ftrace - Function Tracer ======================== Copyright 2008 Red Hat Inc. Author: Steven Rostedt 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 is 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. 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 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 has been compiled into the kernel. The tracers listed here can be configured by echoing in 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 (or non-zero) to enable it. trace : This file holds the output of the trace in a human readable format. latency_trace : This file shows the same trace but the information is organized more to display possible latencies in the system. 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 isn't adding more data, they will display the same information every time they are read. iter_ctrl : 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) trace_entries : This sets or displays the number of trace entries each CPU buffer can hold. The tracer buffers are the same size for each CPU, so care must be taken when modifying the trace_entries. The number of actually entries will be the number given times the number of possible CPUS. The buffers are saved as individual pages, and the actual entries will always be rounded up to entries per page. This can only be updated when the current_tracer is set to "none". NOTE: It is planned on changing the allocated buffers from being the number of possible CPUS to the number of online CPUS. 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, the code is dynamically modified 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 in names of functions into this file will limit the trace to only those files. set_ftrace_notrace: This has the opposite effect that set_ftrace_filter has. 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_ bet traced. available_filter_functions : When a function is encountered the first time by the dynamic tracer, it is recorded and later the call is converted into a nop. This file lists the functions that have been recorded by the dynamic tracer and these functions can be used to set the ftrace filter by the above "set_ftrace_filter" file. The Tracers ----------- Here are the list of current tracers that can be configured. ftrace - function tracer that uses mcount to trace all functions. It is possible to filter out which functions that are traced when dynamic ftrace is configured in. sched_switch - traces the context switches between tasks. irqsoff - traces the areas that disable interrupts and saves off 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 with the latency_trace file. preemptoff - Similar to irqsoff but traces and records the time preemption is disabled. preemptirqsoff - Similar to irqsoff and preemptoff, but traces and records the largest time 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. none - This is not a tracer. To remove all tracers from tracing simply echo "none" into current_tracer. Examples of using the tracer ---------------------------- Here are typical examples of using the tracers with only controlling them with the debugfs interface (without using any user-land utilities). Output format: -------------- Here's an example of the output format of the file "trace" -------- # tracer: ftrace # # 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 trace that is represented. In this case the tracer is "ftrace". Then a header showing the format. Task name "bash", the task PID "4251", the CPU that it was running on "01", the timestamp in . format, the function name that was traced "path_put" and the parent function that called this function "path_walk". The sched_switch tracer also includes tracing of task wake ups 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 show "==>". The format is: Context switches: Previous task Next Task :: ==> :: Wake ups: Current task Task waking up :: + :: The prio is the internal kernel priority, which is inverse to 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 a bit more information to see why a latency happened. Here's 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 # \ / ||||| \ | / -0 0d..1 0us+: trace_hardirqs_off_thunk (apic_timer_interrupt) -0 0d.s. 97us : __do_softirq (do_softirq) -0 0d.s1 98us : trace_hardirqs_on (do_softirq) vim:ft=help This shows that the current tracer is "irqsoff" tracing the time interrupts are disabled. It gives the trace version and the kernel this was executed on (2.6.26-rc8). Then it displays the max latency in microsecs (97 us). The number of trace entries displayed by 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 reserved for later use. #P is the number of online CPUS (#P:2). The task is the process that was running when the latency happened. (swapper pid: 0). The start and stop 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 that the process was running on. irqs-off: 'd' interrupts are disabled. '.' otherwise. need-resched: 'N' task need_resched is set, '.' otherwise. hardirq/softirq: 'H' - hard irq happened 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 output where as the trace output contained a absolute timestamp. This timestamp is relative to the start of the first entry in the 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 is 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. iter_ctrl --------- The iter_ctrl file is used to control what gets printed in the trace output. To see what is available, simply cat the file: cat /debug/tracing/iter_ctrl print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \ noblock nostacktrace nosched-tree To disable one of the options, echo in the option appended with "no". echo noprint-parent > /debug/tracing/iter_ctrl To enable an option, leave off the "no". echo sym-offest > /debug/tracing/iter_ctrl 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 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. sched-tree - TBD (any users??) sched_switch ------------ This tracer simply records schedule switches. Here's an example on how to implement 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 -0 [00] 240.132589: 0:140:R + 4:115:S -0 [00] 240.132591: 0:140:R ==> 4:115:R ksoftirqd/0-4 [00] 240.132595: 4:115:S ==> 0:140:R -0 [00] 240.132598: 0:140:R + 4:115:S -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 only lists the wake ups (represented with '+') and context switches ('==>') with the previous task or current 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 - deep 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 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 interrupts are disabled and when they are re-enabled. When a new maximum latency is hit, it saves off the trace so that it may be retrieved at a later time. Every time a new maximum in reached, the old saved trace is discarded and the new trace is saved. To reset the maximum, echo 0 into tracing_max_latency. Here's 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-rc8 -------------------------------------------------------------------- latency: 6 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) ----------------- | task: bash-4269 (uid:0 nice:0 policy:0 rt_prio:0) ----------------- => started at: copy_page_range => ended at: copy_page_range # _------=> CPU# # / _-----=> irqs-off # | / _----=> need-resched # || / _---=> hardirq/softirq # ||| / _--=> preempt-depth # |||| / # ||||| delay # cmd pid ||||| time | caller # \ / ||||| \ | / bash-4269 1...1 0us+: _spin_lock (copy_page_range) bash-4269 1...1 7us : _spin_unlock (copy_page_range) bash-4269 1...2 7us : trace_preempt_on (copy_page_range) vim:ft=help Here we see that that we had a latency of 6 microsecs (which is very good). The spin_lock in copy_page_range disabled interrupts. The difference between the 6 and the displayed timestamp 7us is because the clock must have incremented between the time of recording the max latency and recording the function that had that latency. Note the above 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) vim:ft=help Here we traced a 50 microsecond latency. But we also see all the functions that were called during that time. Note that enabling function tracing we endure an added overhead. This overhead may extend the latency times. But never the less, this trace has provided some very helpful debugging. preemptoff ---------- When preemption is disabled we may be able to receive interrupts but the task can not 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 disables preemption. Like the irqsoff, it records the maximum latency that preemption was disabled. The control of preemptoff is much like the irqsoff. # 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) vim:ft=help 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 by the functions themselves that this is not the case. Notice that the __do_softirq when called doesn't have a preempt_count. It may seem that we missed a preempt enabled. What really happened is that the preempt count is held on the threads 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 don't need to worry about it. Having a tracer like this is good to let 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. 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 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: 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) vim:ft=help The trace_hardirqs_off_thunk is called from assembly on x86 when interrupts are disabled in the assembly code. Without the function tracing, we don't 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 with the 'N' in the trace. Interrupts are disabled in the spin_lock and the trace started. We see that a schedule took place to run sshd. When the interrupts were enabled we took an interrupt. On return of the interrupt the softirq ran. We took another interrupt while running the softirq as we see with the capital 'H'. wakeup ------ In Real-Time environment it is very important to know the wakeup time it takes for the highest priority task that wakes up to the time 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 is more appropriate for such measurements. Real-Time environments is 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 different 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 # \ / ||||| \ | / -0 1d.h4 0us+: try_to_wake_up (wake_up_process) -0 1d..4 4us : schedule (cpu_idle) vim:ft=help 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 didn't we see the 'N' set early? This may be a harmless bug with x86_32 and 4K stacks. The need_reched() function that tests if we need to reschedule looks on the actual stack. Where as the setting of the NEED_RESCHED bit happens on the task's stack. But because we are in a hard interrupt, the test is with the interrupts stack which has that to be false. We don't see the 'N' until we switch back to the task's stack. ftrace ------ ftrace is not only the name of the tracing infrastructure, but it is also a name of one of the tracers. The 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 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 # | | | | | 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: It is sometimes better to enable or disable tracing directly from a program, because the buffer may be overflowed by the echo commands before you get to the point you want to trace. It is also easier to stop the tracing at the point that you hit the part that you are interested in. Since the ftrace buffer is a ring buffer with the oldest data being overwritten, usually it is sufficient to start the tracer with an echo command but have you code stop it. Something like the following is usually appropriate for this. 