1 ftrace - Function Tracer
2 ========================
4 Copyright 2008 Red Hat Inc.
5 Author: Steven Rostedt <srostedt@redhat.com>
6 License: The GNU Free Documentation License, Version 1.2
7 (dual licensed under the GPL v2)
8 Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton,
9 John Kacur, and David Teigland.
10 Written for: 2.6.28-rc2
16 Ftrace is an internal tracer designed to help out developers and
17 designers of systems to find what is going on inside the kernel.
18 It can be used for debugging or analyzing latencies and
19 performance issues that take place outside of user-space.
21 Although ftrace is typically considered the function tracer, it
22 is really a frame work of several assorted tracing utilities.
23 There's latency tracing to examine what occurs between interrupts
24 disabled and enabled, as well as for preemption and from a time
25 a task is woken to the task is actually scheduled in.
27 One of the most common uses of ftrace is the event tracing.
28 Through out the kernel is hundreds of static event points that
29 can be enabled via the debugfs file system to see what is
30 going on in certain parts of the kernel.
33 Implementation Details
34 ----------------------
36 See ftrace-design.txt for details for arch porters and such.
42 Ftrace uses the debugfs file system to hold the control files as
43 well as the files to display output.
45 When debugfs is configured into the kernel (which selecting any ftrace
46 option will do) the directory /sys/kernel/debug will be created. To mount
47 this directory, you can add to your /etc/fstab file:
49 debugfs /sys/kernel/debug debugfs defaults 0 0
51 Or you can mount it at run time with:
53 mount -t debugfs nodev /sys/kernel/debug
55 For quicker access to that directory you may want to make a soft link to
58 ln -s /sys/kernel/debug /debug
60 Any selected ftrace option will also create a directory called tracing
61 within the debugfs. The rest of the document will assume that you are in
62 the ftrace directory (cd /sys/kernel/debug/tracing) and will only concentrate
63 on the files within that directory and not distract from the content with
64 the extended "/sys/kernel/debug/tracing" path name.
66 That's it! (assuming that you have ftrace configured into your kernel)
68 After mounting debugfs, you can see a directory called
69 "tracing". This directory contains the control and output files
70 of ftrace. Here is a list of some of the key files:
73 Note: all time values are in microseconds.
77 This is used to set or display the current tracer
82 This holds the different types of tracers that
83 have been compiled into the kernel. The
84 tracers listed here can be configured by
85 echoing their name into current_tracer.
89 This sets or displays whether writing to the trace
90 ring buffer is enabled. Echo 0 into this file to disable
91 the tracer or 1 to enable it. Note, this only disables
92 writing to the ring buffer, the tracing overhead may
97 This file holds the output of the trace in a human
98 readable format (described below).
102 The output is the same as the "trace" file but this
103 file is meant to be streamed with live tracing.
104 Reads from this file will block until new data is
105 retrieved. Unlike the "trace" file, this file is a
106 consumer. This means reading from this file causes
107 sequential reads to display more current data. Once
108 data is read from this file, it is consumed, and
109 will not be read again with a sequential read. The
110 "trace" file is static, and if the tracer is not
111 adding more data, it will display the same
112 information every time it is read.
116 This file lets the user control the amount of data
117 that is displayed in one of the above output
118 files. Options also exist to modify how a tracer
119 or events work (stack traces, timestamps, etc).
123 This is a directory that has a file for every available
124 trace option (also in trace_options). Options may also be set
125 or cleared by writing a "1" or "0" respectively into the
126 corresponding file with the option name.
130 Some of the tracers record the max latency.
131 For example, the time interrupts are disabled.
132 This time is saved in this file. The max trace
133 will also be stored, and displayed by "trace".
134 A new max trace will only be recorded if the
135 latency is greater than the value in this
136 file. (in microseconds)
140 Some latency tracers will record a trace whenever the
141 latency is greater than the number in this file.
142 Only active when the file contains a number greater than 0.
147 This sets or displays the number of kilobytes each CPU
148 buffer holds. By default, the trace buffers are the same size
149 for each CPU. The displayed number is the size of the
150 CPU buffer and not total size of all buffers. The
151 trace buffers are allocated in pages (blocks of memory
152 that the kernel uses for allocation, usually 4 KB in size).
153 If the last page allocated has room for more bytes
154 than requested, the rest of the page will be used,
155 making the actual allocation bigger than requested.
156 ( Note, the size may not be a multiple of the page size
157 due to buffer management meta-data. )
159 buffer_total_size_kb:
161 This displays the total combined size of all the trace buffers.
165 If a process is performing the tracing, and the ring buffer
166 should be shrunk "freed" when the process is finished, even
167 if it were to be killed by a signal, this file can be used
168 for that purpose. On close of this file, the ring buffer will
169 be resized to its minimum size. Having a process that is tracing
170 also open this file, when the process exits its file descriptor
171 for this file will be closed, and in doing so, the ring buffer
174 It may also stop tracing if disable_on_free option is set.
178 This is a mask that lets the user only trace
179 on specified CPUs. The format is a hex string
180 representing the CPUs.
184 When dynamic ftrace is configured in (see the
185 section below "dynamic ftrace"), the code is dynamically
186 modified (code text rewrite) to disable calling of the
187 function profiler (mcount). This lets tracing be configured
188 in with practically no overhead in performance. This also
189 has a side effect of enabling or disabling specific functions
190 to be traced. Echoing names of functions into this file
191 will limit the trace to only those functions.
193 This interface also allows for commands to be used. See the
194 "Filter commands" section for more details.
198 This has an effect opposite to that of
199 set_ftrace_filter. Any function that is added here will not
200 be traced. If a function exists in both set_ftrace_filter
201 and set_ftrace_notrace, the function will _not_ be traced.
205 Have the function tracer only trace a single thread.
209 Have the events only trace a task with a PID listed in this file.
210 Note, sched_switch and sched_wake_up will also trace events
215 Set a "trigger" function where tracing should start
216 with the function graph tracer (See the section
217 "dynamic ftrace" for more details).
219 available_filter_functions:
221 This lists the functions that ftrace
222 has processed and can trace. These are the function
223 names that you can pass to "set_ftrace_filter" or
224 "set_ftrace_notrace". (See the section "dynamic ftrace"
225 below for more details.)
229 This file is more for debugging ftrace, but can also be useful
230 in seeing if any function has a callback attached to it.
231 Not only does the trace infrastructure use ftrace function
232 trace utility, but other subsystems might too. This file
233 displays all functions that have a callback attached to them
234 as well as the number of callbacks that have been attached.
235 Note, a callback may also call multiple functions which will
236 not be listed in this count.
238 If the callback registered to be traced by a function with
239 the "save regs" attribute (thus even more overhead), a 'R'
240 will be displayed on the same line as the function that
241 is returning registers.
243 If the callback registered to be traced by a function with
244 the "ip modify" attribute (thus the regs->ip can be changed),
245 an 'I' will be displayed on the same line as the function that
248 function_profile_enabled:
250 When set it will enable all functions with either the function
251 tracer, or if enabled, the function graph tracer. It will
252 keep a histogram of the number of functions that were called
253 and if run with the function graph tracer, it will also keep
254 track of the time spent in those functions. The histogram
255 content can be displayed in the files:
257 trace_stats/function<cpu> ( function0, function1, etc).
261 A directory that holds different tracing stats.
265 Enable dynamic trace points. See kprobetrace.txt.
269 Dynamic trace points stats. See kprobetrace.txt.
273 Used with the function graph tracer. This is the max depth
274 it will trace into a function. Setting this to a value of
275 one will show only the first kernel function that is called
280 This is for tools that read the raw format files. If an event in
281 the ring buffer references a string (currently only trace_printk()
282 does this), only a pointer to the string is recorded into the buffer
283 and not the string itself. This prevents tools from knowing what
284 that string was. This file displays the string and address for
285 the string allowing tools to map the pointers to what the
290 Only the pid of the task is recorded in a trace event unless
291 the event specifically saves the task comm as well. Ftrace
292 makes a cache of pid mappings to comms to try to display
293 comms for events. If a pid for a comm is not listed, then
294 "<...>" is displayed in the output.
298 This displays the "snapshot" buffer and also lets the user
299 take a snapshot of the current running trace.
300 See the "Snapshot" section below for more details.
304 When the stack tracer is activated, this will display the
305 maximum stack size it has encountered.
306 See the "Stack Trace" section below.
310 This displays the stack back trace of the largest stack
311 that was encountered when the stack tracer is activated.
312 See the "Stack Trace" section below.
316 This is similar to "set_ftrace_filter" but it limits what
317 functions the stack tracer will check.
321 Whenever an event is recorded into the ring buffer, a
322 "timestamp" is added. This stamp comes from a specified
323 clock. By default, ftrace uses the "local" clock. This
324 clock is very fast and strictly per cpu, but on some
325 systems it may not be monotonic with respect to other
326 CPUs. In other words, the local clocks may not be in sync
327 with local clocks on other CPUs.
329 Usual clocks for tracing:
332 [local] global counter x86-tsc
334 local: Default clock, but may not be in sync across CPUs
336 global: This clock is in sync with all CPUs but may
337 be a bit slower than the local clock.
339 counter: This is not a clock at all, but literally an atomic
340 counter. It counts up one by one, but is in sync
341 with all CPUs. This is useful when you need to
342 know exactly the order events occurred with respect to
343 each other on different CPUs.
345 uptime: This uses the jiffies counter and the time stamp
346 is relative to the time since boot up.
348 perf: This makes ftrace use the same clock that perf uses.
349 Eventually perf will be able to read ftrace buffers
350 and this will help out in interleaving the data.
352 x86-tsc: Architectures may define their own clocks. For
353 example, x86 uses its own TSC cycle clock here.
355 ppc-tb: This uses the powerpc timebase register value.
356 This is in sync across CPUs and can also be used
357 to correlate events across hypervisor/guest if
360 To set a clock, simply echo the clock name into this file.
362 echo global > trace_clock
366 This is a very useful file for synchronizing user space
367 with events happening in the kernel. Writing strings into
368 this file will be written into the ftrace buffer.
370 It is useful in applications to open this file at the start
371 of the application and just reference the file descriptor
374 void trace_write(const char *fmt, ...)
384 n = vsnprintf(buf, 256, fmt, ap);
387 write(trace_fd, buf, n);
392 trace_fd = open("trace_marker", WR_ONLY);
396 Add dynamic tracepoints in programs.
401 Uprobe statistics. See uprobetrace.txt
405 This is a way to make multiple trace buffers where different
406 events can be recorded in different buffers.
407 See "Instances" section below.
411 This is the trace event directory. It holds event tracepoints
412 (also known as static tracepoints) that have been compiled
413 into the kernel. It shows what event tracepoints exist
414 and how they are grouped by system. There are "enable"
415 files at various levels that can enable the tracepoints
416 when a "1" is written to them.
418 See events.txt for more information.
422 This is a directory that contains the trace per_cpu information.
424 per_cpu/cpu0/buffer_size_kb:
426 The ftrace buffer is defined per_cpu. That is, there's a separate
427 buffer for each CPU to allow writes to be done atomically,
428 and free from cache bouncing. These buffers may have different
429 size buffers. This file is similar to the buffer_size_kb
430 file, but it only displays or sets the buffer size for the
431 specific CPU. (here cpu0).
435 This is similar to the "trace" file, but it will only display
436 the data specific for the CPU. If written to, it only clears
437 the specific CPU buffer.
