace>
1431 => _raw_spin_unlock_irq
1432 => run_timer_softirq
1437 => smp_apic_timer_interrupt
1438 => apic_timer_interrupt
1443 => x86_64_start_reservations
1444 => x86_64_start_kernel
1446 Here we see that we had a latency of 16 microseconds (which is
1447 very good). The _raw_spin_lock_irq in run_timer_softirq disabled
1448 interrupts. The difference between the 16 and the displayed
1449 timestamp 25us occurred because the clock was incremented
1450 between the time of recording the max latency and the time of
1451 recording the function that had that latency.
1453 Note the above example had function-trace not set. If we set
1454 function-trace, we get a much larger output::
1456 with echo 1 > options/function-trace
1460 # irqsoff latency trace v1.1.5 on 3.8.0-test+
1461 # --------------------------------------------------------------------
1462 # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1464 # | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0)
1466 # => started at: ata_scsi_queuecmd
1467 # => ended at: ata_scsi_queuecmd
1471 # / _-----=> irqs-off
1472 # | / _----=> need-resched
1473 # || / _---=> hardirq/softirq
1474 # ||| / _--=> preempt-depth
1476 # cmd pid ||||| time | caller
1478 bash-2042 3d... 0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd
1479 bash-2042 3d... 0us : add_preempt_count <-_raw_spin_lock_irqsave
1480 bash-2042 3d..1 1us : ata_scsi_find_dev <-ata_scsi_queuecmd
1481 bash-2042 3d..1 1us : __ata_scsi_find_dev <-ata_scsi_find_dev
1482 bash-2042 3d..1 2us : ata_find_dev.part.14 <-__ata_scsi_find_dev
1483 bash-2042 3d..1 2us : ata_qc_new_init <-__ata_scsi_queuecmd
1484 bash-2042 3d..1 3us : ata_sg_init <-__ata_scsi_queuecmd
1485 bash-2042 3d..1 4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd
1486 bash-2042 3d..1 4us : ata_build_rw_tf <-ata_scsi_rw_xlat
1488 bash-2042 3d..1 67us : delay_tsc <-__delay
1489 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc
1490 bash-2042 3d..2 67us : sub_preempt_count <-delay_tsc
1491 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc
1492 bash-2042 3d..2 68us : sub_preempt_count <-delay_tsc
1493 bash-2042 3d..1 68us+: ata_bmdma_start <-ata_bmdma_qc_issue
1494 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1495 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1496 bash-2042 3d..1 72us+: trace_hardirqs_on <-ata_scsi_queuecmd
1497 bash-2042 3d..1 120us : <stack trace>
1498 => _raw_spin_unlock_irqrestore
1499 => ata_scsi_queuecmd
1500 => scsi_dispatch_cmd
1502 => __blk_run_queue_uncond
1505 => submit_bio_noacct
1508 => __ext3_get_inode_loc
1517 => user_path_at_empty
1522 => system_call_fastpath
1525 Here we traced a 71 microsecond latency. But we also see all the
1526 functions that were called during that time. Note that by
1527 enabling function tracing, we incur an added overhead. This
1528 overhead may extend the latency times. But nevertheless, this
1529 trace has provided some very helpful debugging information.
1531 If we prefer function graph output instead of function, we can set
1532 display-graph option::
1534 with echo 1 > options/display-graph
1538 # irqsoff latency trace v1.1.5 on 4.20.0-rc6+
1539 # --------------------------------------------------------------------
1540 # latency: 3751 us, #274/274, CPU#0 | (M:desktop VP:0, KP:0, SP:0 HP:0 #P:4)
1542 # | task: bash-1507 (uid:0 nice:0 policy:0 rt_prio:0)
1544 # => started at: free_debug_processing
1545 # => ended at: return_to_handler
1549 # / _----=> need-resched
1550 # | / _---=> hardirq/softirq
1551 # || / _--=> preempt-depth
1553 # REL TIME CPU TASK/PID |||| DURATION FUNCTION CALLS
1554 # | | | | |||| | | | | | |
1555 0 us | 0) bash-1507 | d... | 0.000 us | _raw_spin_lock_irqsave();
1556 0 us | 0) bash-1507 | d..1 | 0.378 us | do_raw_spin_trylock();
1557 1 us | 0) bash-1507 | d..2 | | set_track() {
1558 2 us | 0) bash-1507 | d..2 | | save_stack_trace() {
1559 2 us | 0) bash-1507 | d..2 | | __save_stack_trace() {
1560 3 us | 0) bash-1507 | d..2 | | __unwind_start() {
1561 3 us | 0) bash-1507 | d..2 | | get_stack_info() {
1562 3 us | 0) bash-1507 | d..2 | 0.351 us | in_task_stack();
1563 4 us | 0) bash-1507 | d..2 | 1.107 us | }
1565 3750 us | 0) bash-1507 | d..1 | 0.516 us | do_raw_spin_unlock();
1566 3750 us | 0) bash-1507 | d..1 | 0.000 us | _raw_spin_unlock_irqrestore();
1567 3764 us | 0) bash-1507 | d..1 | 0.000 us | tracer_hardirqs_on();
1568 bash-1507 0d..1 3792us : <stack trace>
1569 => free_debug_processing
1578 => search_binary_handler
1579 => __do_execve_file.isra.32
1582 => entry_SYSCALL_64_after_hwframe
1587 When preemption is disabled, we may be able to receive
1588 interrupts but the task cannot be preempted and a higher
1589 priority task must wait for preemption to be enabled again
1590 before it can preempt a lower priority task.
1592 The preemptoff tracer traces the places that disable preemption.
1593 Like the irqsoff tracer, it records the maximum latency for
1594 which preemption was disabled. The control of preemptoff tracer
1595 is much like the irqsoff tracer.
1598 # echo 0 > options/function-trace
1599 # echo preemptoff > current_tracer
1600 # echo 1 > tracing_on
1601 # echo 0 > tracing_max_latency
1604 # echo 0 > tracing_on
1606 # tracer: preemptoff
1608 # preemptoff latency trace v1.1.5 on 3.8.0-test+
1609 # --------------------------------------------------------------------
1610 # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1612 # | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0)
1614 # => started at: do_IRQ
1615 # => ended at: do_IRQ
1619 # / _-----=> irqs-off
1620 # | / _----=> need-resched
1621 # || / _---=> hardirq/softirq
1622 # ||| / _--=> preempt-depth
1624 # cmd pid ||||| time | caller
1626 sshd-1991 1d.h. 0us+: irq_enter <-do_IRQ
1627 sshd-1991 1d..1 46us : irq_exit <-do_IRQ
1628 sshd-1991 1d..1 47us+: trace_preempt_on <-do_IRQ
1629 sshd-1991 1d..1 52us : <stack trace>
1630 => sub_preempt_count
1636 This has some more changes. Preemption was disabled when an
1637 interrupt came in (notice the 'h'), and was enabled on exit.
1638 But we also see that interrupts have been disabled when entering
1639 the preempt off section and leaving it (the 'd'). We do not know if
1640 interrupts were enabled in the mean time or shortly after this
1644 # tracer: preemptoff
1646 # preemptoff latency trace v1.1.5 on 3.8.0-test+
1647 # --------------------------------------------------------------------
1648 # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1650 # | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0)
1652 # => started at: wake_up_new_task
1653 # => ended at: task_rq_unlock
1657 # / _-----=> irqs-off
1658 # | / _----=> need-resched
1659 # || / _---=> hardirq/softirq
1660 # ||| / _--=> preempt-depth
1662 # cmd pid ||||| time | caller
1664 bash-1994 1d..1 0us : _raw_spin_lock_irqsave <-wake_up_new_task
1665 bash-1994 1d..1 0us : select_task_rq_fair <-select_task_rq
1666 bash-1994 1d..1 1us : __rcu_read_lock <-select_task_rq_fair
1667 bash-1994 1d..1 1us : source_load <-select_task_rq_fair
1668 bash-1994 1d..1 1us : source_load <-select_task_rq_fair
1670 bash-1994 1d..1 12us : irq_enter <-smp_apic_timer_interrupt
1671 bash-1994 1d..1 12us : rcu_irq_enter <-irq_enter
1672 bash-1994 1d..1 13us : add_preempt_count <-irq_enter
1673 bash-1994 1d.h1 13us : exit_idle <-smp_apic_timer_interrupt
1674 bash-1994 1d.h1 13us : hrtimer_interrupt <-smp_apic_timer_interrupt
1675 bash-1994 1d.h1 13us : _raw_spin_lock <-hrtimer_interrupt
1676 bash-1994 1d.h1 14us : add_preempt_count <-_raw_spin_lock
1677 bash-1994 1d.h2 14us : ktime_get_update_offsets <-hrtimer_interrupt
1679 bash-1994 1d.h1 35us : lapic_next_event <-clockevents_program_event
1680 bash-1994 1d.h1 35us : irq_exit <-smp_apic_timer_interrupt
1681 bash-1994 1d.h1 36us : sub_preempt_count <-irq_exit
1682 bash-1994 1d..2 36us : do_softirq <-irq_exit
1683 bash-1994 1d..2 36us : __do_softirq <-call_softirq
1684 bash-1994 1d..2 36us : __local_bh_disable <-__do_softirq
1685 bash-1994 1d.s2 37us : add_preempt_count <-_raw_spin_lock_irq
1686 bash-1994 1d.s3 38us : _raw_spin_unlock <-run_timer_softirq
1687 bash-1994 1d.s3 39us : sub_preempt_count <-_raw_spin_unlock
1688 bash-1994 1d.s2 39us : call_timer_fn <-run_timer_softirq
1690 bash-1994 1dNs2 81us : cpu_needs_another_gp <-rcu_process_callbacks
1691 bash-1994 1dNs2 82us : __local_bh_enable <-__do_softirq
1692 bash-1994 1dNs2 82us : sub_preempt_count <-__local_bh_enable
1693 bash-1994 1dN.2 82us : idle_cpu <-irq_exit
1694 bash-1994 1dN.2 83us : rcu_irq_exit <-irq_exit
1695 bash-1994 1dN.2 83us : sub_preempt_count <-irq_exit
1696 bash-1994 1.N.1 84us : _raw_spin_unlock_irqrestore <-task_rq_unlock
1697 bash-1994 1.N.1 84us+: trace_preempt_on <-task_rq_unlock
1698 bash-1994 1.N.1 104us : <stack trace>
1699 => sub_preempt_count
1700 => _raw_spin_unlock_irqrestore
1708 The above is an example of the preemptoff trace with
1709 function-trace set. Here we see that interrupts were not disabled
1710 the entire time. The irq_enter code lets us know that we entered
1711 an interrupt 'h'. Before that, the functions being traced still
1712 show that it is not in an interrupt, but we can see from the
1713 functions themselves that this is not the case.
