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diff --git a/Documentation/ftrace.txt b/Documentation/ftrace.txt deleted file mode 100644 index 803b1318b13..00000000000 --- a/Documentation/ftrace.txt +++ /dev/null @@ -1,1424 +0,0 @@ - ftrace - Function Tracer - ======================== - -Copyright 2008 Red Hat Inc. - Author: Steven Rostedt <srostedt@redhat.com> - License: The GNU Free Documentation License, Version 1.2 - (dual licensed under the GPL v2) -Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton, - John Kacur, and David Teigland. - -Written for: 2.6.28-rc2 - -Introduction ------------- - -Ftrace is an internal tracer designed to help out developers and -designers of systems to find what is going on inside the kernel. -It can be used for debugging or analyzing latencies and performance -issues that take place outside of user-space. - -Although ftrace is the function tracer, it also includes an -infrastructure that allows for other types of tracing. Some of the -tracers that are currently in ftrace include a tracer to trace -context switches, the time it takes for a high priority task to -run after it was woken up, the time interrupts are disabled, and -more (ftrace allows for tracer plugins, which means that the list of -tracers can always grow). - - -The File System ---------------- - -Ftrace uses the debugfs file system to hold the control files as well -as the files to display output. - -To mount the debugfs system: - - # mkdir /debug - # mount -t debugfs nodev /debug - -(Note: it is more common to mount at /sys/kernel/debug, but for simplicity - this document will use /debug) - -That's it! (assuming that you have ftrace configured into your kernel) - -After mounting the debugfs, you can see a directory called -"tracing". This directory contains the control and output files -of ftrace. Here is a list of some of the key files: - - - Note: all time values are in microseconds. - - current_tracer: This is used to set or display the current tracer - that is configured. - - available_tracers: This holds the different types of tracers that - have been compiled into the kernel. The tracers - listed here can be configured by echoing their name - into current_tracer. - - tracing_enabled: This sets or displays whether the current_tracer - is activated and tracing or not. Echo 0 into this - file to disable the tracer or 1 to enable it. - - trace: This file holds the output of the trace in a human readable - format (described below). - - latency_trace: This file shows the same trace but the information - is organized more to display possible latencies - in the system (described below). - - trace_pipe: The output is the same as the "trace" file but this - file is meant to be streamed with live tracing. - Reads from this file will block until new data - is retrieved. Unlike the "trace" and "latency_trace" - files, this file is a consumer. This means reading - from this file causes sequential reads to display - more current data. Once data is read from this - file, it is consumed, and will not be read - again with a sequential read. The "trace" and - "latency_trace" files are static, and if the - tracer is not adding more data, they will display - the same information every time they are read. - - trace_options: This file lets the user control the amount of data - that is displayed in one of the above output - files. - - trace_max_latency: Some of the tracers record the max latency. - For example, the time interrupts are disabled. - This time is saved in this file. The max trace - will also be stored, and displayed by either - "trace" or "latency_trace". A new max trace will - only be recorded if the latency is greater than - the value in this file. (in microseconds) - - buffer_size_kb: This sets or displays the number of kilobytes each CPU - buffer can hold. The tracer buffers are the same size - for each CPU. The displayed number is the size of the - CPU buffer and not total size of all buffers. The - trace buffers are allocated in pages (blocks of memory - that the kernel uses for allocation, usually 4 KB in size). - If the last page allocated has room for more bytes - than requested, the rest of the page will be used, - making the actual allocation bigger than requested. - (Note, the size may not be a multiple of the page size due - to buffer managment overhead.) - - This can only be updated when the current_tracer - is set to "nop". - - tracing_cpumask: This is a mask that lets the user only trace - on specified CPUS. The format is a hex string - representing the CPUS. - - set_ftrace_filter: When dynamic ftrace is configured in (see the - section below "dynamic ftrace"), the code is dynamically - modified (code text rewrite) to disable calling of the - function profiler (mcount). This lets tracing be configured - in with practically no overhead in performance. This also - has a side effect of enabling or disabling specific functions - to be traced. Echoing names of functions into this file - will limit the trace to only those functions. - - set_ftrace_notrace: This has an effect opposite to that of - set_ftrace_filter. Any function that is added here will not - be traced. If a function exists in both set_ftrace_filter - and set_ftrace_notrace, the function will _not_ be traced. - - set_ftrace_pid: Have the function tracer only trace a single thread. - - available_filter_functions: This lists the functions that ftrace - has processed and can trace. These are the function - names that you can pass to "set_ftrace_filter" or - "set_ftrace_notrace". (See the section "dynamic ftrace" - below for more details.) - - -The Tracers ------------ - -Here is the list of current tracers that may be configured. - - function - function tracer that uses mcount to trace all functions. - - sched_switch - traces the context switches between tasks. - - irqsoff - traces the areas that disable interrupts and saves - the trace with the longest max latency. - See tracing_max_latency. When a new max is recorded, - it replaces the old trace. It is best to view this - trace via the latency_trace file. - - preemptoff - Similar to irqsoff but traces and records the amount of - time for which preemption is disabled. - - preemptirqsoff - Similar to irqsoff and preemptoff, but traces and - records the largest time for which irqs and/or preemption - is disabled. - - wakeup - Traces and records the max latency that it takes for - the highest priority task to get scheduled after - it has been woken up. - - nop - This is not a tracer. To remove all tracers from tracing - simply echo "nop" into current_tracer. - - -Examples of using the tracer ----------------------------- - -Here are typical examples of using the tracers when controlling them only -with the debugfs interface (without using any user-land utilities). - -Output format: --------------- - -Here is an example of the output format of the file "trace" - - -------- -# tracer: function -# -# TASK-PID CPU# TIMESTAMP FUNCTION -# | | | | | - bash-4251 [01] 10152.583854: path_put <-path_walk - bash-4251 [01] 10152.583855: dput <-path_put - bash-4251 [01] 10152.583855: _atomic_dec_and_lock <-dput - -------- - -A header is printed with the tracer name that is represented by the trace. -In this case the tracer is "function". Then a header showing the format. Task -name "bash", the task PID "4251", the CPU that it was running on -"01", the timestamp in <secs>.