439 per_cpu/cpu0/trace_pipe
441 This is similar to the "trace_pipe" file, and is a consuming
442 read, but it will only display (and consume) the data specific
445 per_cpu/cpu0/trace_pipe_raw
447 For tools that can parse the ftrace ring buffer binary format,
448 the trace_pipe_raw file can be used to extract the data
449 from the ring buffer directly. With the use of the splice()
450 system call, the buffer data can be quickly transferred to
451 a file or to the network where a server is collecting the
454 Like trace_pipe, this is a consuming reader, where multiple
455 reads will always produce different data.
457 per_cpu/cpu0/snapshot:
459 This is similar to the main "snapshot" file, but will only
460 snapshot the current CPU (if supported). It only displays
461 the content of the snapshot for a given CPU, and if
462 written to, only clears this CPU buffer.
464 per_cpu/cpu0/snapshot_raw:
466 Similar to the trace_pipe_raw, but will read the binary format
467 from the snapshot buffer for the given CPU.
471 This displays certain stats about the ring buffer:
473 entries: The number of events that are still in the buffer.
475 overrun: The number of lost events due to overwriting when
478 commit overrun: Should always be zero.
479 This gets set if so many events happened within a nested
480 event (ring buffer is re-entrant), that it fills the
481 buffer and starts dropping events.
483 bytes: Bytes actually read (not overwritten).
485 oldest event ts: The oldest timestamp in the buffer
487 now ts: The current timestamp
489 dropped events: Events lost due to overwrite option being off.
491 read events: The number of events read.
496 Here is the list of current tracers that may be configured.
500 Function call tracer to trace all kernel functions.
504 Similar to the function tracer except that the
505 function tracer probes the functions on their entry
506 whereas the function graph tracer traces on both entry
507 and exit of the functions. It then provides the ability
508 to draw a graph of function calls similar to C code
513 Traces the areas that disable interrupts and saves
514 the trace with the longest max latency.
515 See tracing_max_latency. When a new max is recorded,
516 it replaces the old trace. It is best to view this
517 trace with the latency-format option enabled.
521 Similar to irqsoff but traces and records the amount of
522 time for which preemption is disabled.
526 Similar to irqsoff and preemptoff, but traces and
527 records the largest time for which irqs and/or preemption
532 Traces and records the max latency that it takes for
533 the highest priority task to get scheduled after
534 it has been woken up.
535 Traces all tasks as an average developer would expect.
539 Traces and records the max latency that it takes for just
540 RT tasks (as the current "wakeup" does). This is useful
541 for those interested in wake up timings of RT tasks.
545 This is the "trace nothing" tracer. To remove all
546 tracers from tracing simply echo "nop" into
550 Examples of using the tracer
551 ----------------------------
553 Here are typical examples of using the tracers when controlling
554 them only with the debugfs interface (without using any
555 user-land utilities).
560 Here is an example of the output format of the file "trace"
565 # entries-in-buffer/entries-written: 140080/250280 #P:4
568 # / _----=> need-resched
569 # | / _---=> hardirq/softirq
570 # || / _--=> preempt-depth
572 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
574 bash-1977 [000] .... 17284.993652: sys_close <-system_call_fastpath
575 bash-1977 [000] .... 17284.993653: __close_fd <-sys_close
576 bash-1977 [000] .... 17284.993653: _raw_spin_lock <-__close_fd
577 sshd-1974 [003] .... 17284.993653: __srcu_read_unlock <-fsnotify
578 bash-1977 [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock
579 bash-1977 [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd
580 bash-1977 [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock
581 bash-1977 [000] .... 17284.993657: filp_close <-__close_fd
582 bash-1977 [000] .... 17284.993657: dnotify_flush <-filp_close
583 sshd-1974 [003] .... 17284.993658: sys_select <-system_call_fastpath
586 A header is printed with the tracer name that is represented by
587 the trace. In this case the tracer is "function". Then it shows the
588 number of events in the buffer as well as the total number of entries
589 that were written. The difference is the number of entries that were
590 lost due to the buffer filling up (250280 - 140080 = 110200 events
593 The header explains the content of the events. Task name "bash", the task
594 PID "1977", the CPU that it was running on "000", the latency format
595 (explained below), the timestamp in <secs>.<usecs> format, the
596 function name that was traced "sys_close" and the parent function that
597 called this function "system_call_fastpath". The timestamp is the time
598 at which the function was entered.
603 When the latency-format option is enabled or when one of the latency
604 tracers is set, the trace file gives somewhat more information to see
605 why a latency happened. Here is a typical trace.
609 # irqsoff latency trace v1.1.5 on 3.8.0-test+
610 # --------------------------------------------------------------------
611 # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
613 # | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0)
615 # => started at: __lock_task_sighand
616 # => ended at: _raw_spin_unlock_irqrestore
620 # / _-----=> irqs-off
621 # | / _----=> need-resched
622 # || / _---=> hardirq/softirq
623 # ||| / _--=> preempt-depth
625 # cmd pid ||||| time | caller
627 ps-6143 2d... 0us!: trace_hardirqs_off <-__lock_task_sighand
628 ps-6143 2d..1 259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore
629 ps-6143 2d..1 263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore
630 ps-6143 2d..1 306us : <stack trace>
631 => trace_hardirqs_on_caller
633 => _raw_spin_unlock_irqrestore
640 => system_call_fastpath
643 This shows that the current tracer is "irqsoff" tracing the time
644 for which interrupts were disabled. It gives the trace version (which
645 never changes) and the version of the kernel upon which this was executed on
646 (3.10). Then it displays the max latency in microseconds (259 us). The number
647 of trace entries displayed and the total number (both are four: #4/4).
648 VP, KP, SP, and HP are always zero and are reserved for later use.
649 #P is the number of online CPUs (#P:4).
651 The task is the process that was running when the latency
652 occurred. (ps pid: 6143).
654 The start and stop (the functions in which the interrupts were
655 disabled and enabled respectively) that caused the latencies:
657 __lock_task_sighand is where the interrupts were disabled.
658 _raw_spin_unlock_irqrestore is where they were enabled again.
660 The next lines after the header are the trace itself. The header
661 explains which is which.
663 cmd: The name of the process in the trace.
665 pid: The PID of that process.
667 CPU#: The CPU which the process was running on.
669 irqs-off: 'd' interrupts are disabled. '.' otherwise.
670 Note: If the architecture does not support a way to
671 read the irq flags variable, an 'X' will always
675 'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set,
676 'n' only TIF_NEED_RESCHED is set,
677 'p' only PREEMPT_NEED_RESCHED is set,
681 'H' - hard irq occurred inside a softirq.
682 'h' - hard irq is running
683 's' - soft irq is running
684 '.' - normal context.
686 preempt-depth: The level of preempt_disabled
688 The above is mostly meaningful for kernel developers.
690 time: When the latency-format option is enabled, the trace file
691 output includes a timestamp relative to the start of the
692 trace. This differs from the output when latency-format
693 is disabled, which includes an absolute timestamp.
695 delay: This is just to help catch your eye a bit better. And
696 needs to be fixed to be only relative to the same CPU.
697 The marks are determined by the difference between this
698 current trace and the next trace.
699 '$' - greater than 1 second
700 '@' - greater than 100 milisecond
701 '*' - greater than 10 milisecond
702 '#' - greater than 1000 microsecond
703 '!' - greater than 100 microsecond
704 '+' - greater than 10 microsecond
705 ' ' - less than or equal to 10 microsecond.
707 The rest is the same as the 'trace' file.
709 Note, the latency tracers will usually end with a back trace
710 to easily find where the latency occurred.
715 The trace_options file (or the options directory) is used to control
716 what gets printed in the trace output, or manipulate the tracers.
717 To see what is available, simply cat the file:
747 To disable one of the options, echo in the option prepended with
750 echo noprint-parent > trace_options
752 To enable an option, leave off the "no".
754 echo sym-offset > trace_options
756 Here are the available options:
758 print-parent - On function traces, display the calling (parent)
759 function as well as the function being traced.
762 bash-4000 [01] 1477.606694: simple_strtoul <-kstrtoul
765 bash-4000 [01] 1477.606694: simple_strtoul
768 sym-offset - Display not only the function name, but also the
769 offset in the function. For example, instead of
770 seeing just "ktime_get", you will see
771 "ktime_get+0xb/0x20".
774 bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0
776 sym-addr - this will also display the function address as well
777 as the function name.
780 bash-4000 [01] 1477.606694: simple_strtoul <c0339346>
782 verbose - This deals with the trace file when the
783 latency-format option is enabled.
785 bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
786 (+0.000ms): simple_strtoul (kstrtoul)
788 raw - This will display raw numbers. This option is best for
789 use with user applications that can translate the raw
790 numbers better than having it done in the kernel.
792 hex - Similar to raw, but the numbers will be in a hexadecimal
795 bin - This will print out the formats in raw binary.
797 block - When set, reading trace_pipe will not block when polled.
799 stacktrace - This is one of the options that changes the trace
800 itself. When a trace is recorded, so is the stack
801 of functions. This allows for back traces of
804 trace_printk - Can disable trace_printk() from writing into the buffer.
806 branch - Enable branch tracing with the tracer.
808 annotate - It is sometimes confusing when the CPU buffers are full
809 and one CPU buffer had a lot of events recently, thus
810 a shorter time frame, were another CPU may have only had
811 a few events, which lets it have older events. When
812 the trace is reported, it shows the oldest events first,
813 and it may look like only one CPU ran (the one with the
814 oldest events). When the annotate option is set, it will
815 display when a new CPU buffer started:
817 <idle>-0 [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on
818 <idle>-0 [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on
819 <idle>-0 [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore
820 ##### CPU 2 buffer started ####
821 <idle>-0 [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle
822 <idle>-0 [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog
823 <idle>-0 [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock
825 userstacktrace - This option changes the trace. It records a
826 stacktrace of the current userspace thread.
828 sym-userobj - when user stacktrace are enabled, look up which
829 object the address belongs to, and print a
830 relative address. This is especially useful when
831 ASLR is on, otherwise you don't get a chance to
832 resolve the address to object/file/line after
833 the app is no longer running
835 The lookup is performed when you read
836 trace,trace_pipe. Example:
838 a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
839 x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
842 printk-msg-only - When set, trace_printk()s will only show the format
843 and not their parameters (if trace_bprintk() or
844 trace_bputs() was used to save the trace_printk()).
846 context-info - Show only the event data. Hides the comm, PID,
847 timestamp, CPU, and other useful data.
849 latency-format - This option changes the trace. When
850 it is enabled, the trace displays
851 additional information about the
852 latencies, as described in "Latency
855 sleep-time - When running function graph tracer, to include
856 the time a task schedules out in its function.
857 When enabled, it will account time the task has been
858 scheduled out as part of the function call.
860 graph-time - When running function graph tracer, to include the
861 time to call nested functions. When this is not set,
862 the time reported for the function will only include
863 the time the function itself executed for, not the time
864 for functions that it called.
866 record-cmd - When any event or tracer is enabled, a hook is enabled
867 in the sched_switch trace point to fill comm cache
868 with mapped pids and comms. But this may cause some
869 overhead, and if you only care about pids, and not the
870 name of the task, disabling this option can lower the
873 overwrite - This controls what happens when the trace buffer is
874 full. If "1" (default), the oldest events are
875 discarded and overwritten. If "0", then the newest
876 events are discarded.
877 (see per_cpu/cpu0/stats for overrun and dropped)
879 disable_on_free - When the free_buffer is closed, tracing will
880 stop (tracing_on set to 0).
882 irq-info - Shows the interrupt, preempt count, need resched data.