1718 Knowing the locations that have interrupts disabled or
1719 preemption disabled for the longest times is helpful. But
1720 sometimes we would like to know when either preemption and/or
1721 interrupts are disabled.
1723 Consider the following code::
1725 local_irq_disable();
1726 call_function_with_irqs_off();
1728 call_function_with_irqs_and_preemption_off();
1730 call_function_with_preemption_off();
1733 The irqsoff tracer will record the total length of
1734 call_function_with_irqs_off() and
1735 call_function_with_irqs_and_preemption_off().
1737 The preemptoff tracer will record the total length of
1738 call_function_with_irqs_and_preemption_off() and
1739 call_function_with_preemption_off().
1741 But neither will trace the time that interrupts and/or
1742 preemption is disabled. This total time is the time that we can
1743 not schedule. To record this time, use the preemptirqsoff
1746 Again, using this trace is much like the irqsoff and preemptoff
1750 # echo 0 > options/function-trace
1751 # echo preemptirqsoff > current_tracer
1752 # echo 1 > tracing_on
1753 # echo 0 > tracing_max_latency
1756 # echo 0 > tracing_on
1758 # tracer: preemptirqsoff
1760 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1761 # --------------------------------------------------------------------
1762 # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1764 # | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0)
1766 # => started at: ata_scsi_queuecmd
1767 # => ended at: ata_scsi_queuecmd
1771 # / _-----=> irqs-off
1772 # | / _----=> need-resched
1773 # || / _---=> hardirq/softirq
1774 # ||| / _--=> preempt-depth
1776 # cmd pid ||||| time | caller
1778 ls-2230 3d... 0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd
1779 ls-2230 3...1 100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1780 ls-2230 3...1 101us+: trace_preempt_on <-ata_scsi_queuecmd
1781 ls-2230 3...1 111us : <stack trace>
1782 => sub_preempt_count
1783 => _raw_spin_unlock_irqrestore
1784 => ata_scsi_queuecmd
1785 => scsi_dispatch_cmd
1787 => __blk_run_queue_uncond
1790 => submit_bio_noacct
1795 => htree_dirblock_to_tree
1796 => ext3_htree_fill_tree
1800 => system_call_fastpath
1803 The trace_hardirqs_off_thunk is called from assembly on x86 when
1804 interrupts are disabled in the assembly code. Without the
1805 function tracing, we do not know if interrupts were enabled
1806 within the preemption points. We do see that it started with
1809 Here is a trace with function-trace set::
1811 # tracer: preemptirqsoff
1813 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1814 # --------------------------------------------------------------------
1815 # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1817 # | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0)
1819 # => started at: schedule
1820 # => ended at: mutex_unlock
1824 # / _-----=> irqs-off
1825 # | / _----=> need-resched
1826 # || / _---=> hardirq/softirq
1827 # ||| / _--=> preempt-depth
1829 # cmd pid ||||| time | caller
1831 kworker/-59 3...1 0us : __schedule <-schedule
1832 kworker/-59 3d..1 0us : rcu_preempt_qs <-rcu_note_context_switch
1833 kworker/-59 3d..1 1us : add_preempt_count <-_raw_spin_lock_irq
1834 kworker/-59 3d..2 1us : deactivate_task <-__schedule
1835 kworker/-59 3d..2 1us : dequeue_task <-deactivate_task
1836 kworker/-59 3d..2 2us : update_rq_clock <-dequeue_task
1837 kworker/-59 3d..2 2us : dequeue_task_fair <-dequeue_task
1838 kworker/-59 3d..2 2us : update_curr <-dequeue_task_fair
1839 kworker/-59 3d..2 2us : update_min_vruntime <-update_curr
1840 kworker/-59 3d..2 3us : cpuacct_charge <-update_curr
1841 kworker/-59 3d..2 3us : __rcu_read_lock <-cpuacct_charge
1842 kworker/-59 3d..2 3us : __rcu_read_unlock <-cpuacct_charge
1843 kworker/-59 3d..2 3us : update_cfs_rq_blocked_load <-dequeue_task_fair
1844 kworker/-59 3d..2 4us : clear_buddies <-dequeue_task_fair
1845 kworker/-59 3d..2 4us : account_entity_dequeue <-dequeue_task_fair
1846 kworker/-59 3d..2 4us : update_min_vruntime <-dequeue_task_fair
1847 kworker/-59 3d..2 4us : update_cfs_shares <-dequeue_task_fair
1848 kworker/-59 3d..2 5us : hrtick_update <-dequeue_task_fair
1849 kworker/-59 3d..2 5us : wq_worker_sleeping <-__schedule
1850 kworker/-59 3d..2 5us : kthread_data <-wq_worker_sleeping
1851 kworker/-59 3d..2 5us : put_prev_task_fair <-__schedule
1852 kworker/-59 3d..2 6us : pick_next_task_fair <-pick_next_task
1853 kworker/-59 3d..2 6us : clear_buddies <-pick_next_task_fair
1854 kworker/-59 3d..2 6us : set_next_entity <-pick_next_task_fair
1855 kworker/-59 3d..2 6us : update_stats_wait_end <-set_next_entity
1856 ls-2269 3d..2 7us : finish_task_switch <-__schedule
1857 ls-2269 3d..2 7us : _raw_spin_unlock_irq <-finish_task_switch
1858 ls-2269 3d..2 8us : do_IRQ <-ret_from_intr
1859 ls-2269 3d..2 8us : irq_enter <-do_IRQ
1860 ls-2269 3d..2 8us : rcu_irq_enter <-irq_enter
1861 ls-2269 3d..2 9us : add_preempt_count <-irq_enter
1862 ls-2269 3d.h2 9us : exit_idle <-do_IRQ
1864 ls-2269 3d.h3 20us : sub_preempt_count <-_raw_spin_unlock
1865 ls-2269 3d.h2 20us : irq_exit <-do_IRQ
1866 ls-2269 3d.h2 21us : sub_preempt_count <-irq_exit
1867 ls-2269 3d..3 21us : do_softirq <-irq_exit
1868 ls-2269 3d..3 21us : __do_softirq <-call_softirq
1869 ls-2269 3d..3 21us+: __local_bh_disable <-__do_softirq
1870 ls-2269 3d.s4 29us : sub_preempt_count <-_local_bh_enable_ip
1871 ls-2269 3d.s5 29us : sub_preempt_count <-_local_bh_enable_ip
1872 ls-2269 3d.s5 31us : do_IRQ <-ret_from_intr
1873 ls-2269 3d.s5 31us : irq_enter <-do_IRQ
1874 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter
1876 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter
1877 ls-2269 3d.s5 32us : add_preempt_count <-irq_enter
1878 ls-2269 3d.H5 32us : exit_idle <-do_IRQ
1879 ls-2269 3d.H5 32us : handle_irq <-do_IRQ
1880 ls-2269 3d.H5 32us : irq_to_desc <-handle_irq
1881 ls-2269 3d.H5 33us : handle_fasteoi_irq <-handle_irq
1883 ls-2269 3d.s5 158us : _raw_spin_unlock_irqrestore <-rtl8139_poll
1884 ls-2269 3d.s3 158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action
1885 ls-2269 3d.s3 159us : __local_bh_enable <-__do_softirq
1886 ls-2269 3d.s3 159us : sub_preempt_count <-__local_bh_enable
1887 ls-2269 3d..3 159us : idle_cpu <-irq_exit
1888 ls-2269 3d..3 159us : rcu_irq_exit <-irq_exit
1889 ls-2269 3d..3 160us : sub_preempt_count <-irq_exit
1890 ls-2269 3d... 161us : __mutex_unlock_slowpath <-mutex_unlock
1891 ls-2269 3d... 162us+: trace_hardirqs_on <-mutex_unlock
1892 ls-2269 3d... 186us : <stack trace>
1893 => __mutex_unlock_slowpath
1900 => system_call_fastpath
1902 This is an interesting trace. It started with kworker running and
1903 scheduling out and ls taking over. But as soon as ls released the
1904 rq lock and enabled interrupts (but not preemption) an interrupt
1905 triggered. When the interrupt finished, it started running softirqs.
1906 But while the softirq was running, another interrupt triggered.
1907 When an interrupt is running inside a softirq, the annotation is 'H'.
1913 One common case that people are interested in tracing is the
1914 time it takes for a task that is woken to actually wake up.