<usecs> format, the function name that was -traced "path_put" and the parent function that called this function -"path_walk". The timestamp is the time at which the function was -entered. - -The sched_switch tracer also includes tracing of task wakeups and -context switches. - - ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 2916:115:S - ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 10:115:S - ksoftirqd/1-7 [01] 1453.070013: 7:115:R ==> 10:115:R - events/1-10 [01] 1453.070013: 10:115:S ==> 2916:115:R - kondemand/1-2916 [01] 1453.070013: 2916:115:S ==> 7:115:R - ksoftirqd/1-7 [01] 1453.070013: 7:115:S ==> 0:140:R - -Wake ups are represented by a "+" and the context switches are shown as -"==>". The format is: - - Context switches: - - Previous task Next Task - - <pid>:<prio>:<state> ==> <pid>:<prio>:<state> - - Wake ups: - - Current task Task waking up - - <pid>:<prio>:<state> + <pid>:<prio>:<state> - -The prio is the internal kernel priority, which is the inverse of the -priority that is usually displayed by user-space tools. Zero represents -the highest priority (99). Prio 100 starts the "nice" priorities with -100 being equal to nice -20 and 139 being nice 19. The prio "140" is -reserved for the idle task which is the lowest priority thread (pid 0). - - -Latency trace format --------------------- - -For traces that display latency times, the latency_trace file gives -somewhat more information to see why a latency happened. Here is a typical -trace. - -# tracer: irqsoff -# -irqsoff latency trace v1.1.5 on 2.6.26-rc8 --------------------------------------------------------------------- - latency: 97 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) - ----------------- - | task: swapper-0 (uid:0 nice:0 policy:0 rt_prio:0) - ----------------- - => started at: apic_timer_interrupt - => ended at: do_softirq - -# _------=> CPU# -# / _-----=> irqs-off -# | / _----=> need-resched -# || / _---=> hardirq/softirq -# ||| / _--=> preempt-depth -# |||| / -# ||||| delay -# cmd pid ||||| time | caller -# \ / ||||| \ | / - <idle>-0 0d..1 0us+: trace_hardirqs_off_thunk (apic_timer_interrupt) - <idle>-0 0d.s. 97us : __do_softirq (do_softirq) - <idle>-0 0d.s1 98us : trace_hardirqs_on (do_softirq) - - - -This shows that the current tracer is "irqsoff" tracing the time for which -interrupts were disabled. It gives the trace version and the version -of the kernel upon which this was executed on (2.6.26-rc8). Then it displays -the max latency in microsecs (97 us). The number of trace entries displayed -and the total number recorded (both are three: #3/3). The type of -preemption that was used (PREEMPT). VP, KP, SP, and HP are always zero -and are reserved for later use. #P is the number of online CPUS (#P:2). - -The task is the process that was running when the latency occurred. -(swapper pid: 0). - -The start and stop (the functions in which the interrupts were disabled and -enabled respectively) that caused the latencies: - - apic_timer_interrupt is where the interrupts were disabled. - do_softirq is where they were enabled again. - -The next lines after the header are the trace itself. The header -explains which is which. - - cmd: The name of the process in the trace. - - pid: The PID of that process. - - CPU#: The CPU which the process was running on. - - irqs-off: 'd' interrupts are disabled. '.' otherwise. - Note: If the architecture does not support a way to - read the irq flags variable, an 'X' will always - be printed here. - - need-resched: 'N' task need_resched is set, '.' otherwise. - - hardirq/softirq: - 'H' - hard irq occurred inside a softirq. - 'h' - hard irq is running - 's' - soft irq is running - '.' - normal context. - - preempt-depth: The level of preempt_disabled - -The above is mostly meaningful for kernel developers. - - time: This differs from the trace file output. The trace file output - includes an absolute timestamp. The timestamp used by the - latency_trace file is relative to the start of the trace. - - delay: This is just to help catch your eye a bit better. And - needs to be fixed to be only relative to the same CPU. - The marks are determined by the difference between this - current trace and the next trace. - '!' - greater than preempt_mark_thresh (default 100) - '+' - greater than 1 microsecond - ' ' - less than or equal to 1 microsecond. - - The rest is the same as the 'trace' file. - - -trace_options -------------- - -The trace_options file is used to control what gets printed in the trace -output. To see what is available, simply cat the file: - - cat /debug/tracing/trace_options - print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \ - noblock nostacktrace nosched-tree nouserstacktrace nosym-userobj - -To disable one of the options, echo in the option prepended with "no". - - echo noprint-parent > /debug/tracing/trace_options - -To enable an option, leave off the "no". - - echo sym-offset > /debug/tracing/trace_options - -Here are the available options: - - print-parent - On function traces, display the calling function - as well as the function being traced. - - print-parent: - bash-4000 [01] 1477.606694: simple_strtoul <-strict_strtoul - - noprint-parent: - bash-4000 [01] 1477.606694: simple_strtoul - - - sym-offset - Display not only the function name, but also the offset - in the function. For example, instead of seeing just - "ktime_get", you will see "ktime_get+0xb/0x20". - - sym-offset: - bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0 - - sym-addr - this will also display the function address as well as - the function name. - - sym-addr: - bash-4000 [01] 1477.606694: simple_strtoul <c0339346> - - verbose - This deals with the latency_trace file. - - bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \ - (+0.000ms): simple_strtoul (strict_strtoul) - - raw - This will display raw numbers. This option is best for use with - user applications that can translate the raw numbers better than - having it done in the kernel. - - hex - Similar to raw, but the numbers will be in a hexadecimal format. - - bin - This will print out the formats in raw binary. - - block - TBD (needs update) - - stacktrace - This is one of the options that changes the trace itself. - When a trace is recorded, so is the stack of functions. - This allows for back traces of trace sites. - - userstacktrace - This option changes the trace. - It records a stacktrace of the current userspace thread. - - sym-userobj - when user stacktrace are enabled, look up which object the - address belongs to, and print a relative address - This is especially useful when ASLR is on, otherwise you don't - get a chance to resolve the address to object/file/line after the app is no - longer running - - The lookup is performed when you read trace,trace_pipe,latency_trace. Example: - - a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0 -x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6] - - sched-tree - TBD (any users??) - - -sched_switch ------------- - -This tracer simply records schedule switches. Here is an example -of how to use it. - - # echo sched_switch > /debug/tracing/current_tracer - # echo 1 > /debug/tracing/tracing_enabled - # sleep 1 - # echo 0 > /debug/tracing/tracing_enabled - # cat /debug/tracing/trace - -# tracer: sched_switch -# -# TASK-PID CPU# TIMESTAMP FUNCTION -# | | | | | - bash-3997 [01] 240.132281: 3997:120:R + 4055:120:R - bash-3997 [01] 240.132284: 3997:120:R ==> 4055:120:R - sleep-4055 [01] 240.132371: 4055:120:S ==> 3997:120:R - bash-3997 [01] 240.132454: 3997:120:R + 4055:120:S - bash-3997 [01] 240.132457: 3997:120:R ==> 4055:120:R - sleep-4055 [01] 240.132460: 4055:120:D ==> 3997:120:R - bash-3997 [01] 240.132463: 3997:120:R + 4055:120:D - bash-3997 [01] 240.132465: 3997:120:R ==> 4055:120:R - <idle>-0 [00] 240.132589: 0:140:R + 4:115:S - <idle>-0 [00] 240.132591: 0:140:R ==> 4:115:R - ksoftirqd/0-4 [00] 240.132595: 4:115:S ==> 0:140:R - <idle>-0 [00] 240.132598: 0:140:R + 4:115:S - <idle>-0 [00] 240.132599: 0:140:R ==> 4:115:R - ksoftirqd/0-4 [00] 240.132603: 4:115:S ==> 0:140:R - sleep-4055 [01] 240.133058: 4055:120:S ==> 3997:120:R - [...] - - -As we have discussed previously about this format, the header shows -the name of the trace and points to the options. The "FUNCTION" -is a misnomer since here it represents the wake ups and context -switches. - -The sched_switch file only lists the wake ups (represented with '+') -and context switches ('==>') with the previous task or current task -first followed by the next task or task waking up. The format for both -of these is PID:KERNEL-PRIO:TASK-STATE. Remember that the KERNEL-PRIO -is the inverse of the actual priority with zero (0) being the highest -priority and the nice values starting at 100 (nice -20). Below is -a quick chart to map the kernel priority to user land priorities. - - Kernel priority: 0 to 99 ==> user RT priority 99 to 0 - Kernel priority: 100 to 139 ==> user nice -20 to 19 - Kernel priority: 140 ==> idle task priority - -The task states are: - - R - running : wants to run, may not actually be running - S - sleep : process is waiting to be woken up (handles signals) - D - disk sleep (uninterruptible sleep) : process must be woken up - (ignores signals) - T - stopped : process suspended - t - traced : process is being traced (with something like gdb) - Z - zombie : process waiting to be cleaned up - X - unknown - - -ftrace_enabled --------------- - -The following tracers (listed below) give different output depending -on whether or not the sysctl ftrace_enabled is set. To set ftrace_enabled, -one can either use the sysctl function or set it via the proc -file system interface. - - sysctl kernel.ftrace_enabled=1 - - or - - echo 1 > /proc/sys/kernel/ftrace_enabled - -To disable ftrace_enabled simply replace the '1' with '0' in -the above commands. - -When ftrace_enabled is set the tracers will also record the functions -that are within the trace. The descriptions of the tracers -will also show an example with ftrace enabled. - - -irqsoff -------- - -When interrupts are disabled, the CPU can not react to any other -external event (besides NMIs and SMIs). This prevents the timer -interrupt from triggering or the mouse interrupt from letting the -kernel know of a new mouse event. The result is a latency with the -reaction time. - -The irqsoff tracer tracks the time for which interrupts are disabled. -When a new maximum latency is hit, the tracer saves the trace leading up -to that latency point so that every time a new maximum is reached, the old -saved trace is discarded and the new trace is saved. - -To reset the maximum, echo 0 into tracing_max_latency. Here is an -example: - - # echo irqsoff > /debug/tracing/current_tracer - # echo 0 > /debug/tracing/tracing_max_latency - # echo 1 > /debug/tracing/tracing_enabled - # ls -ltr - [...] - # echo 0 > /debug/tracing/tracing_enabled - # cat /debug/tracing/latency_trace -# tracer: irqsoff -# -irqsoff latency trace v1.1.5 on 2.6.26 --------------------------------------------------------------------- - latency: 12 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) - ----------------- - | task: bash-3730 (uid:0 nice:0 policy:0 rt_prio:0) - ----------------- - => started at: sys_setpgid - => ended at: sys_setpgid - -# _------=> CPU# -# / _-----=> irqs-off -# | / _----=> need-resched -# || / _---=> hardirq/softirq -# ||| / _--=> preempt-depth -# |||| / -# ||||| delay -# cmd pid ||||| time | caller -# \ / ||||| \ | / - bash-3730 1d... 0us : _write_lock_irq (sys_setpgid) - bash-3730 1d..1 1us+: _write_unlock_irq (sys_setpgid) - bash-3730 1d..2 14us : trace_hardirqs_on (sys_setpgid) - - -Here we see that that we had a latency of 12 microsecs (which is -very good). The _write_lock_irq in sys_setpgid disabled interrupts. -The difference between the 12 and the displayed timestamp 14us occurred -because the clock was incremented between the time of recording the max -latency and the time of recording the function that had that latency. - -Note the above example had ftrace_enabled not set. If we set the -ftrace_enabled, we get a much larger output: - -# tracer: irqsoff -# -irqsoff latency trace v1.1.5 on 2.6.26-rc8 --------------------------------------------------------------------- - latency: 50 us, #101/101, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) - ----------------- - | task: ls-4339 (uid:0 