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diff --git a/Documentation/ftrace.txt b/Documentation/ftrace.txt new file mode 100644 index 000000000000..f218f616ff6b --- /dev/null +++ b/Documentation/ftrace.txt @@ -0,0 +1,1360 @@ + ftrace - Function Tracer + ======================== + +Copyright 2008 Red Hat Inc. + Author: Steven Rostedt <srostedt@redhat.com> + License: The GNU Free Documentation License, Version 1.2 +Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton, + John Kacur, and David Teigland. + +Written for: 2.6.27-rc1 + +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. + + 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. The displayed number + is the size of the CPU buffer and not total size. The + trace buffers are allocated in pages (blocks of memory + that the kernel uses for allocation, usually 4 KB in size). + Since each entry is smaller than a page, if the last + allocated page has room for more entries than were + requested, the rest of the page is used to allocate + entries. + + 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 (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. + + 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. (See the section "dynamic ftrace" + below for more details). + + +The Tracers +----------- + +Here is the list of current tracers that may be configured. + + ftrace - 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. + + 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 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: 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 tracer name that is represented by the trace. +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 <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. + + 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. + + +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 prepended with "no". + + echo noprint-parent > /debug/tracing/iter_ctrl + +To enable an option, leave off the "no". + + echo sym-offset > /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 <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. + + 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. + +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: ftrace 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. + +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.) + +When dynamic ftrace is initialized, it calls kstop_machine to make +the machine 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 changed 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 measurable 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 +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. + + # 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. + +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 are not any, then +it simply goes back to sleep. But if there are some, it will call +kstop_machine to convert the calls to nops. + +There may be a case in which 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 kstop_machine to update the +mcount calls to nops. Remember that there is a large overhead +to calling mcount. Without this kernel thread, that overhead will +exist. + +If there are recorded calls to mcount, any write to the ftraced_enabled +file will cause the kstop_machine to run. 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 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 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 file 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 +an issue in the kernel. The file trace_entries 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/trace_entries +65620 + +Note, to modify this, you must have tracing completely disabled. To do that, +echo "none" into the current_tracer. If the current_tracer is not set +to "none", an EINVAL error will be returned. + + # 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 in individual pages. It allocates the number of pages it takes +to fulfill the request. If more entries may fit on the last page +then they will be added. + + # echo 1 > /debug/tracing/trace_entries + # cat /debug/tracing/trace_entries +85 + +This shows us that 85 entries can fit in a single page. + +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/trace_entries +-bash: echo: write error: Cannot allocate memory + # cat /debug/tracing/trace_entries +85 + |