1915 Now for non Real-Time tasks, this can be arbitrary. But tracing
1916 it none the less can be interesting.
1918 Without function tracing::
1920 # echo 0 > options/function-trace
1921 # echo wakeup > current_tracer
1922 # echo 1 > tracing_on
1923 # echo 0 > tracing_max_latency
1925 # echo 0 > tracing_on
1929 # wakeup latency trace v1.1.5 on 3.8.0-test+
1930 # --------------------------------------------------------------------
1931 # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1933 # | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0)
1937 # / _-----=> irqs-off
1938 # | / _----=> need-resched
1939 # || / _---=> hardirq/softirq
1940 # ||| / _--=> preempt-depth
1942 # cmd pid ||||| time | caller
1944 <idle>-0 3dNs7 0us : 0:120:R + [003] 312:100:R kworker/3:1H
1945 <idle>-0 3dNs7 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
1946 <idle>-0 3d..3 15us : __schedule <-schedule
1947 <idle>-0 3d..3 15us : 0:120:R ==> [003] 312:100:R kworker/3:1H
1949 The tracer only traces the highest priority task in the system
1950 to avoid tracing the normal circumstances. Here we see that
1951 the kworker with a nice priority of -20 (not very nice), took
1952 just 15 microseconds from the time it woke up, to the time it
1955 Non Real-Time tasks are not that interesting. A more interesting
1956 trace is to concentrate only on Real-Time tasks.
1961 In a Real-Time environment it is very important to know the
1962 wakeup time it takes for the highest priority task that is woken
1963 up to the time that it executes. This is also known as "schedule
1964 latency". I stress the point that this is about RT tasks. It is
1965 also important to know the scheduling latency of non-RT tasks,
1966 but the average schedule latency is better for non-RT tasks.
1967 Tools like LatencyTop are more appropriate for such
1970 Real-Time environments are interested in the worst case latency.
1971 That is the longest latency it takes for something to happen,
1972 and not the average. We can have a very fast scheduler that may
1973 only have a large latency once in a while, but that would not
1974 work well with Real-Time tasks. The wakeup_rt tracer was designed
1975 to record the worst case wakeups of RT tasks. Non-RT tasks are
1976 not recorded because the tracer only records one worst case and
1977 tracing non-RT tasks that are unpredictable will overwrite the
1978 worst case latency of RT tasks (just run the normal wakeup
1979 tracer for a while to see that effect).
1981 Since this tracer only deals with RT tasks, we will run this
1982 slightly differently than we did with the previous tracers.
1983 Instead of performing an 'ls', we will run 'sleep 1' under
1984 'chrt' which changes the priority of the task.
1987 # echo 0 > options/function-trace
1988 # echo wakeup_rt > current_tracer
1989 # echo 1 > tracing_on
1990 # echo 0 > tracing_max_latency
1992 # echo 0 > tracing_on
1998 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1999 # --------------------------------------------------------------------
2000 # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
2002 # | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5)
2006 # / _-----=> irqs-off
2007 # | / _----=> need-resched
2008 # || / _---=> hardirq/softirq
2009 # ||| / _--=> preempt-depth
2011 # cmd pid ||||| time | caller
2013 <idle>-0 3d.h4 0us : 0:120:R + [003] 2389: 94:R sleep
2014 <idle>-0 3d.h4 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
2015 <idle>-0 3d..3 5us : __schedule <-schedule
2016 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep
2019 Running this on an idle system, we see that it only took 5 microseconds
2020 to perform the task switch. Note, since the trace point in the schedule
2021 is before the actual "switch", we stop the tracing when the recorded task
2022 is about to schedule in. This may change if we add a new marker at the
2023 end of the scheduler.
2025 Notice that the recorded task is 'sleep' with the PID of 2389
2026 and it has an rt_prio of 5. This priority is user-space priority
2027 and not the internal kernel priority. The policy is 1 for
2028 SCHED_FIFO and 2 for SCHED_RR.
2030 Note, that the trace data shows the internal priority (99 - rtprio).
2033 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep
2035 The 0:120:R means idle was running with a nice priority of 0 (120 - 120)
2036 and in the running state 'R'. The sleep task was scheduled in with
2037 2389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94)
2038 and it too is in the running state.
2040 Doing the same with chrt -r 5 and function-trace set.
2043 echo 1 > options/function-trace
2047 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
2048 # --------------------------------------------------------------------
2049 # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
2051 # | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5)
2055 # / _-----=> irqs-off
2056 # | / _----=> need-resched
2057 # || / _---=> hardirq/softirq
2058 # ||| / _--=> preempt-depth
2060 # cmd pid ||||| time | caller
2062 <idle>-0 3d.h4 1us+: 0:120:R + [003] 2448: 94:R sleep
2063 <idle>-0 3d.h4 2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
2064 <idle>-0 3d.h3 3us : check_preempt_curr <-ttwu_do_wakeup
2065 <idle>-0 3d.h3 3us : resched_curr <-check_preempt_curr
2066 <idle>-0 3dNh3 4us : task_woken_rt <-ttwu_do_wakeup
2067 <idle>-0 3dNh3 4us : _raw_spin_unlock <-try_to_wake_up
2068 <idle>-0 3dNh3 4us : sub_preempt_count <-_raw_spin_unlock
2069 <idle>-0 3dNh2 5us : ttwu_stat <-try_to_wake_up
2070 <idle>-0 3dNh2 5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
2071 <idle>-0 3dNh2 6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
2072 <idle>-0 3dNh1 6us : _raw_spin_lock <-__run_hrtimer
2073 <idle>-0 3dNh1 6us : add_preempt_count <-_raw_spin_lock
2074 <idle>-0 3dNh2 7us : _raw_spin_unlock <-hrtimer_interrupt
2075 <idle>-0 3dNh2 7us : sub_preempt_count <-_raw_spin_unlock
2076 <idle>-0 3dNh1 7us : tick_program_event <-hrtimer_interrupt
2077 <idle>-0 3dNh1 7us : clockevents_program_event <-tick_program_event
2078 <idle>-0 3dNh1 8us : ktime_get <-clockevents_program_event
2079 <idle>-0 3dNh1 8us : lapic_next_event <-clockevents_program_event
2080 <idle>-0 3dNh1 8us : irq_exit <-smp_apic_timer_interrupt
2081 <idle>-0 3dNh1 9us : sub_preempt_count <-irq_exit
2082 <idle>-0 3dN.2 9us : idle_cpu <-irq_exit
2083 <idle>-0 3dN.2 9us : rcu_irq_exit <-irq_exit
2084 <idle>-0 3dN.2 10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
2085 <idle>-0 3dN.2 10us : sub_preempt_count <-irq_exit
2086 <idle>-0 3.N.1 11us : rcu_idle_exit <-cpu_idle
2087 <idle>-0 3dN.1 11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
2088 <idle>-0 3.N.1 11us : tick_nohz_idle_exit <-cpu_idle
2089 <idle>-0 3dN.1 12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
2090 <idle>-0 3dN.1 12us : ktime_get <-tick_nohz_idle_exit
2091 <idle>-0 3dN.1 12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
2092 <idle>-0 3dN.1 13us : cpu_load_update_nohz <-tick_nohz_idle_exit
2093 <idle>-0 3dN.1 13us : _raw_spin_lock <-cpu_load_update_nohz
2094 <idle>-0 3dN.1 13us : add_preempt_count <-_raw_spin_lock
2095 <idle>-0 3dN.2 13us : __cpu_load_update <-cpu_load_update_nohz
2096 <idle>-0 3dN.2 14us : sched_avg_update <-__cpu_load_update
2097 <idle>-0 3dN.2 14us : _raw_spin_unlock <-cpu_load_update_nohz
2098 <idle>-0 3dN.2 14us : sub_preempt_count <-_raw_spin_unlock
2099 <idle>-0 3dN.1 15us : calc_load_nohz_stop <-tick_nohz_idle_exit
2100 <idle>-0 3dN.1 15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
2101 <idle>-0 3dN.1 15us : hrtimer_cancel <-tick_nohz_idle_exit
2102 <idle>-0 3dN.1 15us : hrtimer_try_to_cancel <-hrtimer_cancel
2103 <idle>-0 3dN.1 16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
2104 <idle>-0 3dN.1 16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
2105 <idle>-0 3dN.1 16us : add_preempt_count <-_raw_spin_lock_irqsave
2106 <idle>-0 3dN.2 17us : __remove_hrtimer <-remove_hrtimer.part.16
2107 <idle>-0 3dN.2 17us : hrtimer_force_reprogram <-__remove_hrtimer
2108 <idle>-0 3dN.2 17us : tick_program_event <-hrtimer_force_reprogram
2109 <idle>-0 3dN.2 18us : clockevents_program_event <-tick_program_event
2110 <idle>-0 3dN.2 18us : ktime_get <-clockevents_program_event
2111 <idle>-0 3dN.2 18us : lapic_next_event <-clockevents_program_event
2112 <idle>-0 3dN.2 19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
2113 <idle>-0 3dN.2 19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
2114 <idle>-0 3dN.1 19us : hrtimer_forward <-tick_nohz_idle_exit
2115 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward
2116 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward
2117 <idle>-0 3dN.1 20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
2118 <idle>-0 3dN.1 20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
2119 <idle>-0 3dN.1 21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
2120 <idle>-0 3dN.1 21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
2121 <idle>-0 3dN.1 21us : add_preempt_count <-_raw_spin_lock_irqsave
2122 <idle>-0 3dN.2 22us : ktime_add_safe <-__hrtimer_start_range_ns
2123 <idle>-0 3dN.2 22us : enqueue_hrtimer <-__hrtimer_start_range_ns
2124 <idle>-0 3dN.2 22us : tick_program_event <-__hrtimer_start_range_ns
2125 <idle>-0 3dN.2 23us : clockevents_program_event <-tick_program_event
2126 <idle>-0 3dN.2 23us : ktime_get <-clockevents_program_event
2127 <idle>-0 3dN.2 23us : lapic_next_event <-clockevents_program_event
2128 <idle>-0 3dN.2 24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
2129 <idle>-0 3dN.2 24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
2130 <idle>-0 3dN.1 24us : account_idle_ticks <-tick_nohz_idle_exit
2131 <idle>-0 3dN.1 24us : account_idle_time <-account_idle_ticks
2132 <idle>-0 3.N.1 25us : sub_preempt_count <-cpu_idle
2133 <idle>-0 3.N.. 25us : schedule <-cpu_idle
2134 <idle>-0 3.N.. 25us : __schedule <-preempt_schedule
2135 <idle>-0 3.N.. 26us : add_preempt_count <-__schedule
2136 <idle>-0 3.N.1 26us : rcu_note_context_switch <-__schedule
2137 <idle>-0 3.N.1 26us : rcu_sched_qs <-rcu_note_context_switch
2138 <idle>-0 3dN.1 27us : rcu_preempt_qs <-rcu_note_context_switch
2139 <idle>-0 3.N.1 27us : _raw_spin_lock_irq <-__schedule
2140 <idle>-0 3dN.1 27us : add_preempt_count <-_raw_spin_lock_irq
2141 <idle>-0 3dN.2 28us : put_prev_task_idle <-__schedule
2142 <idle>-0 3dN.2 28us : pick_next_task_stop <-pick_next_task
2143 <idle>-0 3dN.2 28us : pick_next_task_rt <-pick_next_task
2144 <idle>-0 3dN.2 29us : dequeue_pushable_task <-pick_next_task_rt
2145 <idle>-0 3d..3 29us : __schedule <-preempt_schedule
2146 <idle>-0 3d..3 30us : 0:120:R ==> [003] 2448: 94:R sleep
2148 This isn't that big of a trace, even with function tracing enabled,
2149 so I included the entire trace.