nice:0 policy:0 rt_prio:0) - ----------------- - => started at: __alloc_pages_internal - => ended at: __alloc_pages_internal - -# _------=> CPU# -# / _-----=> irqs-off -# | / _----=> need-resched -# || / _---=> hardirq/softirq -# ||| / _--=> preempt-depth -# |||| / -# ||||| delay -# cmd pid ||||| time | caller -# \ / ||||| \ | / - ls-4339 0...1 0us+: get_page_from_freelist (__alloc_pages_internal) - ls-4339 0d..1 3us : rmqueue_bulk (get_page_from_freelist) - ls-4339 0d..1 3us : _spin_lock (rmqueue_bulk) - ls-4339 0d..1 4us : add_preempt_count (_spin_lock) - ls-4339 0d..2 4us : __rmqueue (rmqueue_bulk) - ls-4339 0d..2 5us : __rmqueue_smallest (__rmqueue) - ls-4339 0d..2 5us : __mod_zone_page_state (__rmqueue_smallest) - ls-4339 0d..2 6us : __rmqueue (rmqueue_bulk) - ls-4339 0d..2 6us : __rmqueue_smallest (__rmqueue) - ls-4339 0d..2 7us : __mod_zone_page_state (__rmqueue_smallest) - ls-4339 0d..2 7us : __rmqueue (rmqueue_bulk) - ls-4339 0d..2 8us : __rmqueue_smallest (__rmqueue) -[...] - ls-4339 0d..2 46us : __rmqueue_smallest (__rmqueue) - ls-4339 0d..2 47us : __mod_zone_page_state (__rmqueue_smallest) - ls-4339 0d..2 47us : __rmqueue (rmqueue_bulk) - ls-4339 0d..2 48us : __rmqueue_smallest (__rmqueue) - ls-4339 0d..2 48us : __mod_zone_page_state (__rmqueue_smallest) - ls-4339 0d..2 49us : _spin_unlock (rmqueue_bulk) - ls-4339 0d..2 49us : sub_preempt_count (_spin_unlock) - ls-4339 0d..1 50us : get_page_from_freelist (__alloc_pages_internal) - ls-4339 0d..2 51us : trace_hardirqs_on (__alloc_pages_internal) - - - -Here we traced a 50 microsecond latency. But we also see all the -functions that were called during that time. Note that by enabling -function tracing, we incur an added overhead. This overhead may -extend the latency times. But nevertheless, this trace has provided -some very helpful debugging information. - - -preemptoff ----------- - -When preemption is disabled, we may be able to receive interrupts but -the task cannot be preempted and a higher priority task must wait -for preemption to be enabled again before it can preempt a lower -priority task. - -The preemptoff tracer traces the places that disable preemption. -Like the irqsoff tracer, it records the maximum latency for which preemption -was disabled. The control of preemptoff tracer is much like the irqsoff -tracer. - - # echo preemptoff > /debug/tracing/current_tracer - # echo 0 > /debug/tracing/tracing_max_latency - # echo 1 > /debug/tracing/tracing_enabled - # ls -ltr - [...] - # echo 0 > /debug/tracing/tracing_enabled - # cat /debug/tracing/latency_trace -# tracer: preemptoff -# -preemptoff latency trace v1.1.5 on 2.6.26-rc8 --------------------------------------------------------------------- - latency: 29 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) - ----------------- - | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) - ----------------- - => started at: do_IRQ - => ended at: __do_softirq - -# _------=> CPU# -# / _-----=> irqs-off -# | / _----=> need-resched -# || / _---=> hardirq/softirq -# ||| / _--=> preempt-depth -# |||| / -# ||||| delay -# cmd pid ||||| time | caller -# \ / ||||| \ | / - sshd-4261 0d.h. 0us+: irq_enter (do_IRQ) - sshd-4261 0d.s. 29us : _local_bh_enable (__do_softirq) - sshd-4261 0d.s1 30us : trace_preempt_on (__do_softirq) - - -This has some more changes. Preemption was disabled when an interrupt -came in (notice the 'h'), and was enabled while doing a softirq. -(notice the 's'). But we also see that interrupts have been disabled -when entering the preempt off section and leaving it (the 'd'). -We do not know if interrupts were enabled in the mean time. - -# tracer: preemptoff -# -preemptoff latency trace v1.1.5 on 2.6.26-rc8 --------------------------------------------------------------------- - latency: 63 us, #87/87, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) - ----------------- - | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) - ----------------- - => started at: remove_wait_queue - => ended at: __do_softirq - -# _------=> CPU# -# / _-----=> irqs-off -# | / _----=> need-resched -# || / _---=> hardirq/softirq -# ||| / _--=> preempt-depth -# |||| / -# ||||| delay -# cmd pid ||||| time | caller -# \ / ||||| \ | / - sshd-4261 0d..1 0us : _spin_lock_irqsave (remove_wait_queue) - sshd-4261 0d..1 1us : _spin_unlock_irqrestore (remove_wait_queue) - sshd-4261 0d..1 2us : do_IRQ (common_interrupt) - sshd-4261 0d..1 2us : irq_enter (do_IRQ) - sshd-4261 0d..1 2us : idle_cpu (irq_enter) - sshd-4261 0d..1 3us : add_preempt_count (irq_enter) - sshd-4261 0d.h1 3us : idle_cpu (irq_enter) - sshd-4261 0d.h. 4us : handle_fasteoi_irq (do_IRQ) -[...] - sshd-4261 0d.h. 12us : add_preempt_count (_spin_lock) - sshd-4261 0d.h1 12us : ack_ioapic_quirk_irq (handle_fasteoi_irq) - sshd-4261 0d.h1 13us : move_native_irq (ack_ioapic_quirk_irq) - sshd-4261 0d.h1 13us : _spin_unlock (handle_fasteoi_irq) - sshd-4261 0d.h1 14us : sub_preempt_count (_spin_unlock) - sshd-4261 0d.h1 14us : irq_exit (do_IRQ) - sshd-4261 0d.h1 15us : sub_preempt_count (irq_exit) - sshd-4261 0d..2 15us : do_softirq (irq_exit) - sshd-4261 0d... 15us : __do_softirq (do_softirq) - sshd-4261 0d... 16us : __local_bh_disable (__do_softirq) - sshd-4261 0d... 16us+: add_preempt_count (__local_bh_disable) - sshd-4261 0d.s4 20us : add_preempt_count (__local_bh_disable) - sshd-4261 0d.s4 21us : sub_preempt_count (local_bh_enable) - sshd-4261 0d.s5 21us : sub_preempt_count (local_bh_enable) -[...] - sshd-4261 0d.s6 41us : add_preempt_count (__local_bh_disable) - sshd-4261 0d.s6 42us : sub_preempt_count (local_bh_enable) - sshd-4261 0d.s7 42us : sub_preempt_count (local_bh_enable) - sshd-4261 0d.s5 43us : add_preempt_count (__local_bh_disable) - sshd-4261 0d.s5 43us : sub_preempt_count (local_bh_enable_ip) - sshd-4261 0d.s6 44us : sub_preempt_count (local_bh_enable_ip) - sshd-4261 0d.s5 44us : add_preempt_count (__local_bh_disable) - sshd-4261 0d.s5 45us : sub_preempt_count (local_bh_enable) -[...] - sshd-4261 0d.s. 63us : _local_bh_enable (__do_softirq) - sshd-4261 0d.s1 64us : trace_preempt_on (__do_softirq) - - -The above is an example of the preemptoff trace with ftrace_enabled -set. Here we see that interrupts were disabled the entire time. -The irq_enter code lets us know that we entered an interrupt 'h'. -Before that, the functions being traced still show that it is not -in an interrupt, but we can see from the functions themselves that -this is not the case. - -Notice that __do_softirq when called does not have a preempt_count. -It may seem that we missed a preempt enabling. What really happened -is that