883 When disabled, the trace looks like:
887 # entries-in-buffer/entries-written: 144405/9452052 #P:4
889 # TASK-PID CPU# TIMESTAMP FUNCTION
891 <idle>-0 [002] 23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up
892 <idle>-0 [002] 23636.756054: activate_task <-ttwu_do_activate.constprop.89
893 <idle>-0 [002] 23636.756055: enqueue_task <-activate_task
896 markers - When set, the trace_marker is writable (only by root).
897 When disabled, the trace_marker will error with EINVAL
901 function-trace - The latency tracers will enable function tracing
902 if this option is enabled (default it is). When
903 it is disabled, the latency tracers do not trace
904 functions. This keeps the overhead of the tracer down
905 when performing latency tests.
907 Note: Some tracers have their own options. They only appear
908 when the tracer is active.
915 When interrupts are disabled, the CPU can not react to any other
916 external event (besides NMIs and SMIs). This prevents the timer
917 interrupt from triggering or the mouse interrupt from letting
918 the kernel know of a new mouse event. The result is a latency
919 with the reaction time.
921 The irqsoff tracer tracks the time for which interrupts are
922 disabled. When a new maximum latency is hit, the tracer saves
923 the trace leading up to that latency point so that every time a
924 new maximum is reached, the old saved trace is discarded and the
927 To reset the maximum, echo 0 into tracing_max_latency. Here is
930 # echo 0 > options/function-trace
931 # echo irqsoff > current_tracer
932 # echo 1 > tracing_on
933 # echo 0 > tracing_max_latency
936 # echo 0 > tracing_on
940 # irqsoff latency trace v1.1.5 on 3.8.0-test+
941 # --------------------------------------------------------------------
942 # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
944 # | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0)
946 # => started at: run_timer_softirq
947 # => ended at: run_timer_softirq
951 # / _-----=> irqs-off
952 # | / _----=> need-resched
953 # || / _---=> hardirq/softirq
954 # ||| / _--=> preempt-depth
956 # cmd pid ||||| time | caller
958 <idle>-0 0d.s2 0us+: _raw_spin_lock_irq <-run_timer_softirq
959 <idle>-0 0dNs3 17us : _raw_spin_unlock_irq <-run_timer_softirq
960 <idle>-0 0dNs3 17us+: trace_hardirqs_on <-run_timer_softirq
961 <idle>-0 0dNs3 25us : <stack trace>
962 => _raw_spin_unlock_irq
968 => smp_apic_timer_interrupt
969 => apic_timer_interrupt
974 => x86_64_start_reservations
975 => x86_64_start_kernel
977 Here we see that that we had a latency of 16 microseconds (which is
978 very good). The _raw_spin_lock_irq in run_timer_softirq disabled
979 interrupts. The difference between the 16 and the displayed
980 timestamp 25us occurred because the clock was incremented
981 between the time of recording the max latency and the time of
982 recording the function that had that latency.
984 Note the above example had function-trace not set. If we set
985 function-trace, we get a much larger output:
987 with echo 1 > options/function-trace
991 # irqsoff latency trace v1.1.5 on 3.8.0-test+
992 # --------------------------------------------------------------------
993 # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
995 # | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0)
997 # => started at: ata_scsi_queuecmd
998 # => ended at: ata_scsi_queuecmd
1002 # / _-----=> irqs-off
1003 # | / _----=> need-resched
1004 # || / _---=> hardirq/softirq
1005 # ||| / _--=> preempt-depth
1007 # cmd pid ||||| time | caller
1009 bash-2042 3d... 0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd
1010 bash-2042 3d... 0us : add_preempt_count <-_raw_spin_lock_irqsave
1011 bash-2042 3d..1 1us : ata_scsi_find_dev <-ata_scsi_queuecmd
1012 bash-2042 3d..1 1us : __ata_scsi_find_dev <-ata_scsi_find_dev
1013 bash-2042 3d..1 2us : ata_find_dev.part.14 <-__ata_scsi_find_dev
1014 bash-2042 3d..1 2us : ata_qc_new_init <-__ata_scsi_queuecmd
1015 bash-2042 3d..1 3us : ata_sg_init <-__ata_scsi_queuecmd
1016 bash-2042 3d..1 4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd
1017 bash-2042 3d..1 4us : ata_build_rw_tf <-ata_scsi_rw_xlat
1019 bash-2042 3d..1 67us : delay_tsc <-__delay
1020 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc
1021 bash-2042 3d..2 67us : sub_preempt_count <-delay_tsc
1022 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc
1023 bash-2042 3d..2 68us : sub_preempt_count <-delay_tsc
1024 bash-2042 3d..1 68us+: ata_bmdma_start <-ata_bmdma_qc_issue
1025 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1026 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1027 bash-2042 3d..1 72us+: trace_hardirqs_on <-ata_scsi_queuecmd
1028 bash-2042 3d..1 120us : <stack trace>
1029 => _raw_spin_unlock_irqrestore
1030 => ata_scsi_queuecmd
1031 => scsi_dispatch_cmd
1033 => __blk_run_queue_uncond
1036 => generic_make_request
1039 => __ext3_get_inode_loc
1048 => user_path_at_empty
1053 => system_call_fastpath
1056 Here we traced a 71 microsecond latency. But we also see all the
1057 functions that were called during that time. Note that by
1058 enabling function tracing, we incur an added overhead. This
1059 overhead may extend the latency times. But nevertheless, this
1060 trace has provided some very helpful debugging information.
1066 When preemption is disabled, we may be able to receive
1067 interrupts but the task cannot be preempted and a higher
1068 priority task must wait for preemption to be enabled again
1069 before it can preempt a lower priority task.
1071 The preemptoff tracer traces the places that disable preemption.
1072 Like the irqsoff tracer, it records the maximum latency for
1073 which preemption was disabled. The control of preemptoff tracer
1074 is much like the irqsoff tracer.
1076 # echo 0 > options/function-trace
1077 # echo preemptoff > current_tracer
1078 # echo 1 > tracing_on
1079 # echo 0 > tracing_max_latency
1082 # echo 0 > tracing_on
1084 # tracer: preemptoff
1086 # preemptoff latency trace v1.1.5 on 3.8.0-test+
1087 # --------------------------------------------------------------------
1088 # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1090 # | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0)
1092 # => started at: do_IRQ
1093 # => ended at: do_IRQ
1097 # / _-----=> irqs-off
1098 # | / _----=> need-resched
1099 # || / _---=> hardirq/softirq
1100 # ||| / _--=> preempt-depth
1102 # cmd pid ||||| time | caller
1104 sshd-1991 1d.h. 0us+: irq_enter <-do_IRQ
1105 sshd-1991 1d..1 46us : irq_exit <-do_IRQ
1106 sshd-1991 1d..1 47us+: trace_preempt_on <-do_IRQ
1107 sshd-1991 1d..1 52us : <stack trace>
1108 => sub_preempt_count
1114 This has some more changes. Preemption was disabled when an
1115 interrupt came in (notice the 'h'), and was enabled on exit.
1116 But we also see that interrupts have been disabled when entering
1117 the preempt off section and leaving it (the 'd'). We do not know if
1118 interrupts were enabled in the mean time or shortly after this
1121 # tracer: preemptoff
1123 # preemptoff latency trace v1.1.5 on 3.8.0-test+
1124 # --------------------------------------------------------------------
1125 # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1127 # | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0)
1129 # => started at: wake_up_new_task
1130 # => ended at: task_rq_unlock
1134 # / _-----=> irqs-off
1135 # | / _----=> need-resched
1136 # || / _---=> hardirq/softirq
1137 # ||| / _--=> preempt-depth
1139 # cmd pid ||||| time | caller
1141 bash-1994 1d..1 0us : _raw_spin_lock_irqsave <-wake_up_new_task
1142 bash-1994 1d..1 0us : select_task_rq_fair <-select_task_rq
1143 bash-1994 1d..1 1us : __rcu_read_lock <-select_task_rq_fair
1144 bash-1994 1d..1 1us : source_load <-select_task_rq_fair
1145 bash-1994 1d..1 1us : source_load <-select_task_rq_fair
1147 bash-1994 1d..1 12us : irq_enter <-smp_apic_timer_interrupt
1148 bash-1994 1d..1 12us : rcu_irq_enter <-irq_enter
1149 bash-1994 1d..1 13us : add_preempt_count <-irq_enter
1150 bash-1994 1d.h1 13us : exit_idle <-smp_apic_timer_interrupt
1151 bash-1994 1d.h1 13us : hrtimer_interrupt <-smp_apic_timer_interrupt
1152 bash-1994 1d.h1 13us : _raw_spin_lock <-hrtimer_interrupt
1153 bash-1994 1d.h1 14us : add_preempt_count <-_raw_spin_lock
1154 bash-1994 1d.h2 14us : ktime_get_update_offsets <-hrtimer_interrupt
1156 bash-1994 1d.h1 35us : lapic_next_event <-clockevents_program_event
1157 bash-1994 1d.h1 35us : irq_exit <-smp_apic_timer_interrupt
1158 bash-1994 1d.h1 36us : sub_preempt_count <-irq_exit
1159 bash-1994 1d..2 36us : do_softirq <-irq_exit
1160 bash-1994 1d..2 36us : __do_softirq <-call_softirq
1161 bash-1994 1d..2 36us : __local_bh_disable <-__do_softirq
1162 bash-1994 1d.s2 37us : add_preempt_count <-_raw_spin_lock_irq
1163 bash-1994 1d.s3 38us : _raw_spin_unlock <-run_timer_softirq
1164 bash-1994 1d.s3 39us : sub_preempt_count <-_raw_spin_unlock
1165 bash-1994 1d.s2 39us : call_timer_fn <-run_timer_softirq
1167 bash-1994 1dNs2 81us : cpu_needs_another_gp <-rcu_process_callbacks
1168 bash-1994 1dNs2 82us : __local_bh_enable <-__do_softirq
1169 bash-1994 1dNs2 82us : sub_preempt_count <-__local_bh_enable
1170 bash-1994 1dN.2 82us : idle_cpu <-irq_exit
1171 bash-1994 1dN.2 83us : rcu_irq_exit <-irq_exit
1172 bash-1994 1dN.2 83us : sub_preempt_count <-irq_exit
1173 bash-1994 1.N.1 84us : _raw_spin_unlock_irqrestore <-task_rq_unlock
1174 bash-1994 1.N.1 84us+: trace_preempt_on <-task_rq_unlock
1175 bash-1994 1.N.1 104us : <stack trace>
1176 => sub_preempt_count
1177 => _raw_spin_unlock_irqrestore
1185 The above is an example of the preemptoff trace with
1186 function-trace set. Here we see that interrupts were not disabled
1187 the entire time. The irq_enter code lets us know that we entered
1188 an interrupt 'h'. Before that, the functions being traced still
1189 show that it is not in an interrupt, but we can see from the
1190 functions themselves that this is not the case.
1195 Knowing the locations that have interrupts disabled or
1196 preemption disabled for the longest times is helpful. But
1197 sometimes we would like to know when either preemption and/or
1198 interrupts are disabled.
1200 Consider the following code:
1202 local_irq_disable();
1203 call_function_with_irqs_off();
1205 call_function_with_irqs_and_preemption_off();
1207 call_function_with_preemption_off();
1210 The irqsoff tracer will record the total length of
1211 call_function_with_irqs_off() and
1212 call_function_with_irqs_and_preemption_off().
1214 The preemptoff tracer will record the total length of
1215 call_function_with_irqs_and_preemption_off() and
1216 call_function_with_preemption_off().