2151 The interrupt went off while when the system was idle. Somewhere
2152 before task_woken_rt() was called, the NEED_RESCHED flag was set,
2153 this is indicated by the first occurrence of the 'N' flag.
2155 Latency tracing and events
2156 --------------------------
2157 As function tracing can induce a much larger latency, but without
2158 seeing what happens within the latency it is hard to know what
2159 caused it. There is a middle ground, and that is with enabling
2163 # echo 0 > options/function-trace
2164 # echo wakeup_rt > current_tracer
2165 # echo 1 > events/enable
2166 # echo 1 > tracing_on
2167 # echo 0 > tracing_max_latency
2169 # echo 0 > tracing_on
2173 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
2174 # --------------------------------------------------------------------
2175 # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
2177 # | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5)
2181 # / _-----=> irqs-off
2182 # | / _----=> need-resched
2183 # || / _---=> hardirq/softirq
2184 # ||| / _--=> preempt-depth
2186 # cmd pid ||||| time | caller
2188 <idle>-0 2d.h4 0us : 0:120:R + [002] 5882: 94:R sleep
2189 <idle>-0 2d.h4 0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
2190 <idle>-0 2d.h4 1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
2191 <idle>-0 2dNh2 1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
2192 <idle>-0 2.N.2 2us : power_end: cpu_id=2
2193 <idle>-0 2.N.2 3us : cpu_idle: state=4294967295 cpu_id=2
2194 <idle>-0 2dN.3 4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
2195 <idle>-0 2dN.3 4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
2196 <idle>-0 2.N.2 5us : rcu_utilization: Start context switch
2197 <idle>-0 2.N.2 5us : rcu_utilization: End context switch
2198 <idle>-0 2d..3 6us : __schedule <-schedule
2199 <idle>-0 2d..3 6us : 0:120:R ==> [002] 5882: 94:R sleep
2202 Hardware Latency Detector
2203 -------------------------
2205 The hardware latency detector is executed by enabling the "hwlat" tracer.
2207 NOTE, this tracer will affect the performance of the system as it will
2208 periodically make a CPU constantly busy with interrupts disabled.
2211 # echo hwlat > current_tracer
2216 # entries-in-buffer/entries-written: 13/13 #P:8
2219 # / _----=> need-resched
2220 # | / _---=> hardirq/softirq
2221 # || / _--=> preempt-depth
2223 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2225 <...>-1729 [001] d... 678.473449: #1 inner/outer(us): 11/12 ts:1581527483.343962693 count:6
2226 <...>-1729 [004] d... 689.556542: #2 inner/outer(us): 16/9 ts:1581527494.889008092 count:1
2227 <...>-1729 [005] d... 714.756290: #3 inner/outer(us): 16/16 ts:1581527519.678961629 count:5
2228 <...>-1729 [001] d... 718.788247: #4 inner/outer(us): 9/17 ts:1581527523.889012713 count:1
2229 <...>-1729 [002] d... 719.796341: #5 inner/outer(us): 13/9 ts:1581527524.912872606 count:1
2230 <...>-1729 [006] d... 844.787091: #6 inner/outer(us): 9/12 ts:1581527649.889048502 count:2
2231 <...>-1729 [003] d... 849.827033: #7 inner/outer(us): 18/9 ts:1581527654.889013793 count:1
2232 <...>-1729 [007] d... 853.859002: #8 inner/outer(us): 9/12 ts:1581527658.889065736 count:1
2233 <...>-1729 [001] d... 855.874978: #9 inner/outer(us): 9/11 ts:1581527660.861991877 count:1
2234 <...>-1729 [001] d... 863.938932: #10 inner/outer(us): 9/11 ts:1581527668.970010500 count:1 nmi-total:7 nmi-count:1
2235 <...>-1729 [007] d... 878.050780: #11 inner/outer(us): 9/12 ts:1581527683.385002600 count:1 nmi-total:5 nmi-count:1
2236 <...>-1729 [007] d... 886.114702: #12 inner/outer(us): 9/12 ts:1581527691.385001600 count:1
2239 The above output is somewhat the same in the header. All events will have
2240 interrupts disabled 'd'. Under the FUNCTION title there is:
2243 This is the count of events recorded that were greater than the
2244 tracing_threshold (See below).
2246 inner/outer(us): 11/11
2248 This shows two numbers as "inner latency" and "outer latency". The test
2249 runs in a loop checking a timestamp twice. The latency detected within
2250 the two timestamps is the "inner latency" and the latency detected
2251 after the previous timestamp and the next timestamp in the loop is
2252 the "outer latency".
2254 ts:1581527483.343962693
2256 The absolute timestamp that the first latency was recorded in the window.
2260 The number of times a latency was detected during the window.
2262 nmi-total:7 nmi-count:1
2264 On architectures that support it, if an NMI comes in during the
2265 test, the time spent in NMI is reported in "nmi-total" (in
2268 All architectures that have NMIs will show the "nmi-count" if an
2269 NMI comes in during the test.
2274 This gets automatically set to "10" to represent 10
2275 microseconds. This is the threshold of latency that
2276 needs to be detected before the trace will be recorded.
2278 Note, when hwlat tracer is finished (another tracer is
2279 written into "current_tracer"), the original value for
2280 tracing_threshold is placed back into this file.
2282 hwlat_detector/width
2283 The length of time the test runs with interrupts disabled.
2285 hwlat_detector/window
2286 The length of time of the window which the test
2287 runs. That is, the test will run for "width"
2288 microseconds per "window" microseconds
2291 When the test is started. A kernel thread is created that
2292 runs the test. This thread will alternate between CPUs
2293 listed in the tracing_cpumask between each period
2294 (one "window"). To limit the test to specific CPUs
2295 set the mask in this file to only the CPUs that the test
2301 This tracer is the function tracer. Enabling the function tracer
2302 can be done from the debug file system. Make sure the
2303 ftrace_enabled is set; otherwise this tracer is a nop.
2304 See the "ftrace_enabled" section below.
2307 # sysctl kernel.ftrace_enabled=1
2308 # echo function > current_tracer
2309 # echo 1 > tracing_on
2311 # echo 0 > tracing_on
2315 # entries-in-buffer/entries-written: 24799/24799 #P:4
2318 # / _----=> need-resched
2319 # | / _---=> hardirq/softirq
2320 # || / _--=> preempt-depth
2322 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2324 bash-1994 [002] .... 3082.063030: mutex_unlock <-rb_simple_write
2325 bash-1994 [002] .... 3082.063031: __mutex_unlock_slowpath <-mutex_unlock
2326 bash-1994 [002] .... 3082.063031: __fsnotify_parent <-fsnotify_modify
2327 bash-1994 [002] .... 3082.063032: fsnotify <-fsnotify_modify
2328 bash-1994 [002] .... 3082.063032: __srcu_read_lock <-fsnotify
2329 bash-1994 [002] .... 3082.063032: add_preempt_count <-__srcu_read_lock
2330 bash-1994 [002] ...1 3082.063032: sub_preempt_count <-__srcu_read_lock
2331 bash-1994 [002] .... 3082.063033: __srcu_read_unlock <-fsnotify
2335 Note: function tracer uses ring buffers to store the above
2336 entries. The newest data may overwrite the oldest data.
