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