1218 But neither will trace the time that interrupts and/or
1219 preemption is disabled. This total time is the time that we can
1220 not schedule. To record this time, use the preemptirqsoff
1223 Again, using this trace is much like the irqsoff and preemptoff
1226 # echo 0 > options/function-trace
1227 # echo preemptirqsoff > current_tracer
1228 # echo 1 > tracing_on
1229 # echo 0 > tracing_max_latency
1232 # echo 0 > tracing_on
1234 # tracer: preemptirqsoff
1236 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1237 # --------------------------------------------------------------------
1238 # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1240 # | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0)
1242 # => started at: ata_scsi_queuecmd
1243 # => ended at: ata_scsi_queuecmd
1247 # / _-----=> irqs-off
1248 # | / _----=> need-resched
1249 # || / _---=> hardirq/softirq
1250 # ||| / _--=> preempt-depth
1252 # cmd pid ||||| time | caller
1254 ls-2230 3d... 0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd
1255 ls-2230 3...1 100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1256 ls-2230 3...1 101us+: trace_preempt_on <-ata_scsi_queuecmd
1257 ls-2230 3...1 111us : <stack trace>
1258 => sub_preempt_count
1259 => _raw_spin_unlock_irqrestore
1260 => ata_scsi_queuecmd
1261 => scsi_dispatch_cmd
1263 => __blk_run_queue_uncond
1266 => generic_make_request
1271 => htree_dirblock_to_tree
1272 => ext3_htree_fill_tree
1276 => system_call_fastpath
1279 The trace_hardirqs_off_thunk is called from assembly on x86 when
1280 interrupts are disabled in the assembly code. Without the
1281 function tracing, we do not know if interrupts were enabled
1282 within the preemption points. We do see that it started with
1285 Here is a trace with function-trace set:
1287 # tracer: preemptirqsoff
1289 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1290 # --------------------------------------------------------------------
1291 # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1293 # | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0)
1295 # => started at: schedule
1296 # => ended at: mutex_unlock
1300 # / _-----=> irqs-off
1301 # | / _----=> need-resched
1302 # || / _---=> hardirq/softirq
1303 # ||| / _--=> preempt-depth
1305 # cmd pid ||||| time | caller
1307 kworker/-59 3...1 0us : __schedule <-schedule
1308 kworker/-59 3d..1 0us : rcu_preempt_qs <-rcu_note_context_switch
1309 kworker/-59 3d..1 1us : add_preempt_count <-_raw_spin_lock_irq
1310 kworker/-59 3d..2 1us : deactivate_task <-__schedule
1311 kworker/-59 3d..2 1us : dequeue_task <-deactivate_task
1312 kworker/-59 3d..2 2us : update_rq_clock <-dequeue_task
1313 kworker/-59 3d..2 2us : dequeue_task_fair <-dequeue_task
1314 kworker/-59 3d..2 2us : update_curr <-dequeue_task_fair
1315 kworker/-59 3d..2 2us : update_min_vruntime <-update_curr
1316 kworker/-59 3d..2 3us : cpuacct_charge <-update_curr
1317 kworker/-59 3d..2 3us : __rcu_read_lock <-cpuacct_charge
1318 kworker/-59 3d..2 3us : __rcu_read_unlock <-cpuacct_charge
1319 kworker/-59 3d..2 3us : update_cfs_rq_blocked_load <-dequeue_task_fair
1320 kworker/-59 3d..2 4us : clear_buddies <-dequeue_task_fair
1321 kworker/-59 3d..2 4us : account_entity_dequeue <-dequeue_task_fair
1322 kworker/-59 3d..2 4us : update_min_vruntime <-dequeue_task_fair
1323 kworker/-59 3d..2 4us : update_cfs_shares <-dequeue_task_fair
1324 kworker/-59 3d..2 5us : hrtick_update <-dequeue_task_fair
1325 kworker/-59 3d..2 5us : wq_worker_sleeping <-__schedule
1326 kworker/-59 3d..2 5us : kthread_data <-wq_worker_sleeping
1327 kworker/-59 3d..2 5us : put_prev_task_fair <-__schedule
1328 kworker/-59 3d..2 6us : pick_next_task_fair <-pick_next_task
1329 kworker/-59 3d..2 6us : clear_buddies <-pick_next_task_fair
1330 kworker/-59 3d..2 6us : set_next_entity <-pick_next_task_fair
1331 kworker/-59 3d..2 6us : update_stats_wait_end <-set_next_entity
1332 ls-2269 3d..2 7us : finish_task_switch <-__schedule
1333 ls-2269 3d..2 7us : _raw_spin_unlock_irq <-finish_task_switch
1334 ls-2269 3d..2 8us : do_IRQ <-ret_from_intr
1335 ls-2269 3d..2 8us : irq_enter <-do_IRQ
1336 ls-2269 3d..2 8us : rcu_irq_enter <-irq_enter
1337 ls-2269 3d..2 9us : add_preempt_count <-irq_enter
1338 ls-2269 3d.h2 9us : exit_idle <-do_IRQ
1340 ls-2269 3d.h3 20us : sub_preempt_count <-_raw_spin_unlock
1341 ls-2269 3d.h2 20us : irq_exit <-do_IRQ
1342 ls-2269 3d.h2 21us : sub_preempt_count <-irq_exit
1343 ls-2269 3d..3 21us : do_softirq <-irq_exit
1344 ls-2269 3d..3 21us : __do_softirq <-call_softirq
1345 ls-2269 3d..3 21us+: __local_bh_disable <-__do_softirq
1346 ls-2269 3d.s4 29us : sub_preempt_count <-_local_bh_enable_ip
1347 ls-2269 3d.s5 29us : sub_preempt_count <-_local_bh_enable_ip
1348 ls-2269 3d.s5 31us : do_IRQ <-ret_from_intr
1349 ls-2269 3d.s5 31us : irq_enter <-do_IRQ
1350 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter
1352 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter
1353 ls-2269 3d.s5 32us : add_preempt_count <-irq_enter
1354 ls-2269 3d.H5 32us : exit_idle <-do_IRQ
1355 ls-2269 3d.H5 32us : handle_irq <-do_IRQ
1356 ls-2269 3d.H5 32us : irq_to_desc <-handle_irq
1357 ls-2269 3d.H5 33us : handle_fasteoi_irq <-handle_irq
1359 ls-2269 3d.s5 158us : _raw_spin_unlock_irqrestore <-rtl8139_poll
1360 ls-2269 3d.s3 158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action
1361 ls-2269 3d.s3 159us : __local_bh_enable <-__do_softirq
1362 ls-2269 3d.s3 159us : sub_preempt_count <-__local_bh_enable
1363 ls-2269 3d..3 159us : idle_cpu <-irq_exit
1364 ls-2269 3d..3 159us : rcu_irq_exit <-irq_exit
1365 ls-2269 3d..3 160us : sub_preempt_count <-irq_exit
1366 ls-2269 3d... 161us : __mutex_unlock_slowpath <-mutex_unlock
1367 ls-2269 3d... 162us+: trace_hardirqs_on <-mutex_unlock
1368 ls-2269 3d... 186us : <stack trace>
1369 => __mutex_unlock_slowpath
1376 => system_call_fastpath
1378 This is an interesting trace. It started with kworker running and
1379 scheduling out and ls taking over. But as soon as ls released the
1380 rq lock and enabled interrupts (but not preemption) an interrupt
1381 triggered. When the interrupt finished, it started running softirqs.
1382 But while the softirq was running, another interrupt triggered.
1383 When an interrupt is running inside a softirq, the annotation is 'H'.
1389 One common case that people are interested in tracing is the
1390 time it takes for a task that is woken to actually wake up.
1391 Now for non Real-Time tasks, this can be arbitrary. But tracing
1392 it none the less can be interesting.
1394 Without function tracing:
1396 # echo 0 > options/function-trace
1397 # echo wakeup > current_tracer
1398 # echo 1 > tracing_on
1399 # echo 0 > tracing_max_latency
1401 # echo 0 > tracing_on
1405 # wakeup latency trace v1.1.5 on 3.8.0-test+
1406 # --------------------------------------------------------------------
1407 # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1409 # | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0)
1413 # / _-----=> irqs-off
1414 # | / _----=> need-resched
1415 # || / _---=> hardirq/softirq
1416 # ||| / _--=> preempt-depth
1418 # cmd pid ||||| time | caller
1420 <idle>-0 3dNs7 0us : 0:120:R + [003] 312:100:R kworker/3:1H
1421 <idle>-0 3dNs7 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
1422 <idle>-0 3d..3 15us : __schedule <-schedule
1423 <idle>-0 3d..3 15us : 0:120:R ==> [003] 312:100:R kworker/3:1H
1425 The tracer only traces the highest priority task in the system
1426 to avoid tracing the normal circumstances. Here we see that
1427 the kworker with a nice priority of -20 (not very nice), took
1428 just 15 microseconds from the time it woke up, to the time it
1431 Non Real-Time tasks are not that interesting. A more interesting
1432 trace is to concentrate only on Real-Time tasks.
1437 In a Real-Time environment it is very important to know the
1438 wakeup time it takes for the highest priority task that is woken
1439 up to the time that it executes. This is also known as "schedule
1440 latency". I stress the point that this is about RT tasks. It is
1441 also important to know the scheduling latency of non-RT tasks,
1442 but the average schedule latency is better for non-RT tasks.
1443 Tools like LatencyTop are more appropriate for such
1446 Real-Time environments are interested in the worst case latency.
1447 That is the longest latency it takes for something to happen,
1448 and not the average. We can have a very fast scheduler that may
1449 only have a large latency once in a while, but that would not
1450 work well with Real-Time tasks. The wakeup_rt tracer was designed
1451 to record the worst case wakeups of RT tasks. Non-RT tasks are
1452 not recorded because the tracer only records one worst case and
1453 tracing non-RT tasks that are unpredictable will overwrite the
1454 worst case latency of RT tasks (just run the normal wakeup
1455 tracer for a while to see that effect).
1457 Since this tracer only deals with RT tasks, we will run this
1458 slightly differently than we did with the previous tracers.
1459 Instead of performing an 'ls', we will run 'sleep 1' under
1460 'chrt' which changes the priority of the task.
1462 # echo 0 > options/function-trace
1463 # echo wakeup_rt > current_tracer
1464 # echo 1 > tracing_on
1465 # echo 0 > tracing_max_latency
1467 # echo 0 > tracing_on
1473 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1474 # --------------------------------------------------------------------
1475 # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1477 # | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5)
1481 # / _-----=> irqs-off
1482 # | / _----=> need-resched
1483 # || / _---=> hardirq/softirq
1484 # ||| / _--=> preempt-depth
1486 # cmd pid ||||| time | caller
1488 <idle>-0 3d.h4 0us : 0:120:R + [003] 2389: 94:R sleep
1489 <idle>-0 3d.h4 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
1490 <idle>-0 3d..3 5us : __schedule <-schedule
1491 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep
1494 Running this on an idle system, we see that it only took 5 microseconds
1495 to perform the task switch. Note, since the trace point in the schedule
1496 is before the actual "switch", we stop the tracing when the recorded task
1497 is about to schedule in. This may change if we add a new marker at the
1498 end of the scheduler.
1500 Notice that the recorded task is 'sleep' with the PID of 2389
1501 and it has an rt_prio of 5. This priority is user-space priority
1502 and not the internal kernel priority. The policy is 1 for
1503 SCHED_FIFO and 2 for SCHED_RR.
1505 Note, that the trace data shows the internal priority (99 - rtprio).
1507 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep
1509 The 0:120:R means idle was running with a nice priority of 0 (120 - 20)
1510 and in the running state 'R'. The sleep task was scheduled in with
1511 2389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94)
1512 and it too is in the running state.