2337 Sometimes using echo to stop the trace is not sufficient because
2338 the tracing could have overwritten the data that you wanted to
2339 record. For this reason, it is sometimes better to disable
2340 tracing directly from a program. This allows you to stop the
2341 tracing at the point that you hit the part that you are
2342 interested in. To disable the tracing directly from a C program,
2343 something like following code snippet can be used::
2347 int main(int argc, char *argv[]) {
2349 trace_fd = open(tracing_file("tracing_on"), O_WRONLY);
2351 if (condition_hit()) {
2352 write(trace_fd, "0", 1);
2358 Single thread tracing
2359 ---------------------
2361 By writing into set_ftrace_pid you can trace a
2362 single thread. For example::
2364 # cat set_ftrace_pid
2366 # echo 3111 > set_ftrace_pid
2367 # cat set_ftrace_pid
2369 # echo function > current_tracer
2373 # TASK-PID CPU# TIMESTAMP FUNCTION
2375 yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return
2376 yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
2377 yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
2378 yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
2379 yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll
2380 yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll
2381 # echo > set_ftrace_pid
2385 # TASK-PID CPU# TIMESTAMP FUNCTION
2387 ##### CPU 3 buffer started ####
2388 yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait
2389 yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry
2390 yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry
2391 yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit
2392 yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit
2394 If you want to trace a function when executing, you could use
2395 something like this simple program.
2400 #include <sys/types.h>
2401 #include <sys/stat.h>
2407 #define STR(x) _STR(x)
2408 #define MAX_PATH 256
2410 const char *find_tracefs(void)
2412 static char tracefs[MAX_PATH+1];
2413 static int tracefs_found;
2420 if ((fp = fopen("/proc/mounts","r")) == NULL) {
2421 perror("/proc/mounts");
2425 while (fscanf(fp, "%*s %"
2427 "s %99s %*s %*d %*d\n",
2428 tracefs, type) == 2) {
2429 if (strcmp(type, "tracefs") == 0)
2434 if (strcmp(type, "tracefs") != 0) {
2435 fprintf(stderr, "tracefs not mounted");
2439 strcat(tracefs, "/tracing/");
2445 const char *tracing_file(const char *file_name)
2447 static char trace_file[MAX_PATH+1];
2448 snprintf(trace_file, MAX_PATH, "%s/%s", find_tracefs(), file_name);
2452 int main (int argc, char **argv)
2462 ffd = open(tracing_file("current_tracer"), O_WRONLY);
2465 write(ffd, "nop", 3);
2467 fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
2468 s = sprintf(line, "%d\n", getpid());
2471 write(ffd, "function", 8);
2476 execvp(argv[1], argv+1);
2482 Or this simple script!
2487 tracefs=`sed -ne 's/^tracefs \(.*\) tracefs.*/\1/p' /proc/mounts`
2488 echo 0 > $tracefs/tracing_on
2489 echo $$ > $tracefs/set_ftrace_pid
2490 echo function > $tracefs/current_tracer
2491 echo 1 > $tracefs/tracing_on
2495 function graph tracer
2496 ---------------------------
2498 This tracer is similar to the function tracer except that it
2499 probes a function on its entry and its exit. This is done by
2500 using a dynamically allocated stack of return addresses in each
2501 task_struct. On function entry the tracer overwrites the return
2502 address of each function traced to set a custom probe. Thus the
2503 original return address is stored on the stack of return address
2506 Probing on both ends of a function leads to special features
2509 - measure of a function's time execution
2510 - having a reliable call stack to draw function calls graph
2512 This tracer is useful in several situations:
2514 - you want to find the reason of a strange kernel behavior and
2515 need to see what happens in detail on any areas (or specific
2518 - you are experiencing weird latencies but it's difficult to
2521 - you want to find quickly which path is taken by a specific
2524 - you just want to peek inside a working kernel and want to see
2529 # tracer: function_graph
2531 # CPU DURATION FUNCTION CALLS
2535 0) | do_sys_open() {
2537 0) | kmem_cache_alloc() {
2538 0) 1.382 us | __might_sleep();
2540 0) | strncpy_from_user() {
2541 0) | might_fault() {
2542 0) 1.389 us | __might_sleep();
2547 0) 0.668 us | _spin_lock();
2548 0) 0.570 us | expand_files();
2549 0) 0.586 us | _spin_unlock();
2552 There are several columns that can be dynamically
2553 enabled/disabled. You can use every combination of options you
2554 want, depending on your needs.
2556 - The cpu number on which the function executed is default
2557 enabled. It is sometimes better to only trace one cpu (see
2558 tracing_cpu_mask file) or you might sometimes see unordered
2559 function calls while cpu tracing switch.
2561 - hide: echo nofuncgraph-cpu > trace_options
2562 - show: echo funcgraph-cpu > trace_options
2564 - The duration (function's time of execution) is displayed on
2565 the closing bracket line of a function or on the same line
2566 than the current function in case of a leaf one. It is default
2569 - hide: echo nofuncgraph-duration > trace_options
2570 - show: echo funcgraph-duration > trace_options
2572 - The overhead field precedes the duration field in case of
2573 reached duration thresholds.
2575 - hide: echo nofuncgraph-overhead > trace_options
2576 - show: echo funcgraph-overhead > trace_options
2577 - depends on: funcgraph-duration
2581 3) # 1837.709 us | } /* __switch_to */
2582 3) | finish_task_switch() {
2583 3) 0.313 us | _raw_spin_unlock_irq();
2585 3) # 1889.063 us | } /* __schedule */
2586 3) ! 140.417 us | } /* __schedule */
2587 3) # 2034.948 us | } /* schedule */
2588 3) * 33998.59 us | } /* schedule_preempt_disabled */
2592 1) 0.260 us | msecs_to_jiffies();
2593 1) 0.313 us | __rcu_read_unlock();
2596 1) 0.313 us | rcu_bh_qs();
2597 1) 0.313 us | __local_bh_enable();
2599 1) 0.365 us | idle_cpu();
2600 1) | rcu_irq_exit() {
2601 1) 0.417 us | rcu_eqs_enter_common.isra.47();
2605 1) @ 119760.2 us | }
2611 2) 0.417 us | scheduler_ipi();
2621 + means that the function exceeded 10 usecs.
2622 ! means that the function exceeded 100 usecs.
2623 # means that the function exceeded 1000 usecs.
2624 * means that the function exceeded 10 msecs.
2625 @ means that the function exceeded 100 msecs.
2626 $ means that the function exceeded 1 sec.
2629 - The task/pid field displays the thread cmdline and pid which
2630 executed the function. It is default disabled.
2632 - hide: echo nofuncgraph-proc > trace_options
2633 - show: echo funcgraph-proc > trace_options
2637 # tracer: function_graph
2639 # CPU TASK/PID DURATION FUNCTION CALLS
2641 0) sh-4802 | | d_free() {
2642 0) sh-4802 | | call_rcu() {
2643 0) sh-4802 | | __call_rcu() {
2644 0) sh-4802 | 0.616 us | rcu_process_gp_end();
2645 0) sh-4802 | 0.586 us | check_for_new_grace_period();
2646 0) sh-4802 | 2.899 us | }
2647 0) sh-4802 | 4.040 us | }
2648 0) sh-4802 | 5.151 us | }
2649 0) sh-4802 | + 49.370 us | }
2652 - The absolute time field is an absolute timestamp given by the
2653 system clock since it started. A snapshot of this time is
2654 given on each entry/exit of functions
2656 - hide: echo nofuncgraph-abstime > trace_options
2657 - show: echo funcgraph-abstime > trace_options
2662 # TIME CPU DURATION FUNCTION CALLS
2664 360.774522 | 1) 0.541 us | }
2665 360.774522 | 1) 4.663 us | }
2666 360.774523 | 1) 0.541 us | __wake_up_bit();
2667 360.774524 | 1) 6.796 us | }
2668 360.774524 | 1) 7.952 us | }
2669 360.774525 | 1) 9.063 us | }
2670 360.774525 | 1) 0.615 us | journal_mark_dirty();
2671 360.774527 | 1) 0.578 us | __brelse();
2672 360.774528 | 1) | reiserfs_prepare_for_journal() {
2673 360.774528 | 1) | unlock_buffer() {
2674 360.774529 | 1) | wake_up_bit() {
2675 360.774529 | 1) | bit_waitqueue() {
2676 360.774530 | 1) 0.594 us | __phys_addr();
2679 The function name is always displayed after the closing bracket
2680 for a function if the start of that function is not in the
2683 Display of the function name after the closing bracket may be
2684 enabled for functions whose start is in the trace buffer,
2685 allowing easier searching with grep for function durations.
2686 It is default disabled.
2688 - hide: echo nofuncgraph-tail > trace_options
2689 - show: echo funcgraph-tail > trace_options
2691 Example with nofuncgraph-tail (default)::
2694 0) | kmem_cache_free() {
2695 0) 0.518 us | __phys_addr();
2699 Example with funcgraph-tail::
2702 0) | kmem_cache_free() {
2703 0) 0.518 us | __phys_addr();
2704 0) 1.757 us | } /* kmem_cache_free() */
2705 0) 2.861 us | } /* putname() */
2707 You can put some comments on specific functions by using
2708 trace_printk() For example, if you want to put a comment inside
2709 the __might_sleep() function, you just have to include
2710 <linux/ftrace.h> and call trace_printk() inside __might_sleep()::
2712 trace_printk("I'm a comment!\n")
2716 1) | __might_sleep() {
2717 1) | /* I'm a comment! */
2721 You might find other useful features for this tracer in the
2722 following "dynamic ftrace" section such as tracing only specific
2728 If CONFIG_DYNAMIC_FTRACE is set, the system will run with
2729 virtually no overhead when function tracing is disabled. The way
2730 this works is the mcount function call (placed at the start of
2731 every kernel function, produced by the -pg switch in gcc),
2732 starts of pointing to a simple return. (Enabling FTRACE will
2733 include the -pg switch in the compiling of the kernel.)