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); } [...] } dynamic ftrace -------------- If CONFIG_DYNAMIC_FTRACE is set, then 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. When dynamic ftrace is initialized, it calls kstop_machine to make it act like a uniprocessor so that it can freely modify code without worrying about other processors executing that same code. At initialization, the mcount calls are change to call a "record_ip" function. After this, the first time a kernel function is called, it has the calling address saved in a hash table. Later on the ftraced kernel thread is awoken and will again call kstop_machine if new functions have been recorded. The ftraced thread will change all calls to mcount to "nop". Just calling mcount and having mcount return has shown a 10% overhead. By converting it to a nop, there is no recordable overhead to the system. One special side-effect to the recording of the functions being traced, is that we can now selectively choose which functions we want to trace and which ones we want the mcount calls to remain as nops. Two files that contain to the enabling and disabling of recorded functions are: set_ftrace_filter and set_ftrace_notrace A list of available functions that you can add to this 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'm 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 -0 [00] 1317.070115: hrtimer_interrupt <-smp_apic_timer_interrupt To see what functions are being traced, you can cat the file: # cat /debug/tracing/set_ftrace_filter hrtimer_interrupt sys_nanosleep Perhaps this isn't enough. The filters also allow simple wild cards. Only the following is currently available * - will match functions that begins with * - will match functions that end with ** - will match functions that have in it Thats all the wild cards that are allowed. * will not work. # 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 -0 [00] 1480.612019: hrtimer_get_next_event <-get_next_timer_interrupt -0 [00] 1480.612025: hrtimer_get_next_event <-get_next_timer_interrupt -0 [00] 1480.612032: hrtimer_get_next_event <-get_next_timer_interrupt -0 [00] 1480.612037: hrtimer_get_next_event <-get_next_timer_interrupt -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. ftraced ------- As mentioned above, when dynamic ftrace is configured in, a kernel thread wakes up once a second and checks to see if there are mcount calls that need to be converted into nops. If there is not, then it simply goes back to sleep. But if there is, it will call kstop_machine to convert the calls to nops. There may be a case that you do not want this added latency. Perhaps you are doing some audio recording and this activity might cause skips in the playback. There is an interface to disable and enable the ftraced kernel thread. # echo 0 > /debug/tracing/ftraced_enabled This will disable the calling of the kstop_machine to update the mcount calls to nops. Remember that there's a large overhead to calling mcount. Without this kernel thread, that overhead will exist. Any write to the ftraced_enabled file will cause the kstop_machine to run if there are recorded calls to mcount. This means that a user can manually perform the updates when they want to by simply echoing a '0' into the ftraced_enabled file. The updates are also done at the beginning of enabling a tracer that uses ftrace function recording. trace_pipe ---------- The trace_pipe outputs the same as trace, 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 ftrace > /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: ftrace # # 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 will block until more input is added. By changing the tracer, trace_pipe will issue an EOF. We needed to set the ftrace tracer _before_ cating the trace_pipe file. trace entries ------------- Having too much or not enough data can be troublesome in diagnosing some issue in the kernel. The file trace_entries is used to modify the size of the internal trace buffers. The numbers 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/trace_entries 65620 Note, to modify this you must have tracing fulling disabled. To do that, echo "none" into the current_tracer. # echo none > /debug/tracing/current_tracer # echo 100000 > /debug/tracing/trace_entries # cat /debug/tracing/trace_entries 100045 Notice that we echoed in 100,000 but the size is 100,045. The entries are held by individual pages. It allocates the number of pages it takes to fulfill the request. If more entries may fit on the last page it will add them. # echo 1 > /debug/tracing/trace_entries # cat /debug/tracing/trace_entries 85 This shows us that 85 entries can fit on a single page. The number of pages that will be allocated is a percentage of available memory. Allocating too much will produces an error. # echo 1000000000000 > /debug/tracing/trace_entries -bash: echo: write error: Cannot allocate memory # cat /debug/tracing/trace_entries 85