1514 Doing the same with chrt -r 5 and function-trace set.
1516 echo 1 > options/function-trace
1520 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1521 # --------------------------------------------------------------------
1522 # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1524 # | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5)
1528 # / _-----=> irqs-off
1529 # | / _----=> need-resched
1530 # || / _---=> hardirq/softirq
1531 # ||| / _--=> preempt-depth
1533 # cmd pid ||||| time | caller
1535 <idle>-0 3d.h4 1us+: 0:120:R + [003] 2448: 94:R sleep
1536 <idle>-0 3d.h4 2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
1537 <idle>-0 3d.h3 3us : check_preempt_curr <-ttwu_do_wakeup
1538 <idle>-0 3d.h3 3us : resched_curr <-check_preempt_curr
1539 <idle>-0 3dNh3 4us : task_woken_rt <-ttwu_do_wakeup
1540 <idle>-0 3dNh3 4us : _raw_spin_unlock <-try_to_wake_up
1541 <idle>-0 3dNh3 4us : sub_preempt_count <-_raw_spin_unlock
1542 <idle>-0 3dNh2 5us : ttwu_stat <-try_to_wake_up
1543 <idle>-0 3dNh2 5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
1544 <idle>-0 3dNh2 6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1545 <idle>-0 3dNh1 6us : _raw_spin_lock <-__run_hrtimer
1546 <idle>-0 3dNh1 6us : add_preempt_count <-_raw_spin_lock
1547 <idle>-0 3dNh2 7us : _raw_spin_unlock <-hrtimer_interrupt
1548 <idle>-0 3dNh2 7us : sub_preempt_count <-_raw_spin_unlock
1549 <idle>-0 3dNh1 7us : tick_program_event <-hrtimer_interrupt
1550 <idle>-0 3dNh1 7us : clockevents_program_event <-tick_program_event
1551 <idle>-0 3dNh1 8us : ktime_get <-clockevents_program_event
1552 <idle>-0 3dNh1 8us : lapic_next_event <-clockevents_program_event
1553 <idle>-0 3dNh1 8us : irq_exit <-smp_apic_timer_interrupt
1554 <idle>-0 3dNh1 9us : sub_preempt_count <-irq_exit
1555 <idle>-0 3dN.2 9us : idle_cpu <-irq_exit
1556 <idle>-0 3dN.2 9us : rcu_irq_exit <-irq_exit
1557 <idle>-0 3dN.2 10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
1558 <idle>-0 3dN.2 10us : sub_preempt_count <-irq_exit
1559 <idle>-0 3.N.1 11us : rcu_idle_exit <-cpu_idle
1560 <idle>-0 3dN.1 11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
1561 <idle>-0 3.N.1 11us : tick_nohz_idle_exit <-cpu_idle
1562 <idle>-0 3dN.1 12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
1563 <idle>-0 3dN.1 12us : ktime_get <-tick_nohz_idle_exit
1564 <idle>-0 3dN.1 12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
1565 <idle>-0 3dN.1 13us : update_cpu_load_nohz <-tick_nohz_idle_exit
1566 <idle>-0 3dN.1 13us : _raw_spin_lock <-update_cpu_load_nohz
1567 <idle>-0 3dN.1 13us : add_preempt_count <-_raw_spin_lock
1568 <idle>-0 3dN.2 13us : __update_cpu_load <-update_cpu_load_nohz
1569 <idle>-0 3dN.2 14us : sched_avg_update <-__update_cpu_load
1570 <idle>-0 3dN.2 14us : _raw_spin_unlock <-update_cpu_load_nohz
1571 <idle>-0 3dN.2 14us : sub_preempt_count <-_raw_spin_unlock
1572 <idle>-0 3dN.1 15us : calc_load_exit_idle <-tick_nohz_idle_exit
1573 <idle>-0 3dN.1 15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
1574 <idle>-0 3dN.1 15us : hrtimer_cancel <-tick_nohz_idle_exit
1575 <idle>-0 3dN.1 15us : hrtimer_try_to_cancel <-hrtimer_cancel
1576 <idle>-0 3dN.1 16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
1577 <idle>-0 3dN.1 16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
1578 <idle>-0 3dN.1 16us : add_preempt_count <-_raw_spin_lock_irqsave
1579 <idle>-0 3dN.2 17us : __remove_hrtimer <-remove_hrtimer.part.16
1580 <idle>-0 3dN.2 17us : hrtimer_force_reprogram <-__remove_hrtimer
1581 <idle>-0 3dN.2 17us : tick_program_event <-hrtimer_force_reprogram
1582 <idle>-0 3dN.2 18us : clockevents_program_event <-tick_program_event
1583 <idle>-0 3dN.2 18us : ktime_get <-clockevents_program_event
1584 <idle>-0 3dN.2 18us : lapic_next_event <-clockevents_program_event
1585 <idle>-0 3dN.2 19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
1586 <idle>-0 3dN.2 19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1587 <idle>-0 3dN.1 19us : hrtimer_forward <-tick_nohz_idle_exit
1588 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward
1589 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward
1590 <idle>-0 3dN.1 20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
1591 <idle>-0 3dN.1 20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
1592 <idle>-0 3dN.1 21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
1593 <idle>-0 3dN.1 21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
1594 <idle>-0 3dN.1 21us : add_preempt_count <-_raw_spin_lock_irqsave
1595 <idle>-0 3dN.2 22us : ktime_add_safe <-__hrtimer_start_range_ns
1596 <idle>-0 3dN.2 22us : enqueue_hrtimer <-__hrtimer_start_range_ns
1597 <idle>-0 3dN.2 22us : tick_program_event <-__hrtimer_start_range_ns
1598 <idle>-0 3dN.2 23us : clockevents_program_event <-tick_program_event
1599 <idle>-0 3dN.2 23us : ktime_get <-clockevents_program_event
1600 <idle>-0 3dN.2 23us : lapic_next_event <-clockevents_program_event
1601 <idle>-0 3dN.2 24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
1602 <idle>-0 3dN.2 24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
1603 <idle>-0 3dN.1 24us : account_idle_ticks <-tick_nohz_idle_exit
1604 <idle>-0 3dN.1 24us : account_idle_time <-account_idle_ticks
1605 <idle>-0 3.N.1 25us : sub_preempt_count <-cpu_idle
1606 <idle>-0 3.N.. 25us : schedule <-cpu_idle
1607 <idle>-0 3.N.. 25us : __schedule <-preempt_schedule
1608 <idle>-0 3.N.. 26us : add_preempt_count <-__schedule
1609 <idle>-0 3.N.1 26us : rcu_note_context_switch <-__schedule
1610 <idle>-0 3.N.1 26us : rcu_sched_qs <-rcu_note_context_switch
1611 <idle>-0 3dN.1 27us : rcu_preempt_qs <-rcu_note_context_switch
1612 <idle>-0 3.N.1 27us : _raw_spin_lock_irq <-__schedule
1613 <idle>-0 3dN.1 27us : add_preempt_count <-_raw_spin_lock_irq
1614 <idle>-0 3dN.2 28us : put_prev_task_idle <-__schedule
1615 <idle>-0 3dN.2 28us : pick_next_task_stop <-pick_next_task
1616 <idle>-0 3dN.2 28us : pick_next_task_rt <-pick_next_task
1617 <idle>-0 3dN.2 29us : dequeue_pushable_task <-pick_next_task_rt
1618 <idle>-0 3d..3 29us : __schedule <-preempt_schedule
1619 <idle>-0 3d..3 30us : 0:120:R ==> [003] 2448: 94:R sleep
1621 This isn't that big of a trace, even with function tracing enabled,
1622 so I included the entire trace.
1624 The interrupt went off while when the system was idle. Somewhere
1625 before task_woken_rt() was called, the NEED_RESCHED flag was set,
1626 this is indicated by the first occurrence of the 'N' flag.
1628 Latency tracing and events
1629 --------------------------
1630 As function tracing can induce a much larger latency, but without
1631 seeing what happens within the latency it is hard to know what
1632 caused it. There is a middle ground, and that is with enabling
1635 # echo 0 > options/function-trace
1636 # echo wakeup_rt > current_tracer
1637 # echo 1 > events/enable
1638 # echo 1 > tracing_on
1639 # echo 0 > tracing_max_latency
1641 # echo 0 > tracing_on
1645 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
1646 # --------------------------------------------------------------------
1647 # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1649 # | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5)
1653 # / _-----=> irqs-off
1654 # | / _----=> need-resched
1655 # || / _---=> hardirq/softirq
1656 # ||| / _--=> preempt-depth
1658 # cmd pid ||||| time | caller
1660 <idle>-0 2d.h4 0us : 0:120:R + [002] 5882: 94:R sleep
1661 <idle>-0 2d.h4 0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
1662 <idle>-0 2d.h4 1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
1663 <idle>-0 2dNh2 1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
1664 <idle>-0 2.N.2 2us : power_end: cpu_id=2
1665 <idle>-0 2.N.2 3us : cpu_idle: state=4294967295 cpu_id=2
1666 <idle>-0 2dN.3 4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
1667 <idle>-0 2dN.3 4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
1668 <idle>-0 2.N.2 5us : rcu_utilization: Start context switch
1669 <idle>-0 2.N.2 5us : rcu_utilization: End context switch
1670 <idle>-0 2d..3 6us : __schedule <-schedule
1671 <idle>-0 2d..3 6us : 0:120:R ==> [002] 5882: 94:R sleep
1677 This tracer is the function tracer. Enabling the function tracer
1678 can be done from the debug file system. Make sure the
1679 ftrace_enabled is set; otherwise this tracer is a nop.
1680 See the "ftrace_enabled" section below.
1682 # sysctl kernel.ftrace_enabled=1
1683 # echo function > current_tracer
1684 # echo 1 > tracing_on
1686 # echo 0 > tracing_on
1690 # entries-in-buffer/entries-written: 24799/24799 #P:4
1693 # / _----=> need-resched
1694 # | / _---=> hardirq/softirq
1695 # || / _--=> preempt-depth
1697 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
1699 bash-1994 [002] .... 3082.063030: mutex_unlock <-rb_simple_write
1700 bash-1994 [002] .... 3082.063031: __mutex_unlock_slowpath <-mutex_unlock
1701 bash-1994 [002] .... 3082.063031: __fsnotify_parent <-fsnotify_modify
1702 bash-1994 [002] .... 3082.063032: fsnotify <-fsnotify_modify
1703 bash-1994 [002] .... 3082.063032: __srcu_read_lock <-fsnotify
1704 bash-1994 [002] .... 3082.063032: add_preempt_count <-__srcu_read_lock
1705 bash-1994 [002] ...1 3082.063032: sub_preempt_count <-__srcu_read_lock
1706 bash-1994 [002] .... 3082.063033: __srcu_read_unlock <-fsnotify
1710 Note: function tracer uses ring buffers to store the above
1711 entries. The newest data may overwrite the oldest data.