2735 At compile time every C file object is run through the
2736 recordmcount program (located in the scripts directory). This
2737 program will parse the ELF headers in the C object to find all
2738 the locations in the .text section that call mcount. Starting
2739 with gcc version 4.6, the -mfentry has been added for x86, which
2740 calls "__fentry__" instead of "mcount". Which is called before
2741 the creation of the stack frame.
2743 Note, not all sections are traced. They may be prevented by either
2744 a notrace, or blocked another way and all inline functions are not
2745 traced. Check the "available_filter_functions" file to see what functions
2748 A section called "__mcount_loc" is created that holds
2749 references to all the mcount/fentry call sites in the .text section.
2750 The recordmcount program re-links this section back into the
2751 original object. The final linking stage of the kernel will add all these
2752 references into a single table.
2754 On boot up, before SMP is initialized, the dynamic ftrace code
2755 scans this table and updates all the locations into nops. It
2756 also records the locations, which are added to the
2757 available_filter_functions list. Modules are processed as they
2758 are loaded and before they are executed. When a module is
2759 unloaded, it also removes its functions from the ftrace function
2760 list. This is automatic in the module unload code, and the
2761 module author does not need to worry about it.
2763 When tracing is enabled, the process of modifying the function
2764 tracepoints is dependent on architecture. The old method is to use
2765 kstop_machine to prevent races with the CPUs executing code being
2766 modified (which can cause the CPU to do undesirable things, especially
2767 if the modified code crosses cache (or page) boundaries), and the nops are
2768 patched back to calls. But this time, they do not call mcount
2769 (which is just a function stub). They now call into the ftrace
2772 The new method of modifying the function tracepoints is to place
2773 a breakpoint at the location to be modified, sync all CPUs, modify
2774 the rest of the instruction not covered by the breakpoint. Sync
2775 all CPUs again, and then remove the breakpoint with the finished
2776 version to the ftrace call site.
2778 Some archs do not even need to monkey around with the synchronization,
2779 and can just slap the new code on top of the old without any
2780 problems with other CPUs executing it at the same time.
2782 One special side-effect to the recording of the functions being
2783 traced is that we can now selectively choose which functions we
2784 wish to trace and which ones we want the mcount calls to remain
2787 Two files are used, one for enabling and one for disabling the
2788 tracing of specified functions. They are:
2796 A list of available functions that you can add to these files is
2799 available_filter_functions
2803 # cat available_filter_functions
2812 If I am only interested in sys_nanosleep and hrtimer_interrupt::
2814 # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter
2815 # echo function > current_tracer
2816 # echo 1 > tracing_on
2818 # echo 0 > tracing_on
2822 # entries-in-buffer/entries-written: 5/5 #P:4
2825 # / _----=> need-resched
2826 # | / _---=> hardirq/softirq
2827 # || / _--=> preempt-depth
2829 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2831 usleep-2665 [001] .... 4186.475355: sys_nanosleep <-system_call_fastpath
2832 <idle>-0 [001] d.h1 4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt
2833 usleep-2665 [001] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
2834 <idle>-0 [003] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
2835 <idle>-0 [002] d.h1 4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt
2837 To see which functions are being traced, you can cat the file:
2840 # cat set_ftrace_filter
2845 Perhaps this is not enough. The filters also allow glob(7) matching.
2848 will match functions that begin with <match>
2850 will match functions that end with <match>
2852 will match functions that have <match> in it
2853 ``<match1>*<match2>``
2854 will match functions that begin with <match1> and end with <match2>
2857 It is better to use quotes to enclose the wild cards,
2858 otherwise the shell may expand the parameters into names
2859 of files in the local directory.
2863 # echo 'hrtimer_*' > set_ftrace_filter
2869 # entries-in-buffer/entries-written: 897/897 #P:4
2872 # / _----=> need-resched
2873 # | / _---=> hardirq/softirq
2874 # || / _--=> preempt-depth
2876 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2878 <idle>-0 [003] dN.1 4228.547803: hrtimer_cancel <-tick_nohz_idle_exit
2879 <idle>-0 [003] dN.1 4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel
2880 <idle>-0 [003] dN.2 4228.547805: hrtimer_force_reprogram <-__remove_hrtimer
2881 <idle>-0 [003] dN.1 4228.547805: hrtimer_forward <-tick_nohz_idle_exit
2882 <idle>-0 [003] dN.1 4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
2883 <idle>-0 [003] d..1 4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt
2884 <idle>-0 [003] d..1 4228.547859: hrtimer_start <-__tick_nohz_idle_enter
2885 <idle>-0 [003] d..2 4228.547860: hrtimer_force_reprogram <-__rem
2887 Notice that we lost the sys_nanosleep.
2890 # cat set_ftrace_filter
2895 hrtimer_try_to_cancel
2899 hrtimer_force_reprogram
2900 hrtimer_get_next_event
2904 hrtimer_get_remaining
2906 hrtimer_init_sleeper
2909 This is because the '>' and '>>' act just like they do in bash.
2910 To rewrite the filters, use '>'
2911 To append to the filters, use '>>'
2913 To clear out a filter so that all functions will be recorded
2916 # echo > set_ftrace_filter
2917 # cat set_ftrace_filter
2920 Again, now we want to append.
2924 # echo sys_nanosleep > set_ftrace_filter
2925 # cat set_ftrace_filter
2927 # echo 'hrtimer_*' >> set_ftrace_filter
2928 # cat set_ftrace_filter
2933 hrtimer_try_to_cancel
2937 hrtimer_force_reprogram
2938 hrtimer_get_next_event
2943 hrtimer_get_remaining
2945 hrtimer_init_sleeper
2948 The set_ftrace_notrace prevents those functions from being
2952 # echo '*preempt*' '*lock*' > set_ftrace_notrace
2958 # entries-in-buffer/entries-written: 39608/39608 #P:4
2961 # / _----=> need-resched
2962 # | / _---=> hardirq/softirq
2963 # || / _--=> preempt-depth
2965 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2967 bash-1994 [000] .... 4342.324896: file_ra_state_init <-do_dentry_open
2968 bash-1994 [000] .... 4342.324897: open_check_o_direct <-do_last
2969 bash-1994 [000] .... 4342.324897: ima_file_check <-do_last
2970 bash-1994 [000] .... 4342.324898: process_measurement <-ima_file_check
2971 bash-1994 [000] .... 4342.324898: ima_get_action <-process_measurement
2972 bash-1994 [000] .... 4342.324898: ima_match_policy <-ima_get_action
2973 bash-1994 [000] .... 4342.324899: do_truncate <-do_last
2974 bash-1994 [000] .... 4342.324899: setattr_should_drop_suidgid <-do_truncate
2975 bash-1994 [000] .... 4342.324899: notify_change <-do_truncate
2976 bash-1994 [000] .... 4342.324900: current_fs_time <-notify_change
2977 bash-1994 [000] .... 4342.324900: current_kernel_time <-current_fs_time
2978 bash-1994 [000] .... 4342.324900: timespec_trunc <-current_fs_time
2980 We can see that there's no more lock or preempt tracing.
2982 Selecting function filters via index
2983 ------------------------------------
2985 Because processing of strings is expensive (the address of the function
2986 needs to be looked up before comparing to the string being passed in),
2987 an index can be used as well to enable functions. This is useful in the
2988 case of setting thousands of specific functions at a time. By passing
2989 in a list of numbers, no string processing will occur. Instead, the function
2990 at the specific location in the internal array (which corresponds to the
2991 functions in the "available_filter_functions" file), is selected.
2995 # echo 1 > set_ftrace_filter
2997 Will select the first function listed in "available_filter_functions"
3001 # head -1 available_filter_functions
3002 trace_initcall_finish_cb
3004 # cat set_ftrace_filter
3005 trace_initcall_finish_cb
3007 # head -50 available_filter_functions | tail -1
3010 # echo 1 50 > set_ftrace_filter
3011 # cat set_ftrace_filter
3012 trace_initcall_finish_cb
3015 Dynamic ftrace with the function graph tracer
3016 ---------------------------------------------
3018 Although what has been explained above concerns both the
3019 function tracer and the function-graph-tracer, there are some
3020 special features only available in the function-graph tracer.