the preempt count is held on the thread's stack and we -switched to the softirq stack (4K stacks in effect). The code -does not copy the preempt count, but because interrupts are disabled, -we do not need to worry about it. Having a tracer like this is good -for letting people know what really happens inside the kernel. - - -preemptirqsoff --------------- - -Knowing the locations that have interrupts disabled or preemption -disabled for the longest times is helpful. But sometimes we would -like to know when either preemption and/or interrupts are disabled. - -Consider the following code: - - local_irq_disable(); - call_function_with_irqs_off(); - preempt_disable(); - call_function_with_irqs_and_preemption_off(); - local_irq_enable(); - call_function_with_preemption_off(); - preempt_enable(); - -The irqsoff tracer will record the total length of -call_function_with_irqs_off() and -call_function_with_irqs_and_preemption_off(). - -The preemptoff tracer will record the total length of -call_function_with_irqs_and_preemption_off() and -call_function_with_preemption_off(). - -But neither will trace the time that interrupts and/or preemption -is disabled. This total time is the time that we can not schedule. -To record this time, use the preemptirqsoff tracer. - -Again, using this trace is much like the irqsoff and preemptoff tracers. - - # echo preemptirqsoff > /debug/tracing/current_tracer - # echo 0 > /debug/tracing/tracing_max_latency - # echo 1 > /debug/tracing/tracing_enabled - # ls -ltr - [...] - # echo 0 > /debug/tracing/tracing_enabled - # cat /debug/tracing/latency_trace -# tracer: preemptirqsoff -# -preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8 --------------------------------------------------------------------- - latency: 293 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) - ----------------- - | task: ls-4860 (uid:0 nice:0 policy:0 rt_prio:0) - ----------------- - => started at: apic_timer_interrupt - => ended at: __do_softirq - -# _------=> CPU# -# / _-----=> irqs-off -# | / _----=> need-resched -# || / _---=> hardirq/softirq -# ||| / _--=> preempt-depth -# |||| / -# ||||| delay -# cmd pid ||||| time | caller -# \ / ||||| \ | / - ls-4860 0d... 0us!: trace_hardirqs_off_thunk (apic_timer_interrupt) - ls-4860 0d.s. 294us : _local_bh_enable (__do_softirq) - ls-4860 0d.s1 294us : trace_preempt_on (__do_softirq) - - - -The trace_hardirqs_off_thunk is called from assembly on x86 when -interrupts are disabled in the assembly code. Without the function -tracing, we do not know if interrupts were enabled within the preemption -points. We do see that it started with preemption enabled. - -Here is a trace with ftrace_enabled set: - - -# tracer: preemptirqsoff -# -preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8 --------------------------------------------------------------------- - latency: 105 us, #183/183, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) - ----------------- - | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) - ----------------- - => started at: write_chan - => ended at: __do_softirq - -# _------=> CPU# -# / _-----=> irqs-off -# | / _----=> need-resched -# || / _---=> hardirq/softirq -# ||| / _--=> preempt-depth -# |||| / -# ||||| delay -# cmd pid ||||| time | caller -# \ / ||||| \ | / - ls-4473 0.N.. 0us : preempt_schedule (write_chan) - ls-4473 0dN.1 1us : _spin_lock (schedule) - ls-4473 0dN.1 2us : add_preempt_count (_spin_lock) - ls-4473 0d..2 2us : put_prev_task_fair (schedule) -[...] - ls-4473 0d..2 13us : set_normalized_timespec (ktime_get_ts) - ls-4473 0d..2 13us : __switch_to (schedule) - sshd-4261 0d..2 14us : finish_task_switch (schedule) - sshd-4261 0d..2 14us : _spin_unlock_irq (finish_task_switch) - sshd-4261 0d..1 15us : add_preempt_count (_spin_lock_irqsave) - sshd-4261 0d..2 16us : _spin_unlock_irqrestore (hrtick_set) - sshd-4261 0d..2 16us : do_IRQ (common_interrupt) - sshd-4261 0d..2 17us : irq_enter (do_IRQ) - sshd-4261 0d..2 17us : idle_cpu (irq_enter) - sshd-4261 0d..2 18us : add_preempt_count (irq_enter) - sshd-4261 0d.h2 18us : idle_cpu (irq_enter) - sshd-4261 0d.h. 18us : handle_fasteoi_irq (do_IRQ) - sshd-4261 0d.h. 19us : _spin_lock (handle_fasteoi_irq) - sshd-4261 0d.h. 19us : add_preempt_count (_spin_lock) - sshd-4261 0d.h1 20us : _spin_unlock (handle_fasteoi_irq) - sshd-4261 0d.h1 20us : sub_preempt_count (_spin_unlock) -[...] - sshd-4261 0d.h1 28us : _spin_unlock (handle_fasteoi_irq) - sshd-4261 0d.h1 29us : sub_preempt_count (_spin_unlock) - sshd-4261 0d.h2 29us : irq_exit (do_IRQ) - sshd-4261 0d.h2 29us : sub_preempt_count (irq_exit) - sshd-4261 0d..3 30us : do_softirq (irq_exit) - sshd-4261 0d... 30us : __do_softirq (do_softirq) - sshd-4261 0d... 31us : __local_bh_disable (__do_softirq) - sshd-4261 0d... 31us+: add_preempt_count (__local_bh_disable) - sshd-4261 0d.s4 34us : add_preempt_count (__local_bh_disable) -[...] - sshd-4261 0d.s3 43us : sub_preempt_count (local_bh_enable_ip) - sshd-4261 0d.s4 44us : sub_preempt_count (local_bh_enable_ip) - sshd-4261 0d.s3 44us : smp_apic_timer_interrupt (apic_timer_interrupt) - sshd-4261 0d.s3 45us : irq_enter (smp_apic_timer_interrupt) - sshd-4261 0d.s3 45us : idle_cpu (irq_enter) - sshd-4261 0d.s3 46us : add_preempt_count (irq_enter) - sshd-4261 0d.H3 46us : idle_cpu (irq_enter) - sshd-4261 0d.H3 47us : hrtimer_interrupt (smp_apic_timer_interrupt) - sshd-4261 0d.H3 47us : ktime_get (hrtimer_interrupt) -[...] - sshd-4261 0d.H3 81us : tick_program_event (hrtimer_interrupt) - sshd-4261 0d.H3 82us : ktime_get (tick_program_event) - sshd-4261 0d.H3 82us : ktime_get_ts (ktime_get) - sshd-4261 0d.H3 83us : getnstimeofday (ktime_get_ts) - sshd-4261 0d.H3 83us : set_normalized_timespec (ktime_get_ts) - sshd-4261 0d.H3 84us : clockevents_program_event (tick_program_event) - sshd-4261 0d.H3 84us : lapic_next_event (clockevents_program_event) - sshd-4261 0d.H3 85us : irq_exit (smp_apic_timer_interrupt) - sshd-4261 0d.H3 85us : sub_preempt_count (irq_exit) - sshd-4261 0d.s4 86us : sub_preempt_count (irq_exit) - sshd-4261 0d.s3 86us : add_preempt_count (__local_bh_disable) -[...] - sshd-4261 0d.s1 98us : sub_preempt_count (net_rx_action) - sshd-4261 0d.s. 99us : add_preempt_count (_spin_lock_irq) - sshd-4261 0d.s1 99us+: _spin_unlock_irq (run_timer_softirq) - sshd-4261 0d.s. 104us : _local_bh_enable (__do_softirq) - sshd-4261 0d.s. 104us : sub_preempt_count (_local_bh_enable) - sshd-4261 0d.s. 105us : _local_bh_enable (__do_softirq) - sshd-4261 0d.s1 105us : trace_preempt_on (__do_softirq) - - -This is a very interesting trace. It started with the preemption of -the