1712 Sometimes using echo to stop the trace is not sufficient because
1713 the tracing could have overwritten the data that you wanted to
1714 record. For this reason, it is sometimes better to disable
1715 tracing directly from a program. This allows you to stop the
1716 tracing at the point that you hit the part that you are
1717 interested in. To disable the tracing directly from a C program,
1718 something like following code snippet can be used:
1722 int main(int argc, char *argv[]) {
1724 trace_fd = open(tracing_file("tracing_on"), O_WRONLY);
1726 if (condition_hit()) {
1727 write(trace_fd, "0", 1);
1733 Single thread tracing
1734 ---------------------
1736 By writing into set_ftrace_pid you can trace a
1737 single thread. For example:
1739 # cat set_ftrace_pid
1741 # echo 3111 > set_ftrace_pid
1742 # cat set_ftrace_pid
1744 # echo function > current_tracer
1748 # TASK-PID CPU# TIMESTAMP FUNCTION
1750 yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return
1751 yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
1752 yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
1753 yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
1754 yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll
1755 yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll
1756 # echo > set_ftrace_pid
1760 # TASK-PID CPU# TIMESTAMP FUNCTION
1762 ##### CPU 3 buffer started ####
1763 yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait
1764 yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry
1765 yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry
1766 yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit
1767 yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit
1769 If you want to trace a function when executing, you could use
1770 something like this simple program:
1774 #include <sys/types.h>
1775 #include <sys/stat.h>
1781 #define STR(x) _STR(x)
1782 #define MAX_PATH 256
1784 const char *find_debugfs(void)
1786 static char debugfs[MAX_PATH+1];
1787 static int debugfs_found;
1794 if ((fp = fopen("/proc/mounts","r")) == NULL) {
1795 perror("/proc/mounts");
1799 while (fscanf(fp, "%*s %"
1801 "s %99s %*s %*d %*d\n",
1802 debugfs, type) == 2) {
1803 if (strcmp(type, "debugfs") == 0)
1808 if (strcmp(type, "debugfs") != 0) {
1809 fprintf(stderr, "debugfs not mounted");
1813 strcat(debugfs, "/tracing/");
1819 const char *tracing_file(const char *file_name)
1821 static char trace_file[MAX_PATH+1];
1822 snprintf(trace_file, MAX_PATH, "%s/%s", find_debugfs(), file_name);
1826 int main (int argc, char **argv)
1836 ffd = open(tracing_file("current_tracer"), O_WRONLY);
1839 write(ffd, "nop", 3);
1841 fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
1842 s = sprintf(line, "%d\n", getpid());
1845 write(ffd, "function", 8);
1850 execvp(argv[1], argv+1);
1856 Or this simple script!
1861 debugfs=`sed -ne 's/^debugfs \(.*\) debugfs.*/\1/p' /proc/mounts`
1862 echo nop > $debugfs/tracing/current_tracer
1863 echo 0 > $debugfs/tracing/tracing_on
1864 echo $$ > $debugfs/tracing/set_ftrace_pid
1865 echo function > $debugfs/tracing/current_tracer
1866 echo 1 > $debugfs/tracing/tracing_on
1871 function graph tracer
1872 ---------------------------
1874 This tracer is similar to the function tracer except that it
1875 probes a function on its entry and its exit. This is done by
1876 using a dynamically allocated stack of return addresses in each
1877 task_struct. On function entry the tracer overwrites the return
1878 address of each function traced to set a custom probe. Thus the
1879 original return address is stored on the stack of return address
1882 Probing on both ends of a function leads to special features
1885 - measure of a function's time execution
1886 - having a reliable call stack to draw function calls graph
1888 This tracer is useful in several situations:
1890 - you want to find the reason of a strange kernel behavior and
1891 need to see what happens in detail on any areas (or specific
1894 - you are experiencing weird latencies but it's difficult to
1897 - you want to find quickly which path is taken by a specific
1900 - you just want to peek inside a working kernel and want to see
1903 # tracer: function_graph
1905 # CPU DURATION FUNCTION CALLS
1909 0) | do_sys_open() {
1911 0) | kmem_cache_alloc() {
1912 0) 1.382 us | __might_sleep();
1914 0) | strncpy_from_user() {
1915 0) | might_fault() {
1916 0) 1.389 us | __might_sleep();
1921 0) 0.668 us | _spin_lock();
1922 0) 0.570 us | expand_files();
1923 0) 0.586 us | _spin_unlock();
1926 There are several columns that can be dynamically
1927 enabled/disabled. You can use every combination of options you
1928 want, depending on your needs.
1930 - The cpu number on which the function executed is default
1931 enabled. It is sometimes better to only trace one cpu (see
1932 tracing_cpu_mask file) or you might sometimes see unordered
1933 function calls while cpu tracing switch.
1935 hide: echo nofuncgraph-cpu > trace_options
1936 show: echo funcgraph-cpu > trace_options
1938 - The duration (function's time of execution) is displayed on
1939 the closing bracket line of a function or on the same line
1940 than the current function in case of a leaf one. It is default
1943 hide: echo nofuncgraph-duration > trace_options
1944 show: echo funcgraph-duration > trace_options
1946 - The overhead field precedes the duration field in case of
1947 reached duration thresholds.
1949 hide: echo nofuncgraph-overhead > trace_options
1950 show: echo funcgraph-overhead > trace_options
1951 depends on: funcgraph-duration
1955 3) # 1837.709 us | } /* __switch_to */
1956 3) | finish_task_switch() {
1957 3) 0.313 us | _raw_spin_unlock_irq();
1959 3) # 1889.063 us | } /* __schedule */
1960 3) ! 140.417 us | } /* __schedule */
1961 3) # 2034.948 us | } /* schedule */
1962 3) * 33998.59 us | } /* schedule_preempt_disabled */
1966 1) 0.260 us | msecs_to_jiffies();
1967 1) 0.313 us | __rcu_read_unlock();
1970 1) 0.313 us | rcu_bh_qs();
1971 1) 0.313 us | __local_bh_enable();
1973 1) 0.365 us | idle_cpu();
1974 1) | rcu_irq_exit() {
1975 1) 0.417 us | rcu_eqs_enter_common.isra.47();
1979 1) @ 119760.2 us | }
1985 2) 0.417 us | scheduler_ipi();
1993 + means that the function exceeded 10 usecs.
1994 ! means that the function exceeded 100 usecs.
1995 # means that the function exceeded 1000 usecs.
1996 * means that the function exceeded 10 msecs.
1997 @ means that the function exceeded 100 msecs.
1998 $ means that the function exceeded 1 sec.
2001 - The task/pid field displays the thread cmdline and pid which
2002 executed the function. It is default disabled.
2004 hide: echo nofuncgraph-proc > trace_options
2005 show: echo funcgraph-proc > trace_options
2009 # tracer: function_graph
2011 # CPU TASK/PID DURATION FUNCTION CALLS
2013 0) sh-4802 | | d_free() {
2014 0) sh-4802 | | call_rcu() {
2015 0) sh-4802 | | __call_rcu() {
2016 0) sh-4802 | 0.616 us | rcu_process_gp_end();
2017 0) sh-4802 | 0.586 us | check_for_new_grace_period();
2018 0) sh-4802 | 2.899 us | }
2019 0) sh-4802 | 4.040 us | }
2020 0) sh-4802 | 5.151 us | }
2021 0) sh-4802 | + 49.370 us | }
2024 - The absolute time field is an absolute timestamp given by the
2025 system clock since it started. A snapshot of this time is
2026 given on each entry/exit of functions
2028 hide: echo nofuncgraph-abstime > trace_options
2029 show: echo funcgraph-abstime > trace_options
2034 # TIME CPU DURATION FUNCTION CALLS
2036 360.774522 | 1) 0.541 us | }
2037 360.774522 | 1) 4.663 us | }
2038 360.774523 | 1) 0.541 us | __wake_up_bit();
2039 360.774524 | 1) 6.796 us | }
2040 360.774524 | 1) 7.952 us | }
2041 360.774525 | 1) 9.063 us | }
2042 360.774525 | 1) 0.615 us | journal_mark_dirty();
2043 360.774527 | 1) 0.578 us | __brelse();
2044 360.774528 | 1) | reiserfs_prepare_for_journal() {
2045 360.774528 | 1) | unlock_buffer() {
2046 360.774529 | 1) | wake_up_bit() {
2047 360.774529 | 1) | bit_waitqueue() {
2048 360.774530 | 1) 0.594 us | __phys_addr();
2051 The function name is always displayed after the closing bracket
2052 for a function if the start of that function is not in the
2055 Display of the function name after the closing bracket may be
2056 enabled for functions whose start is in the trace buffer,
2057 allowing easier searching with grep for function durations.
2058 It is default disabled.
2060 hide: echo nofuncgraph-tail > trace_options
2061 show: echo funcgraph-tail > trace_options
2063 Example with nofuncgraph-tail (default):
2065 0) | kmem_cache_free() {
2066 0) 0.518 us | __phys_addr();
2070 Example with funcgraph-tail:
2072 0) | kmem_cache_free() {
2073 0) 0.518 us | __phys_addr();
2074 0) 1.757 us | } /* kmem_cache_free() */
2075 0) 2.861 us | } /* putname() */
2077 You can put some comments on specific functions by using
2078 trace_printk() For example, if you want to put a comment inside
2079 the __might_sleep() function, you just have to include
2080 <linux/ftrace.h> and call trace_printk() inside __might_sleep()
2082 trace_printk("I'm a comment!\n")
2086 1) | __might_sleep() {
2087 1) | /* I'm a comment! */
2091 You might find other useful features for this tracer in the
2092 following "dynamic ftrace" section such as tracing only specific
2098 If CONFIG_DYNAMIC_FTRACE is set, the system will run with
2099 virtually no overhead when function tracing is disabled. The way
2100 this works is the mcount function call (placed at the start of
2101 every kernel function, produced by the -pg switch in gcc),
2102 starts of pointing to a simple return. (Enabling FTRACE will
2103 include the -pg switch in the compiling of the kernel.)
2105 At compile time every C file object is run through the
2106 recordmcount program (located in the scripts directory). This
2107 program will parse the ELF headers in the C object to find all
2108 the locations in the .text section that call mcount. (Note, only
2109 white listed .text sections are processed, since processing other
2110 sections like .init.text may cause races due to those sections
2111 being freed unexpectedly).
2113 A new section called "__mcount_loc" is created that holds
2114 references to all the mcount call sites in the .text section.
2115 The recordmcount program re-links this section back into the
2116 original object. The final linking stage of the kernel will add all these
2117 references into a single table.
2119 On boot up, before SMP is initialized, the dynamic ftrace code
2120 scans this table and updates all the locations into nops. It
2121 also records the locations, which are added to the
2122 available_filter_functions list. Modules are processed as they
2123 are loaded and before they are executed. When a module is
2124 unloaded, it also removes its functions from the ftrace function
2125 list. This is automatic in the module unload code, and the
2126 module author does not need to worry about it.
2128 When tracing is enabled, the process of modifying the function
2129 tracepoints is dependent on architecture. The old method is to use
2130 kstop_machine to prevent races with the CPUs executing code being
2131 modified (which can cause the CPU to do undesirable things, especially
2132 if the modified code crosses cache (or page) boundaries), and the nops are
2133 patched back to calls. But this time, they do not call mcount
2134 (which is just a function stub). They now call into the ftrace
2137 The new method of modifying the function tracepoints is to place
2138 a breakpoint at the location to be modified, sync all CPUs, modify
2139 the rest of the instruction not covered by the breakpoint. Sync
2140 all CPUs again, and then remove the breakpoint with the finished
2141 version to the ftrace call site.
2143 Some archs do not even need to monkey around with the synchronization,
2144 and can just slap the new code on top of the old without any
2145 problems with other CPUs executing it at the same time.