3022 If you want to trace only one function and all of its children,
3023 you just have to echo its name into set_graph_function::
3025 echo __do_fault > set_graph_function
3027 will produce the following "expanded" trace of the __do_fault()
3031 0) | filemap_fault() {
3032 0) | find_lock_page() {
3033 0) 0.804 us | find_get_page();
3034 0) | __might_sleep() {
3038 0) 0.653 us | _spin_lock();
3039 0) 0.578 us | page_add_file_rmap();
3040 0) 0.525 us | native_set_pte_at();
3041 0) 0.585 us | _spin_unlock();
3042 0) | unlock_page() {
3043 0) 0.541 us | page_waitqueue();
3044 0) 0.639 us | __wake_up_bit();
3048 0) | filemap_fault() {
3049 0) | find_lock_page() {
3050 0) 0.698 us | find_get_page();
3051 0) | __might_sleep() {
3055 0) 0.631 us | _spin_lock();
3056 0) 0.571 us | page_add_file_rmap();
3057 0) 0.526 us | native_set_pte_at();
3058 0) 0.586 us | _spin_unlock();
3059 0) | unlock_page() {
3060 0) 0.533 us | page_waitqueue();
3061 0) 0.638 us | __wake_up_bit();
3065 You can also expand several functions at once::
3067 echo sys_open > set_graph_function
3068 echo sys_close >> set_graph_function
3070 Now if you want to go back to trace all functions you can clear
3071 this special filter via::
3073 echo > set_graph_function
3079 Note, the proc sysctl ftrace_enable is a big on/off switch for the
3080 function tracer. By default it is enabled (when function tracing is
3081 enabled in the kernel). If it is disabled, all function tracing is
3082 disabled. This includes not only the function tracers for ftrace, but
3083 also for any other uses (perf, kprobes, stack tracing, profiling, etc). It
3084 cannot be disabled if there is a callback with FTRACE_OPS_FL_PERMANENT set
3087 Please disable this with care.
3089 This can be disable (and enabled) with::
3091 sysctl kernel.ftrace_enabled=0
3092 sysctl kernel.ftrace_enabled=1
3096 echo 0 > /proc/sys/kernel/ftrace_enabled
3097 echo 1 > /proc/sys/kernel/ftrace_enabled
3103 A few commands are supported by the set_ftrace_filter interface.
3104 Trace commands have the following format::
3106 <function>:<command>:<parameter>
3108 The following commands are supported:
3111 This command enables function filtering per module. The
3112 parameter defines the module. For example, if only the write*
3113 functions in the ext3 module are desired, run:
3115 echo 'write*:mod:ext3' > set_ftrace_filter
3117 This command interacts with the filter in the same way as
3118 filtering based on function names. Thus, adding more functions
3119 in a different module is accomplished by appending (>>) to the
3120 filter file. Remove specific module functions by prepending
3123 echo '!writeback*:mod:ext3' >> set_ftrace_filter
3125 Mod command supports module globbing. Disable tracing for all
3126 functions except a specific module::
3128 echo '!*:mod:!ext3' >> set_ftrace_filter
3130 Disable tracing for all modules, but still trace kernel::
3132 echo '!*:mod:*' >> set_ftrace_filter
3134 Enable filter only for kernel::
3136 echo '*write*:mod:!*' >> set_ftrace_filter
3138 Enable filter for module globbing::
3140 echo '*write*:mod:*snd*' >> set_ftrace_filter
3143 These commands turn tracing on and off when the specified
3144 functions are hit. The parameter determines how many times the
3145 tracing system is turned on and off. If unspecified, there is
3146 no limit. For example, to disable tracing when a schedule bug
3147 is hit the first 5 times, run::
3149 echo '__schedule_bug:traceoff:5' > set_ftrace_filter
3151 To always disable tracing when __schedule_bug is hit::
3153 echo '__schedule_bug:traceoff' > set_ftrace_filter
3155 These commands are cumulative whether or not they are appended
3156 to set_ftrace_filter. To remove a command, prepend it by '!'
3157 and drop the parameter::
3159 echo '!__schedule_bug:traceoff:0' > set_ftrace_filter
3161 The above removes the traceoff command for __schedule_bug
3162 that have a counter. To remove commands without counters::
3164 echo '!__schedule_bug:traceoff' > set_ftrace_filter
3167 Will cause a snapshot to be triggered when the function is hit.
3170 echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter
3172 To only snapshot once:
3175 echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter
3177 To remove the above commands::
3179 echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter
3180 echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter
3182 - enable_event/disable_event:
3183 These commands can enable or disable a trace event. Note, because
3184 function tracing callbacks are very sensitive, when these commands
3185 are registered, the trace point is activated, but disabled in
3186 a "soft" mode. That is, the tracepoint will be called, but
3187 just will not be traced. The event tracepoint stays in this mode
3188 as long as there's a command that triggers it.
3191 echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \
3196 <function>:enable_event:<system>:<event>[:count]
3197 <function>:disable_event:<system>:<event>[:count]
3199 To remove the events commands::
3201 echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \
3203 echo '!schedule:disable_event:sched:sched_switch' > \
3207 When the function is hit, it will dump the contents of the ftrace
3208 ring buffer to the console. This is useful if you need to debug
3209 something, and want to dump the trace when a certain function
3210 is hit. Perhaps it's a function that is called before a triple
3211 fault happens and does not allow you to get a regular dump.
3214 When the function is hit, it will dump the contents of the ftrace
3215 ring buffer for the current CPU to the console. Unlike the "dump"
3216 command, it only prints out the contents of the ring buffer for the
3217 CPU that executed the function that triggered the dump.
3220 When the function is hit, a stack trace is recorded.
3225 The trace_pipe outputs the same content as the trace file, but
3226 the effect on the tracing is different. Every read from
3227 trace_pipe is consumed. This means that subsequent reads will be
3228 different. The trace is live.
3231 # echo function > current_tracer
3232 # cat trace_pipe > /tmp/trace.out &
3234 # echo 1 > tracing_on
3236 # echo 0 > tracing_on
3240 # entries-in-buffer/entries-written: 0/0 #P:4
3243 # / _----=> need-resched
3244 # | / _---=> hardirq/softirq
3245 # || / _--=> preempt-depth
3247 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
3251 # cat /tmp/trace.out
3252 bash-1994 [000] .... 5281.568961: mutex_unlock <-rb_simple_write
3253 bash-1994 [000] .... 5281.568963: __mutex_unlock_slowpath <-mutex_unlock
3254 bash-1994 [000] .... 5281.568963: __fsnotify_parent <-fsnotify_modify
3255 bash-1994 [000] .... 5281.568964: fsnotify <-fsnotify_modify
3256 bash-1994 [000] .... 5281.568964: __srcu_read_lock <-fsnotify
3257 bash-1994 [000] .... 5281.568964: add_preempt_count <-__srcu_read_lock
3258 bash-1994 [000] ...1 5281.568965: sub_preempt_count <-__srcu_read_lock
3259 bash-1994 [000] .... 5281.568965: __srcu_read_unlock <-fsnotify
3260 bash-1994 [000] .... 5281.568967: sys_dup2 <-system_call_fastpath
3263 Note, reading the trace_pipe file will block until more input is
3264 added. This is contrary to the trace file. If any process opened
3265 the trace file for reading, it will actually disable tracing and
3266 prevent new entries from being added. The trace_pipe file does
3267 not have this limitation.
3272 Having too much or not enough data can be troublesome in
3273 diagnosing an issue in the kernel. The file buffer_size_kb is
3274 used to modify the size of the internal trace buffers. The
3275 number listed is the number of entries that can be recorded per
3276 CPU. To know the full size, multiply the number of possible CPUs
3277 with the number of entries.
3280 # cat buffer_size_kb
3281 1408 (units kilobytes)
3283 Or simply read buffer_total_size_kb
3286 # cat buffer_total_size_kb
3289 To modify the buffer, simple echo in a number (in 1024 byte segments).
3292 # echo 10000 > buffer_size_kb
3293 # cat buffer_size_kb
3294 10000 (units kilobytes)
3296 It will try to allocate as much as possible. If you allocate too
3297 much, it can cause Out-Of-Memory to trigger.
3300 # echo 1000000000000 > buffer_size_kb
3301 -bash: echo: write error: Cannot allocate memory
3302 # cat buffer_size_kb
3305 The per_cpu buffers can be changed individually as well:
3308 # echo 10000 > per_cpu/cpu0/buffer_size_kb
3309 # echo 100 > per_cpu/cpu1/buffer_size_kb
3311 When the per_cpu buffers are not the same, the buffer_size_kb
3312 at the top level will just show an X
3315 # cat buffer_size_kb
3318 This is where the buffer_total_size_kb is useful:
3321 # cat buffer_total_size_kb
3324 Writing to the top level buffer_size_kb will reset all the buffers
3325 to be the same again.
3329 CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature
3330 available to all non latency tracers. (Latency tracers which
3331 record max latency, such as "irqsoff" or "wakeup", can't use
3332 this feature, since those are already using the snapshot
3333 mechanism internally.)
3335 Snapshot preserves a current trace buffer at a particular point
3336 in time without stopping tracing. Ftrace swaps the current
3337 buffer with a spare buffer, and tracing continues in the new
3338 current (=previous spare) buffer.
3340 The following tracefs files in "tracing" are related to this
3345 This is used to take a snapshot and to read the output
3346 of the snapshot. Echo 1 into this file to allocate a
3347 spare buffer and to take a snapshot (swap), then read
3348 the snapshot from this file in the same format as
3349 "trace" (described above in the section "The File
3350 System"). Both reads snapshot and tracing are executable
3351 in parallel. When the spare buffer is allocated, echoing
3352 0 frees it, and echoing else (positive) values clear the
3354 More details are shown in the table below.
3356 +--------------+------------+------------+------------+
3357 |status\\input | 0 | 1 | else |
3358 +==============+============+============+============+
3359 |not allocated |(do nothing)| alloc+swap |(do nothing)|
3360 +--------------+------------+------------+------------+
3361 |allocated | free | swap | clear |
3362 +--------------+------------+------------+------------+
3364 Here is an example of using the snapshot feature.
3367 # echo 1 > events/sched/enable
3372 # entries-in-buffer/entries-written: 71/71 #P:8
3375 # / _----=> need-resched
3376 # | / _---=> hardirq/softirq
3377 # || / _--=> preempt-depth
3379 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
3381 <idle>-0 [005] d... 2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120
3382 sleep-2242 [005] d... 2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120 prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120
3384 <idle>-0 [002] d... 2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120
3389 # entries-in-buffer/entries-written: 77/77 #P:8
3392 # / _----=> need-resched
3393 # | / _---=> hardirq/softirq
3394 # || / _--=> preempt-depth
3396 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
3398 <idle>-0 [007] d... 2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120
3399 snapshot-test-2-2229 [002] d... 2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120
3403 If you try to use this snapshot feature when current tracer is
3404 one of the latency tracers, you will get the following results.