ls task. We see that the task had the "need_resched" bit set -via the 'N' in the trace. Interrupts were disabled before the spin_lock -at the beginning of the trace. We see that a schedule took place to run -sshd. When the interrupts were enabled, we took an interrupt. -On return from the interrupt handler, the softirq ran. We took another -interrupt while running the softirq as we see from the capital 'H'. - - -wakeup ------- - -In a Real-Time environment it is very important to know the wakeup -time it takes for the highest priority task that is woken up to the -time that it executes. This is also known as "schedule latency". -I stress the point that this is about RT tasks. It is also important -to know the scheduling latency of non-RT tasks, but the average -schedule latency is better for non-RT tasks. Tools like -LatencyTop are more appropriate for such measurements. - -Real-Time environments are interested in the worst case latency. -That is the longest latency it takes for something to happen, and -not the average. We can have a very fast scheduler that may only -have a large latency once in a while, but that would not work well -with Real-Time tasks. The wakeup tracer was designed to record -the worst case wakeups of RT tasks. Non-RT tasks are not recorded -because the tracer only records one worst case and tracing non-RT -tasks that are unpredictable will overwrite the worst case latency -of RT tasks. - -Since this tracer only deals with RT tasks, we will run this slightly -differently than we did with the previous tracers. Instead of performing -an 'ls', we will run 'sleep 1' under 'chrt' which changes the -priority of the task. - - # echo wakeup > /debug/tracing/current_tracer - # echo 0 > /debug/tracing/tracing_max_latency - # echo 1 > /debug/tracing/tracing_enabled - # chrt -f 5 sleep 1 - # echo 0 > /debug/tracing/tracing_enabled - # cat /debug/tracing/latency_trace -# tracer: wakeup -# -wakeup latency trace v1.1.5 on 2.6.26-rc8 --------------------------------------------------------------------- - latency: 4 us, #2/2, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) - ----------------- - | task: sleep-4901 (uid:0 nice:0 policy:1 rt_prio:5) - ----------------- - -# _------=> CPU# -# / _-----=> irqs-off -# | / _----=> need-resched -# || / _---=> hardirq/softirq -# ||| / _--=> preempt-depth -# |||| / -# ||||| delay -# cmd pid ||||| time | caller -# \ / ||||| \ | / - <idle>-0 1d.h4 0us+: try_to_wake_up (wake_up_process) - <idle>-0 1d..4 4us : schedule (cpu_idle) - - - -Running this on an idle system, we see that it only took 4 microseconds -to perform the task switch. Note, since the trace marker in the -schedule is before the actual "switch", we stop the tracing when -the recorded task is about to schedule in. This may change if -we add a new marker at the end of the scheduler. - -Notice that the recorded task is 'sleep' with the PID of 4901 and it -has an rt_prio of 5. This priority is user-space priority and not -the internal kernel priority. The policy is 1 for SCHED_FIFO and 2 -for SCHED_RR. - -Doing the same with chrt -r 5 and ftrace_enabled set. - -# tracer: wakeup -# -wakeup latency trace v1.1.5 on 2.6.26-rc8 --------------------------------------------------------------------- - latency: 50 us, #60/60, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) - ----------------- - | task: sleep-4068 (uid:0 nice:0 policy:2 rt_prio:5) - ----------------- - -# _------=> CPU# -# / _-----=> irqs-off -# | / _----=> need-resched -# || / _---=> hardirq/softirq -# ||| / _--=> preempt-depth -# |||| / -# ||||| delay -# cmd pid ||||| time | caller -# \ / ||||| \ | / -ksoftirq-7 1d.H3 0us : try_to_wake_up (wake_up_process) -ksoftirq-7 1d.H4 1us : sub_preempt_count (marker_probe_cb) -ksoftirq-7 1d.H3 2us : check_preempt_wakeup (try_to_wake_up) -ksoftirq-7 1d.H3 3us : update_curr (check_preempt_wakeup) -ksoftirq-7 1d.H3 4us : calc_delta_mine (update_curr) -ksoftirq-7 1d.H3 5us : __resched_task (check_preempt_wakeup) -ksoftirq-7 1d.H3 6us : task_wake_up_rt (try_to_wake_up) -ksoftirq-7 1d.H3 7us : _spin_unlock_irqrestore (try_to_wake_up) -[...] -ksoftirq-7 1d.H2 17us : irq_exit (smp_apic_timer_interrupt) -ksoftirq-7 1d.H2 18us : sub_preempt_count (irq_exit) -ksoftirq-7 1d.s3 19us : sub_preempt_count (irq_exit) -ksoftirq-7 1..s2 20us : rcu_process_callbacks (__do_softirq) -[...] -ksoftirq-7 1..s2 26us : __rcu_process_callbacks (rcu_process_callbacks) -ksoftirq-7 1d.s2 27us : _local_bh_enable (__do_softirq) -ksoftirq-7 1d.s2 28us : sub_preempt_count (_local_bh_enable) -ksoftirq-7 1.N.3 29us : sub_preempt_count (ksoftirqd) -ksoftirq-7 1.N.2 30us : _cond_resched (ksoftirqd) -ksoftirq-7 1.N.2 31us : __cond_resched (_cond_resched) -ksoftirq-7 1.N.2 32us : add_preempt_count (__cond_resched) -ksoftirq-7 1.N.2 33us : schedule (__cond_resched) -ksoftirq-7 1.N.2 33us : add_preempt_count (schedule) -ksoftirq-7 1.N.3 34us : hrtick_clear (schedule) -ksoftirq-7 1dN.3 35us : _spin_lock (schedule) -ksoftirq-7 1dN.3 36us : add_preempt_count (_spin_lock) -ksoftirq-7 1d..4 37us : put_prev_task_fair (schedule) -ksoftirq-7 1d..4 38us : update_curr (put_prev_task_fair) -[...] -ksoftirq-7 1d..5 47us : _spin_trylock (tracing_record_cmdline) -ksoftirq-7 1d..5 48us : add_preempt_count (_spin_trylock) -ksoftirq-7 1d..6 49us : _spin_unlock (tracing_record_cmdline) -ksoftirq-7 1d..6 49us : sub_preempt_count (_spin_unlock) -ksoftirq-7 1d..4 50us : schedule (__cond_resched) - -The interrupt went off while running ksoftirqd. This task runs at -SCHED_OTHER. Why did not we see the 'N' set early? This may be -a harmless bug with x86_32 and 4K stacks. On x86_32 with 4K stacks -configured, the interrupt and softirq run with their own stack. -Some information is held on the top of the task's stack (need_resched -and preempt_count are both stored there). The setting of the NEED_RESCHED -bit is done directly to the task's stack, but the reading of the -NEED_RESCHED is done by looking at the current stack, which in this case -is the stack for the hard interrupt. This hides the fact that NEED_RESCHED -has been set. We do not see the 'N' until we switch back to the task's -assigned stack. - -function --------- - -This tracer is the function tracer. Enabling the function tracer -can be done from the debug file system. Make sure the ftrace_enabled is -set; otherwise this tracer is a nop. - - # sysctl kernel.ftrace_enabled=1 - # echo function > /debug/tracing/current_tracer - # echo 1 > /debug/tracing/tracing_enabled - # usleep 1 - # echo 0 > /debug/tracing/tracing_enabled - # cat /debug/tracing/trace -# tracer: function -# -# TASK-PID CPU# TIMESTAMP FUNCTION -# | | | | | - bash-4003 [00] 123.638713: finish_task_switch <-schedule - bash-4003 [00] 123.638714: _spin_unlock_irq <-finish_task_switch - bash-4003 [00] 123.638714: sub_preempt_count <-_spin_unlock_irq - bash-4003 [00] 123.638715: hrtick_set <-schedule - bash-4003 [00] 123.638715: _spin_lock_irqsave <-hrtick_set - bash-4003 [00] 123.638716: add_preempt_count <-_spin_lock_irqsave - bash-4003 [00] 123.638716: _spin_unlock_irqrestore <-hrtick_set - bash-4003 [00] 123.638717: sub_preempt_count <-_spin_unlock_irqrestore - bash-4003 [00] 123.638717: hrtick_clear <-hrtick_set - bash-4003 [00] 123.638718: sub_preempt_count <-schedule - bash-4003 [00] 123.638718: sub_preempt_count <-preempt_schedule - bash-4003 [00] 123.638719: wait_for_completion <-__stop_machine_run - bash-4003 [00] 123.638719: wait_for_common <-wait_for_completion - bash-4003 [00] 123.638720: _spin_lock_irq <-wait_for_common - bash-4003 [00] 123.638720: add_preempt_count <-_spin_lock_irq -[...] - - -Note: function tracer uses ring buffers to store the above entries. -The newest data may overwrite the oldest data. Sometimes using echo to -stop the trace is not sufficient because the tracing could have overwritten -the data that you wanted to record. For this reason, it is sometimes better to -disable tracing directly from a program. This allows you to stop the -tracing at the point that you hit the part that you are interested in. -To disable the tracing directly from a C program, something like following -code snippet can be used: - -int trace_fd; -[...] -int main(int argc, char *argv[]) { - [...] - trace_fd = open("/debug/tracing/tracing_enabled", O_WRONLY); - [...] - if (condition_hit()) { - write(trace_fd, "0", 1); - } - [...] -} - -Note: Here we hard coded the path name. The debugfs mount is not -guaranteed to be at /debug (and is more commonly at /sys/kernel/debug). -For simple one time traces, the above is sufficent. For anything else, -a search through /proc/mounts may be needed to find where the debugfs -file-system is mounted. - - -Single thread tracing ---------------------- - -By writing into /debug/tracing/set_ftrace_pid you can trace a -single thread. For example: - -# cat /debug/tracing/set_ftrace_pid -no pid -# echo 3111 > /debug/tracing/set_ftrace_pid -# cat /debug/tracing/set_ftrace_pid -3111 -# echo function > /debug/tracing/current_tracer -# cat /debug/tracing/trace | head - # tracer: function - # - # TASK-PID CPU# TIMESTAMP FUNCTION - # | | | | | - yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return - yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range - yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel - yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel - yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll - yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll -# echo -1 > /debug/tracing/set_ftrace_pid -# cat /debug/tracing/trace |head - # tracer: function - # - # TASK-PID CPU# TIMESTAMP FUNCTION - # | | | | | - ##### CPU 3 buffer started #### - yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait - yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry - yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry - yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit - yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit - -If you want to trace a function when executing, you could use -something like this simple program: - -#include <stdio.h> -#include <stdlib.h> -#include <sys/types.h> -#include <sys/stat.h> -#include <fcntl.h> -#include <unistd.h> - -int main (int argc, char **argv) -{ - if (argc < 1) - exit(-1); - - if (fork() > 0) { - int fd, ffd; - char line[64]; - int s; - - ffd = open("/debug/tracing/current_tracer", O_WRONLY); - if (ffd < 0) - exit(-1); - write(ffd, "nop", 3); - - fd = open("/debug/tracing/set_ftrace_pid", O_WRONLY); - s = sprintf(line, "%d\n", getpid()); - write(fd, line, s); - - write(ffd, "function", 8); - - close(fd); - close(ffd); - - execvp(argv[1], argv+1); - } - - return 0; -} - -dynamic ftrace --------------- - -If CONFIG_DYNAMIC_FTRACE is set, the system will run with -virtually no overhead when function tracing is disabled. The way -this works is the mcount function call (placed at the start of -every kernel function, produced by the -pg switch in gcc), starts -of pointing to a simple return. (Enabling FTRACE will include the --pg switch in the compiling of the kernel.) - -At compile time every C file object is run through the -recordmcount.pl script (located in the scripts directory). This -script will process the C object using objdump to find all the -locations in the .text section that call mcount. (Note, only -the .text section is processed, since processing other sections -like .init.text may cause races due to those sections being freed). - -A new section called "__mcount_loc" is created that holds references -to all the mcount call sites in the .text section. This section is -compiled back into the original object. The final linker will add -all these references into a single table. - -On boot up, before SMP is initialized, the dynamic ftrace code -scans this table and updates all the locations into nops. It also -records the locations, which are added to the available_filter_functions -list. Modules are processed as they are loaded and before they are -executed. When a module is unloaded, it also removes its functions from -the ftrace function list. This is automatic in the module unload -code, and the module author does not need to worry about it. - -When tracing is enabled, kstop_machine is called to prevent races -with the CPUS executing code being modified (which can cause the -CPU to do undesireable things), and the nops are patched back -to calls. But this time, they do not call mcount (which is just -a function stub). They now call into the ftrace infrastructure. - -One special side-effect to the recording of the functions being -traced is that we can now selectively choose which functions we -wish to trace and which ones we want the mcount calls to remain as -nops. - -Two files are used, one for enabling and one for disabling the tracing -of specified functions. They are: - - set_ftrace_filter - -and - - set_ftrace_notrace - -A list of available functions that you can add to these files is listed -in: - - available_filter_functions - - # cat /debug/tracing/available_filter_functions -put_prev_task_idle -kmem_cache_create -pick_next_task_rt -get_online_cpus -pick_next_task_fair -mutex_lock -[...] - -If I am only interested in sys_nanosleep and hrtimer_interrupt: - - # echo sys_nanosleep hrtimer_interrupt \ - > /debug/tracing/set_ftrace_filter - # echo ftrace > /debug/tracing/current_tracer - # echo 1 > /debug/tracing/tracing_enabled - # usleep 1 - # echo 0 > /debug/tracing/tracing_enabled - # cat /debug/tracing/trace -# tracer: ftrace -# -# TASK-PID CPU# TIMESTAMP FUNCTION -# | | | | | - usleep-4134 [00] 1317.070017: hrtimer_interrupt <-smp_apic_timer_interrupt - usleep-4134 [00] 1317.070111: sys_nanosleep <-syscall_call - <idle>-0 [00] 1317.070115: hrtimer_interrupt <-smp_apic_timer_interrupt - -To see which functions are being traced, you can cat the file: - - # cat /debug/tracing/set_ftrace_filter -hrtimer_interrupt -sys_nanosleep - - -Perhaps this is not enough. The filters also allow simple wild cards. -Only the following are currently available - - <match>* - will match functions that begin with <match> - *<match> - will match functions that end with <match> - *<match>* - will match functions that have <match> in it - -These are the only wild cards which are supported. - - <match>*<match> will not work. - -Note: It is better to use quotes to enclose the wild cards, otherwise - the shell may expand the parameters into names of files in the local - directory. - - # echo 'hrtimer_*' > /debug/tracing/set_ftrace_filter - -Produces: - -# tracer: ftrace -# -# TASK-PID CPU# TIMESTAMP FUNCTION -# | | | | | - bash-4003 [00] 1480.611794: hrtimer_init <-copy_process - bash-4003 [00] 1480.611941: hrtimer_start <-hrtick_set - bash-4003 [00] 1480.611956: hrtimer_cancel <-hrtick_clear - bash-4003 [00] 1480.611956: hrtimer_try_to_cancel <-hrtimer_cancel - <idle>-0 [00] 1480.612019: hrtimer_get_next_event <-get_next_timer_interrupt - <idle>-0 [00] 1480.612025: hrtimer_get_next_event <-get_next_timer_interrupt - <idle>-0 [00] 1480.612032: hrtimer_get_next_event <-get_next_timer_interrupt - <idle>-0 [00] 1480.612037: hrtimer_get_next_event <-get_next_timer_interrupt - <idle>-0 [00] 1480.612382: hrtimer_get_next_event <-get_next_timer_interrupt - - -Notice that we lost the sys_nanosleep. - - # cat /debug/tracing/set_ftrace_filter -hrtimer_run_queues -hrtimer_run_pending -hrtimer_init -hrtimer_cancel -hrtimer_try_to_cancel -hrtimer_forward -hrtimer_start -hrtimer_reprogram -hrtimer_force_reprogram -hrtimer_get_next_event -hrtimer_interrupt -hrtimer_nanosleep -hrtimer_wakeup -hrtimer_get_remaining -hrtimer_get_res -hrtimer_init_sleeper - - -This is because the '>' and '>>' act just like they do in bash. -To rewrite the filters, use '>' -To append to the filters, use '>>' - -To clear out a filter so that all functions will be recorded again: - - # echo > /debug/tracing/set_ftrace_filter - # cat /debug/tracing/set_ftrace_filter - # - -Again, now we want to append. - - # echo sys_nanosleep > /debug/tracing/set_ftrace_filter - # cat /debug/tracing/set_ftrace_filter -sys_nanosleep - # echo 'hrtimer_*' >> /debug/tracing/set_ftrace_filter - # cat /debug/tracing/set_ftrace_filter -hrtimer_run_queues -hrtimer_run_pending -hrtimer_init -hrtimer_cancel -hrtimer_try_to_cancel -hrtimer_forward -hrtimer_start -hrtimer_reprogram -hrtimer_force_reprogram -hrtimer_get_next_event -hrtimer_interrupt -sys_nanosleep -hrtimer_nanosleep -hrtimer_wakeup -hrtimer_get_remaining -hrtimer_get_res -hrtimer_init_sleeper - - -The set_ftrace_notrace prevents those functions from being traced. - - # echo '*preempt*' '*lock*' > /debug/tracing/set_ftrace_notrace - -Produces: - -# tracer: ftrace -# -# TASK-PID CPU# TIMESTAMP FUNCTION -# | | | | | - bash-4043 [01] 115.281644: finish_task_switch <-schedule - bash-4043 [01] 115.281645: hrtick_set <-schedule - bash-4043 [01] 115.281645: hrtick_clear <-hrtick_set - bash-4043 [01] 115.281646: wait_for_completion <-__stop_machine_run - bash-4043 [01] 115.281647: wait_for_common <-wait_for_completion - bash-4043 [01] 115.281647: kthread_stop <-stop_machine_run - bash-4043 [01] 115.281648: init_waitqueue_head <-kthread_stop - bash-4043 [01] 115.281648: wake_up_process <-kthread_stop - bash-4043 [01] 115.281649: try_to_wake_up <-wake_up_process - -We can see that there's no more lock or preempt tracing. - -trace_pipe ----------- - -The trace_pipe outputs the same content as the trace file, but the effect -on the tracing is different. Every read from trace_pipe is consumed. -This means that subsequent reads will be different. The trace -is live. - - # echo function > /debug/tracing/current_tracer - # cat /debug/tracing/trace_pipe > /tmp/trace.out & -[1] 4153 - # echo 1 > /debug/tracing/tracing_enabled - # usleep 1 - # echo 0 > /debug/tracing/tracing_enabled - # cat /debug/tracing/trace -# tracer: function -# -# TASK-PID CPU# TIMESTAMP FUNCTION -# | | | | | - - # - # cat /tmp/trace.out - bash-4043 [00] 41.267106: finish_task_switch <-schedule - bash-4043 [00] 41.267106: hrtick_set <-schedule - bash-4043 [00] 41.267107: hrtick_clear <-hrtick_set - bash-4043 [00] 41.267108: wait_for_completion <-__stop_machine_run - bash-4043 [00] 41.267108: wait_for_common <-wait_for_completion - bash-4043 [00] 41.267109: kthread_stop <-stop_machine_run - bash-4043 [00] 41.267109: init_waitqueue_head <-kthread_stop - bash-4043 [00] 41.267110: wake_up_process <-kthread_stop - bash-4043 [00] 41.267110: try_to_wake_up <-wake_up_process - bash-4043 [00] 41.267111: select_task_rq_rt <-try_to_wake_up - - -Note, reading the trace_pipe file will block until more input is added. -By changing the tracer, trace_pipe will issue an EOF. We needed -to set the function tracer _before_ we "cat" the trace_pipe file. - - -trace entries -------------- - -Having too much or not enough data can be troublesome in diagnosing -an issue in the kernel. The file buffer_size_kb is used to modify -the size of the internal trace buffers. The number listed -is the number of entries that can be recorded per CPU. To know -the full size, multiply the number of possible CPUS with the -number of entries. - - # cat /debug/tracing/buffer_size_kb -1408 (units kilobytes) - -Note, to modify this, you must have tracing completely disabled. To do that, -echo "nop" into the current_tracer. If the current_tracer is not set -to "nop", an EINVAL error will be returned. - - # echo nop > /debug/tracing/current_tracer - # echo 10000 > /debug/tracing/buffer_size_kb - # cat /debug/tracing/buffer_size_kb -10000 (units kilobytes) - -The number of pages which will be allocated is limited to a percentage -of available memory. Allocating too much will produce an error. - - # echo 1000000000000 > /debug/tracing/buffer_size_kb --bash: echo: write error: Cannot allocate memory - # cat /debug/tracing/buffer_size_kb -85 - |