2147 One special side-effect to the recording of the functions being
2148 traced is that we can now selectively choose which functions we
2149 wish to trace and which ones we want the mcount calls to remain
2152 Two files are used, one for enabling and one for disabling the
2153 tracing of specified functions. They are:
2161 A list of available functions that you can add to these files is
2164 available_filter_functions
2166 # cat available_filter_functions
2175 If I am only interested in sys_nanosleep and hrtimer_interrupt:
2177 # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter
2178 # echo function > current_tracer
2179 # echo 1 > tracing_on
2181 # echo 0 > tracing_on
2185 # entries-in-buffer/entries-written: 5/5 #P:4
2188 # / _----=> need-resched
2189 # | / _---=> hardirq/softirq
2190 # || / _--=> preempt-depth
2192 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2194 usleep-2665 [001] .... 4186.475355: sys_nanosleep <-system_call_fastpath
2195 <idle>-0 [001] d.h1 4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt
2196 usleep-2665 [001] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
2197 <idle>-0 [003] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
2198 <idle>-0 [002] d.h1 4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt
2200 To see which functions are being traced, you can cat the file:
2202 # cat set_ftrace_filter
2207 Perhaps this is not enough. The filters also allow simple wild
2208 cards. Only the following are currently available
2210 <match>* - will match functions that begin with <match>
2211 *<match> - will match functions that end with <match>
2212 *<match>* - will match functions that have <match> in it
2214 These are the only wild cards which are supported.
2216 <match>*<match> will not work.
2218 Note: It is better to use quotes to enclose the wild cards,
2219 otherwise the shell may expand the parameters into names
2220 of files in the local directory.
2222 # echo 'hrtimer_*' > set_ftrace_filter
2228 # entries-in-buffer/entries-written: 897/897 #P:4
2231 # / _----=> need-resched
2232 # | / _---=> hardirq/softirq
2233 # || / _--=> preempt-depth
2235 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2237 <idle>-0 [003] dN.1 4228.547803: hrtimer_cancel <-tick_nohz_idle_exit
2238 <idle>-0 [003] dN.1 4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel
2239 <idle>-0 [003] dN.2 4228.547805: hrtimer_force_reprogram <-__remove_hrtimer
2240 <idle>-0 [003] dN.1 4228.547805: hrtimer_forward <-tick_nohz_idle_exit
2241 <idle>-0 [003] dN.1 4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
2242 <idle>-0 [003] d..1 4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt
2243 <idle>-0 [003] d..1 4228.547859: hrtimer_start <-__tick_nohz_idle_enter
2244 <idle>-0 [003] d..2 4228.547860: hrtimer_force_reprogram <-__rem
2246 Notice that we lost the sys_nanosleep.
2248 # cat set_ftrace_filter
2253 hrtimer_try_to_cancel
2257 hrtimer_force_reprogram
2258 hrtimer_get_next_event
2262 hrtimer_get_remaining
2264 hrtimer_init_sleeper
2267 This is because the '>' and '>>' act just like they do in bash.
2268 To rewrite the filters, use '>'
2269 To append to the filters, use '>>'
2271 To clear out a filter so that all functions will be recorded
2274 # echo > set_ftrace_filter
2275 # cat set_ftrace_filter
2278 Again, now we want to append.
2280 # echo sys_nanosleep > set_ftrace_filter
2281 # cat set_ftrace_filter
2283 # echo 'hrtimer_*' >> set_ftrace_filter
2284 # cat set_ftrace_filter
2289 hrtimer_try_to_cancel
2293 hrtimer_force_reprogram
2294 hrtimer_get_next_event
2299 hrtimer_get_remaining
2301 hrtimer_init_sleeper
2304 The set_ftrace_notrace prevents those functions from being
2307 # echo '*preempt*' '*lock*' > set_ftrace_notrace
2313 # entries-in-buffer/entries-written: 39608/39608 #P:4
2316 # / _----=> need-resched
2317 # | / _---=> hardirq/softirq
2318 # || / _--=> preempt-depth
2320 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2322 bash-1994 [000] .... 4342.324896: file_ra_state_init <-do_dentry_open
2323 bash-1994 [000] .... 4342.324897: open_check_o_direct <-do_last
2324 bash-1994 [000] .... 4342.324897: ima_file_check <-do_last
2325 bash-1994 [000] .... 4342.324898: process_measurement <-ima_file_check
2326 bash-1994 [000] .... 4342.324898: ima_get_action <-process_measurement
2327 bash-1994 [000] .... 4342.324898: ima_match_policy <-ima_get_action
2328 bash-1994 [000] .... 4342.324899: do_truncate <-do_last
2329 bash-1994 [000] .... 4342.324899: should_remove_suid <-do_truncate
2330 bash-1994 [000] .... 4342.324899: notify_change <-do_truncate
2331 bash-1994 [000] .... 4342.324900: current_fs_time <-notify_change
2332 bash-1994 [000] .... 4342.324900: current_kernel_time <-current_fs_time
2333 bash-1994 [000] .... 4342.324900: timespec_trunc <-current_fs_time
2335 We can see that there's no more lock or preempt tracing.
2338 Dynamic ftrace with the function graph tracer
2339 ---------------------------------------------
2341 Although what has been explained above concerns both the
2342 function tracer and the function-graph-tracer, there are some
2343 special features only available in the function-graph tracer.
2345 If you want to trace only one function and all of its children,
2346 you just have to echo its name into set_graph_function:
2348 echo __do_fault > set_graph_function
2350 will produce the following "expanded" trace of the __do_fault()
2354 0) | filemap_fault() {
2355 0) | find_lock_page() {
2356 0) 0.804 us | find_get_page();
2357 0) | __might_sleep() {
2361 0) 0.653 us | _spin_lock();
2362 0) 0.578 us | page_add_file_rmap();
2363 0) 0.525 us | native_set_pte_at();
2364 0) 0.585 us | _spin_unlock();
2365 0) | unlock_page() {
2366 0) 0.541 us | page_waitqueue();
2367 0) 0.639 us | __wake_up_bit();
2371 0) | filemap_fault() {
2372 0) | find_lock_page() {
2373 0) 0.698 us | find_get_page();
2374 0) | __might_sleep() {
2378 0) 0.631 us | _spin_lock();
2379 0) 0.571 us | page_add_file_rmap();
2380 0) 0.526 us | native_set_pte_at();
2381 0) 0.586 us | _spin_unlock();
2382 0) | unlock_page() {
2383 0) 0.533 us | page_waitqueue();
2384 0) 0.638 us | __wake_up_bit();
2388 You can also expand several functions at once:
2390 echo sys_open > set_graph_function
2391 echo sys_close >> set_graph_function
2393 Now if you want to go back to trace all functions you can clear
2394 this special filter via:
2396 echo > set_graph_function
2402 Note, the proc sysctl ftrace_enable is a big on/off switch for the
2403 function tracer. By default it is enabled (when function tracing is
2404 enabled in the kernel). If it is disabled, all function tracing is
2405 disabled. This includes not only the function tracers for ftrace, but
2406 also for any other uses (perf, kprobes, stack tracing, profiling, etc).
2408 Please disable this with care.
2410 This can be disable (and enabled) with:
2412 sysctl kernel.ftrace_enabled=0
2413 sysctl kernel.ftrace_enabled=1
2417 echo 0 > /proc/sys/kernel/ftrace_enabled
2418 echo 1 > /proc/sys/kernel/ftrace_enabled
2424 A few commands are supported by the set_ftrace_filter interface.
2425 Trace commands have the following format:
2427 <function>:<command>:<parameter>
2429 The following commands are supported:
2432 This command enables function filtering per module. The
2433 parameter defines the module. For example, if only the write*
2434 functions in the ext3 module are desired, run:
2436 echo 'write*:mod:ext3' > set_ftrace_filter
2438 This command interacts with the filter in the same way as
2439 filtering based on function names. Thus, adding more functions
2440 in a different module is accomplished by appending (>>) to the
2441 filter file. Remove specific module functions by prepending
2444 echo '!writeback*:mod:ext3' >> set_ftrace_filter
2446 Mod command supports module globbing. Disable tracing for all
2447 functions except a specific module:
2449 echo '!*:mod:!ext3' >> set_ftrace_filter
2451 Disable tracing for all modules, but still trace kernel:
2453 echo '!*:mod:*' >> set_ftrace_filter
2455 Enable filter only for kernel:
2457 echo '*write*:mod:!*' >> set_ftrace_filter
2459 Enable filter for module globbing:
2461 echo '*write*:mod:*snd*' >> set_ftrace_filter
2464 These commands turn tracing on and off when the specified
2465 functions are hit. The parameter determines how many times the
2466 tracing system is turned on and off. If unspecified, there is
2467 no limit. For example, to disable tracing when a schedule bug
2468 is hit the first 5 times, run:
2470 echo '__schedule_bug:traceoff:5' > set_ftrace_filter
2472 To always disable tracing when __schedule_bug is hit:
2474 echo '__schedule_bug:traceoff' > set_ftrace_filter
2476 These commands are cumulative whether or not they are appended
2477 to set_ftrace_filter. To remove a command, prepend it by '!'
2478 and drop the parameter:
2480 echo '!__schedule_bug:traceoff:0' > set_ftrace_filter
2482 The above removes the traceoff command for __schedule_bug
2483 that have a counter. To remove commands without counters:
2485 echo '!__schedule_bug:traceoff' > set_ftrace_filter
2488 Will cause a snapshot to be triggered when the function is hit.
2490 echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter
2492 To only snapshot once:
2494 echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter
2496 To remove the above commands:
2498 echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter
2499 echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter
2501 - enable_event/disable_event
2502 These commands can enable or disable a trace event. Note, because
2503 function tracing callbacks are very sensitive, when these commands
2504 are registered, the trace point is activated, but disabled in
2505 a "soft" mode. That is, the tracepoint will be called, but
2506 just will not be traced. The event tracepoint stays in this mode
2507 as long as there's a command that triggers it.
2509 echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \
2514 <function>:enable_event:<system>:<event>[:count]
2515 <function>:disable_event:<system>:<event>[:count]
2517 To remove the events commands:
2520 echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \
2522 echo '!schedule:disable_event:sched:sched_switch' > \
2526 When the function is hit, it will dump the contents of the ftrace
2527 ring buffer to the console. This is useful if you need to debug
2528 something, and want to dump the trace when a certain function
2529 is hit. Perhaps its a function that is called before a tripple
2530 fault happens and does not allow you to get a regular dump.
2533 When the function is hit, it will dump the contents of the ftrace
2534 ring buffer for the current CPU to the console. Unlike the "dump"
2535 command, it only prints out the contents of the ring buffer for the
2536 CPU that executed the function that triggered the dump.
2541 The trace_pipe outputs the same content as the trace file, but
2542 the effect on the tracing is different. Every read from
2543 trace_pipe is consumed. This means that subsequent reads will be
2544 different. The trace is live.
2546 # echo function > current_tracer
2547 # cat trace_pipe > /tmp/trace.out &
2549 # echo 1 > tracing_on
2551 # echo 0 > tracing_on
2555 # entries-in-buffer/entries-written: 0/0 #P:4
2558 # / _----=> need-resched
2559 # | / _---=> hardirq/softirq
2560 # || / _--=> preempt-depth
2562 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2566 # cat /tmp/trace.out
2567 bash-1994 [000] .... 5281.568961: mutex_unlock <-rb_simple_write
2568 bash-1994 [000] .... 5281.568963: __mutex_unlock_slowpath <-mutex_unlock
2569 bash-1994 [000] .... 5281.568963: __fsnotify_parent <-fsnotify_modify
2570 bash-1994 [000] .... 5281.568964: fsnotify <-fsnotify_modify
2571 bash-1994 [000] .... 5281.568964: __srcu_read_lock <-fsnotify
2572 bash-1994 [000] .... 5281.568964: add_preempt_count <-__srcu_read_lock
2573 bash-1994 [000] ...1 5281.568965: sub_preempt_count <-__srcu_read_lock
2574 bash-1994 [000] .... 5281.568965: __srcu_read_unlock <-fsnotify
2575 bash-1994 [000] .... 5281.568967: sys_dup2 <-system_call_fastpath
2578 Note, reading the trace_pipe file will block until more input is
2584 Having too much or not enough data can be troublesome in
2585 diagnosing an issue in the kernel. The file buffer_size_kb is
2586 used to modify the size of the internal trace buffers. The
2587 number listed is the number of entries that can be recorded per
2588 CPU. To know the full size, multiply the number of possible CPUs
2589 with the number of entries.