3407 # echo wakeup > current_tracer
3409 bash: echo: write error: Device or resource busy
3411 cat: snapshot: Device or resource busy
3416 In the tracefs tracing directory, there is a directory called "instances".
3417 This directory can have new directories created inside of it using
3418 mkdir, and removing directories with rmdir. The directory created
3419 with mkdir in this directory will already contain files and other
3420 directories after it is created.
3423 # mkdir instances/foo
3425 buffer_size_kb buffer_total_size_kb events free_buffer per_cpu
3426 set_event snapshot trace trace_clock trace_marker trace_options
3427 trace_pipe tracing_on
3429 As you can see, the new directory looks similar to the tracing directory
3430 itself. In fact, it is very similar, except that the buffer and
3431 events are agnostic from the main directory, or from any other
3432 instances that are created.
3434 The files in the new directory work just like the files with the
3435 same name in the tracing directory except the buffer that is used
3436 is a separate and new buffer. The files affect that buffer but do not
3437 affect the main buffer with the exception of trace_options. Currently,
3438 the trace_options affect all instances and the top level buffer
3439 the same, but this may change in future releases. That is, options
3440 may become specific to the instance they reside in.
3442 Notice that none of the function tracer files are there, nor is
3443 current_tracer and available_tracers. This is because the buffers
3444 can currently only have events enabled for them.
3447 # mkdir instances/foo
3448 # mkdir instances/bar
3449 # mkdir instances/zoot
3450 # echo 100000 > buffer_size_kb
3451 # echo 1000 > instances/foo/buffer_size_kb
3452 # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb
3453 # echo function > current_trace
3454 # echo 1 > instances/foo/events/sched/sched_wakeup/enable
3455 # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable
3456 # echo 1 > instances/foo/events/sched/sched_switch/enable
3457 # echo 1 > instances/bar/events/irq/enable
3458 # echo 1 > instances/zoot/events/syscalls/enable
3460 CPU:2 [LOST 11745 EVENTS]
3461 bash-2044 [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist
3462 bash-2044 [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave
3463 bash-2044 [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist
3464 bash-2044 [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist
3465 bash-2044 [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock
3466 bash-2044 [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype
3467 bash-2044 [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist
3468 bash-2044 [002] d... 10594.481034: zone_statistics <-get_page_from_freelist
3469 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics
3470 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics
3471 bash-2044 [002] .... 10594.481035: arch_dup_task_struct <-copy_process
3474 # cat instances/foo/trace_pipe
3475 bash-1998 [000] d..4 136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
3476 bash-1998 [000] dN.4 136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
3477 <idle>-0 [003] d.h3 136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003
3478 <idle>-0 [003] d..3 136.676909: sched_switch: prev_comm=swapper/3 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_preempt next_pid=9 next_prio=120
3479 rcu_preempt-9 [003] d..3 136.676916: sched_switch: prev_comm=rcu_preempt prev_pid=9 prev_prio=120 prev_state=S ==> next_comm=swapper/3 next_pid=0 next_prio=120
3480 bash-1998 [000] d..4 136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
3481 bash-1998 [000] dN.4 136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
3482 bash-1998 [000] d..3 136.677018: sched_switch: prev_comm=bash prev_pid=1998 prev_prio=120 prev_state=R+ ==> next_comm=kworker/0:1 next_pid=59 next_prio=120
3483 kworker/0:1-59 [000] d..4 136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001
3484 kworker/0:1-59 [000] d..3 136.677025: sched_switch: prev_comm=kworker/0:1 prev_pid=59 prev_prio=120 prev_state=S ==> next_comm=bash next_pid=1998 next_prio=120
3487 # cat instances/bar/trace_pipe
3488 migration/1-14 [001] d.h3 138.732674: softirq_raise: vec=3 [action=NET_RX]
3489 <idle>-0 [001] dNh3 138.732725: softirq_raise: vec=3 [action=NET_RX]
3490 bash-1998 [000] d.h1 138.733101: softirq_raise: vec=1 [action=TIMER]
3491 bash-1998 [000] d.h1 138.733102: softirq_raise: vec=9 [action=RCU]
3492 bash-1998 [000] ..s2 138.733105: softirq_entry: vec=1 [action=TIMER]
3493 bash-1998 [000] ..s2 138.733106: softirq_exit: vec=1 [action=TIMER]
3494 bash-1998 [000] ..s2 138.733106: softirq_entry: vec=9 [action=RCU]
3495 bash-1998 [000] ..s2 138.733109: softirq_exit: vec=9 [action=RCU]
3496 sshd-1995 [001] d.h1 138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4
3497 sshd-1995 [001] d.h1 138.733280: irq_handler_exit: irq=21 ret=unhandled
3498 sshd-1995 [001] d.h1 138.733281: irq_handler_entry: irq=21 name=eth0
3499 sshd-1995 [001] d.h1 138.733283: irq_handler_exit: irq=21 ret=handled
3502 # cat instances/zoot/trace
3505 # entries-in-buffer/entries-written: 18996/18996 #P:4
3508 # / _----=> need-resched
3509 # | / _---=> hardirq/softirq
3510 # || / _--=> preempt-depth
3512 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
3514 bash-1998 [000] d... 140.733501: sys_write -> 0x2
3515 bash-1998 [000] d... 140.733504: sys_dup2(oldfd: a, newfd: 1)
3516 bash-1998 [000] d... 140.733506: sys_dup2 -> 0x1
3517 bash-1998 [000] d... 140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0)
3518 bash-1998 [000] d... 140.733509: sys_fcntl -> 0x1
3519 bash-1998 [000] d... 140.733510: sys_close(fd: a)
3520 bash-1998 [000] d... 140.733510: sys_close -> 0x0
3521 bash-1998 [000] d... 140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8)
3522 bash-1998 [000] d... 140.733515: sys_rt_sigprocmask -> 0x0
3523 bash-1998 [000] d... 140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8)
3524 bash-1998 [000] d... 140.733516: sys_rt_sigaction -> 0x0
3526 You can see that the trace of the top most trace buffer shows only
3527 the function tracing. The foo instance displays wakeups and task
3530 To remove the instances, simply delete their directories:
3533 # rmdir instances/foo
3534 # rmdir instances/bar
3535 # rmdir instances/zoot
3537 Note, if a process has a trace file open in one of the instance
3538 directories, the rmdir will fail with EBUSY.
3543 Since the kernel has a fixed sized stack, it is important not to
3544 waste it in functions. A kernel developer must be conscious of
3545 what they allocate on the stack. If they add too much, the system
3546 can be in danger of a stack overflow, and corruption will occur,
3547 usually leading to a system panic.
3549 There are some tools that check this, usually with interrupts
3550 periodically checking usage. But if you can perform a check
3551 at every function call that will become very useful. As ftrace provides
3552 a function tracer, it makes it convenient to check the stack size
3553 at every function call. This is enabled via the stack tracer.
3555 CONFIG_STACK_TRACER enables the ftrace stack tracing functionality.
3556 To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled.
3559 # echo 1 > /proc/sys/kernel/stack_tracer_enabled
3561 You can also enable it from the kernel command line to trace
3562 the stack size of the kernel during boot up, by adding "stacktrace"
3563 to the kernel command line parameter.
3565 After running it for a few minutes, the output looks like:
3568 # cat stack_max_size
3572 Depth Size Location (18 entries)
3574 0) 2928 224 update_sd_lb_stats+0xbc/0x4ac
3575 1) 2704 160 find_busiest_group+0x31/0x1f1
3576 2) 2544 256 load_balance+0xd9/0x662
3577 3) 2288 80 idle_balance+0xbb/0x130
3578 4) 2208 128 __schedule+0x26e/0x5b9
3579 5) 2080 16 schedule+0x64/0x66
3580 6) 2064 128 schedule_timeout+0x34/0xe0
3581 7) 1936 112 wait_for_common+0x97/0xf1
3582 8) 1824 16 wait_for_completion+0x1d/0x1f
3583 9) 1808 128 flush_work+0xfe/0x119
3584 10) 1680 16 tty_flush_to_ldisc+0x1e/0x20
3585 11) 1664 48 input_available_p+0x1d/0x5c
3586 12) 1616 48 n_tty_poll+0x6d/0x134
3587 13) 1568 64 tty_poll+0x64/0x7f
3588 14) 1504 880 do_select+0x31e/0x511
3589 15) 624 400 core_sys_select+0x177/0x216
3590 16) 224 96 sys_select+0x91/0xb9
3591 17) 128 128 system_call_fastpath+0x16/0x1b
3593 Note, if -mfentry is being used by gcc, functions get traced before
3594 they set up the stack frame. This means that leaf level functions
3595 are not tested by the stack tracer when -mfentry is used.
3597 Currently, -mfentry is used by gcc 4.6.0 and above on x86 only.
3601 More details can be found in the source code, in the `kernel/trace/*.c` files.