2591 # cat buffer_size_kb
2592 1408 (units kilobytes)
2594 Or simply read buffer_total_size_kb
2596 # cat buffer_total_size_kb
2599 To modify the buffer, simple echo in a number (in 1024 byte segments).
2601 # echo 10000 > buffer_size_kb
2602 # cat buffer_size_kb
2603 10000 (units kilobytes)
2605 It will try to allocate as much as possible. If you allocate too
2606 much, it can cause Out-Of-Memory to trigger.
2608 # echo 1000000000000 > buffer_size_kb
2609 -bash: echo: write error: Cannot allocate memory
2610 # cat buffer_size_kb
2613 The per_cpu buffers can be changed individually as well:
2615 # echo 10000 > per_cpu/cpu0/buffer_size_kb
2616 # echo 100 > per_cpu/cpu1/buffer_size_kb
2618 When the per_cpu buffers are not the same, the buffer_size_kb
2619 at the top level will just show an X
2621 # cat buffer_size_kb
2624 This is where the buffer_total_size_kb is useful:
2626 # cat buffer_total_size_kb
2629 Writing to the top level buffer_size_kb will reset all the buffers
2630 to be the same again.
2634 CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature
2635 available to all non latency tracers. (Latency tracers which
2636 record max latency, such as "irqsoff" or "wakeup", can't use
2637 this feature, since those are already using the snapshot
2638 mechanism internally.)
2640 Snapshot preserves a current trace buffer at a particular point
2641 in time without stopping tracing. Ftrace swaps the current
2642 buffer with a spare buffer, and tracing continues in the new
2643 current (=previous spare) buffer.
2645 The following debugfs files in "tracing" are related to this
2650 This is used to take a snapshot and to read the output
2651 of the snapshot. Echo 1 into this file to allocate a
2652 spare buffer and to take a snapshot (swap), then read
2653 the snapshot from this file in the same format as
2654 "trace" (described above in the section "The File
2655 System"). Both reads snapshot and tracing are executable
2656 in parallel. When the spare buffer is allocated, echoing
2657 0 frees it, and echoing else (positive) values clear the
2659 More details are shown in the table below.
2661 status\input | 0 | 1 | else |
2662 --------------+------------+------------+------------+
2663 not allocated |(do nothing)| alloc+swap |(do nothing)|
2664 --------------+------------+------------+------------+
2665 allocated | free | swap | clear |
2666 --------------+------------+------------+------------+
2668 Here is an example of using the snapshot feature.
2670 # echo 1 > events/sched/enable
2675 # entries-in-buffer/entries-written: 71/71 #P:8
2678 # / _----=> need-resched
2679 # | / _---=> hardirq/softirq
2680 # || / _--=> preempt-depth
2682 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2684 <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
2685 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
2687 <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
2692 # entries-in-buffer/entries-written: 77/77 #P:8
2695 # / _----=> need-resched
2696 # | / _---=> hardirq/softirq
2697 # || / _--=> preempt-depth
2699 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2701 <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
2702 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
2706 If you try to use this snapshot feature when current tracer is
2707 one of the latency tracers, you will get the following results.
2709 # echo wakeup > current_tracer
2711 bash: echo: write error: Device or resource busy
2713 cat: snapshot: Device or resource busy
2718 In the debugfs tracing directory is a directory called "instances".
2719 This directory can have new directories created inside of it using
2720 mkdir, and removing directories with rmdir. The directory created
2721 with mkdir in this directory will already contain files and other
2722 directories after it is created.
2724 # mkdir instances/foo
2726 buffer_size_kb buffer_total_size_kb events free_buffer per_cpu
2727 set_event snapshot trace trace_clock trace_marker trace_options
2728 trace_pipe tracing_on
2730 As you can see, the new directory looks similar to the tracing directory
2731 itself. In fact, it is very similar, except that the buffer and
2732 events are agnostic from the main director, or from any other
2733 instances that are created.
2735 The files in the new directory work just like the files with the
2736 same name in the tracing directory except the buffer that is used
2737 is a separate and new buffer. The files affect that buffer but do not
2738 affect the main buffer with the exception of trace_options. Currently,
2739 the trace_options affect all instances and the top level buffer
2740 the same, but this may change in future releases. That is, options
2741 may become specific to the instance they reside in.
2743 Notice that none of the function tracer files are there, nor is
2744 current_tracer and available_tracers. This is because the buffers
2745 can currently only have events enabled for them.
2747 # mkdir instances/foo
2748 # mkdir instances/bar
2749 # mkdir instances/zoot
2750 # echo 100000 > buffer_size_kb
2751 # echo 1000 > instances/foo/buffer_size_kb
2752 # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb
2753 # echo function > current_trace
2754 # echo 1 > instances/foo/events/sched/sched_wakeup/enable
2755 # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable
2756 # echo 1 > instances/foo/events/sched/sched_switch/enable
2757 # echo 1 > instances/bar/events/irq/enable
2758 # echo 1 > instances/zoot/events/syscalls/enable
2760 CPU:2 [LOST 11745 EVENTS]
2761 bash-2044 [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist
2762 bash-2044 [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave
2763 bash-2044 [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist
2764 bash-2044 [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist
2765 bash-2044 [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock
2766 bash-2044 [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype
2767 bash-2044 [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist
2768 bash-2044 [002] d... 10594.481034: zone_statistics <-get_page_from_freelist
2769 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics
2770 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics
2771 bash-2044 [002] .... 10594.481035: arch_dup_task_struct <-copy_process
2774 # cat instances/foo/trace_pipe
2775 bash-1998 [000] d..4 136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
2776 bash-1998 [000] dN.4 136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
2777 <idle>-0 [003] d.h3 136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003
2778 <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
2779 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
2780 bash-1998 [000] d..4 136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
2781 bash-1998 [000] dN.4 136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
2782 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
2783 kworker/0:1-59 [000] d..4 136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001
2784 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
2787 # cat instances/bar/trace_pipe
2788 migration/1-14 [001] d.h3 138.732674: softirq_raise: vec=3 [action=NET_RX]
2789 <idle>-0 [001] dNh3 138.732725: softirq_raise: vec=3 [action=NET_RX]
2790 bash-1998 [000] d.h1 138.733101: softirq_raise: vec=1 [action=TIMER]
2791 bash-1998 [000] d.h1 138.733102: softirq_raise: vec=9 [action=RCU]
2792 bash-1998 [000] ..s2 138.733105: softirq_entry: vec=1 [action=TIMER]
2793 bash-1998 [000] ..s2 138.733106: softirq_exit: vec=1 [action=TIMER]
2794 bash-1998 [000] ..s2 138.733106: softirq_entry: vec=9 [action=RCU]
2795 bash-1998 [000] ..s2 138.733109: softirq_exit: vec=9 [action=RCU]
2796 sshd-1995 [001] d.h1 138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4
2797 sshd-1995 [001] d.h1 138.733280: irq_handler_exit: irq=21 ret=unhandled
2798 sshd-1995 [001] d.h1 138.733281: irq_handler_entry: irq=21 name=eth0
2799 sshd-1995 [001] d.h1 138.733283: irq_handler_exit: irq=21 ret=handled
2802 # cat instances/zoot/trace
2805 # entries-in-buffer/entries-written: 18996/18996 #P:4
2808 # / _----=> need-resched
2809 # | / _---=> hardirq/softirq
2810 # || / _--=> preempt-depth
2812 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2814 bash-1998 [000] d... 140.733501: sys_write -> 0x2
2815 bash-1998 [000] d... 140.733504: sys_dup2(oldfd: a, newfd: 1)
2816 bash-1998 [000] d... 140.733506: sys_dup2 -> 0x1
2817 bash-1998 [000] d... 140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0)
2818 bash-1998 [000] d... 140.733509: sys_fcntl -> 0x1
2819 bash-1998 [000] d... 140.733510: sys_close(fd: a)
2820 bash-1998 [000] d... 140.733510: sys_close -> 0x0
2821 bash-1998 [000] d... 140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8)
2822 bash-1998 [000] d... 140.733515: sys_rt_sigprocmask -> 0x0
2823 bash-1998 [000] d... 140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8)
2824 bash-1998 [000] d... 140.733516: sys_rt_sigaction -> 0x0
2826 You can see that the trace of the top most trace buffer shows only
2827 the function tracing. The foo instance displays wakeups and task
2830 To remove the instances, simply delete their directories:
2832 # rmdir instances/foo
2833 # rmdir instances/bar
2834 # rmdir instances/zoot
2836 Note, if a process has a trace file open in one of the instance
2837 directories, the rmdir will fail with EBUSY.
2842 Since the kernel has a fixed sized stack, it is important not to
2843 waste it in functions. A kernel developer must be conscience of
2844 what they allocate on the stack. If they add too much, the system
2845 can be in danger of a stack overflow, and corruption will occur,
2846 usually leading to a system panic.
2848 There are some tools that check this, usually with interrupts
2849 periodically checking usage. But if you can perform a check
2850 at every function call that will become very useful. As ftrace provides
2851 a function tracer, it makes it convenient to check the stack size
2852 at every function call. This is enabled via the stack tracer.
2854 CONFIG_STACK_TRACER enables the ftrace stack tracing functionality.
2855 To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled.
2857 # echo 1 > /proc/sys/kernel/stack_tracer_enabled
2859 You can also enable it from the kernel command line to trace
2860 the stack size of the kernel during boot up, by adding "stacktrace"
2861 to the kernel command line parameter.
2863 After running it for a few minutes, the output looks like:
2865 # cat stack_max_size
2869 Depth Size Location (18 entries)
2871 0) 2928 224 update_sd_lb_stats+0xbc/0x4ac
2872 1) 2704 160 find_busiest_group+0x31/0x1f1
2873 2) 2544 256 load_balance+0xd9/0x662
2874 3) 2288 80 idle_balance+0xbb/0x130
2875 4) 2208 128 __schedule+0x26e/0x5b9
2876 5) 2080 16 schedule+0x64/0x66
2877 6) 2064 128 schedule_timeout+0x34/0xe0
2878 7) 1936 112 wait_for_common+0x97/0xf1
2879 8) 1824 16 wait_for_completion+0x1d/0x1f
2880 9) 1808 128 flush_work+0xfe/0x119
2881 10) 1680 16 tty_flush_to_ldisc+0x1e/0x20
2882 11) 1664 48 input_available_p+0x1d/0x5c
2883 12) 1616 48 n_tty_poll+0x6d/0x134
2884 13) 1568 64 tty_poll+0x64/0x7f
2885 14) 1504 880 do_select+0x31e/0x511
2886 15) 624 400 core_sys_select+0x177/0x216
2887 16) 224 96 sys_select+0x91/0xb9
2888 17) 128 128 system_call_fastpath+0x16/0x1b
2890 Note, if -mfentry is being used by gcc, functions get traced before
2891 they set up the stack frame. This means that leaf level functions
2892 are not tested by the stack tracer when -mfentry is used.
2894 Currently, -mfentry is used by gcc 4.6.0 and above on x86 only.
2898 More details can be found in the source code, in the
2899 kernel/trace/*.c files.
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