| 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. |
| |
| When debugfs is configured into the kernel (which selecting any ftrace |
| option will do) the directory /sys/kernel/debug will be created. To mount |
| this directory, you can add to your /etc/fstab file: |
| |
| debugfs /sys/kernel/debug debugfs defaults 0 0 |
| |
| Or you can mount it at run time with: |
| |
| mount -t debugfs nodev /sys/kernel/debug |
| |
| For quicker access to that directory you may want to make a soft link to |
| it: |
| |
| ln -s /sys/kernel/debug /debug |
| |
| Any selected ftrace option will also create a directory called tracing |
| within the debugfs. The rest of the document will assume that you are in |
| the ftrace directory (cd /sys/kernel/debug/tracing) and will only concentrate |
| on the files within that directory and not distract from the content with |
| the extended "/sys/kernel/debug/tracing" path name. |
| |
| 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). |
| |
| 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" file, 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" file is 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. |
| |
| tracing_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 "trace". |
| A new max trace will only be recorded if the |
| latency is greater than the value in this |
| file. (in microseconds) |
| |
| buffer_size_kb: |
| |
| This sets or displays the number of kilobytes each CPU |
| buffer can hold. The tracer buffers are the same size |
| for each CPU. The displayed number is the size of the |
| CPU buffer and not total size of all buffers. The |
| trace buffers are allocated in pages (blocks of memory |
| that the kernel uses for allocation, usually 4 KB in size). |
| If the last page allocated has room for more bytes |
| than requested, the rest of the page will be used, |
| making the actual allocation bigger than requested. |
| ( Note, the size may not be a multiple of the page size |
| due to buffer managment overhead. ) |
| |
| This can only be updated when the current_tracer |
| is set to "nop". |
| |
| tracing_cpumask: |
| |
| This is a mask that lets the user only trace |
| on specified CPUS. The format is a hex string |
| representing the CPUS. |
| |
| set_ftrace_filter: |
| |
| When dynamic ftrace is configured in (see the |
| section below "dynamic ftrace"), the code is dynamically |
| modified (code text rewrite) to disable calling of the |
| function profiler (mcount). This lets tracing be configured |
| in with practically no overhead in performance. This also |
| has a side effect of enabling or disabling specific functions |
| to be traced. Echoing names of functions into this file |
| will limit the trace to only those functions. |
| |
| set_ftrace_notrace: |
| |
| This has an effect opposite to that of |
| set_ftrace_filter. Any function that is added here will not |
| be traced. If a function exists in both set_ftrace_filter |
| and set_ftrace_notrace, the function will _not_ be traced. |
| |
| set_ftrace_pid: |
| |
| Have the function tracer only trace a single thread. |
| |
| set_graph_function: |
| |
| Set a "trigger" function where tracing should start |
| with the function graph tracer (See the section |
| "dynamic ftrace" for more details). |
| |
| 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 call tracer to trace all kernel functions. |
| |
| "function_graph" |
| |
| Similar to the function tracer except that the |
| function tracer probes the functions on their entry |
| whereas the function graph tracer traces on both entry |
| and exit of the functions. It then provides the ability |
| to draw a graph of function calls similar to C code |
| source. |
| |
| "sched_switch" |
| |
| Traces the context switches and wakeups 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 with the latency-format option enabled. |
| |
| "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. |
| |
| "hw-branch-tracer" |
| |
| Uses the BTS CPU feature on x86 CPUs to traces all |
| branches executed. |
| |
| "nop" |
| |
| This is the "trace nothing" 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 |
| -------------------- |
| |
| When the latency-format option is enabled, the 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: When the latency-format option is enabled, the trace file |
| output includes a timestamp relative to the start of the |
| trace. This differs from the output when latency-format |
| is disabled, which includes an absolute timestamp. |
| |
| 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 trace_options |
| print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \ |
| noblock nostacktrace nosched-tree nouserstacktrace nosym-userobj |
| |
| To disable one of the options, echo in the option prepended with |
| "no". |
| |
| echo noprint-parent > trace_options |
| |
| To enable an option, leave off the "no". |
| |
| echo sym-offset > trace_options |
| |
| Here are the available options: |
| |
| print-parent - On function traces, display the calling (parent) |
| 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 trace file when the |
| latency-format option is enabled. |
| |
| bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \ |
| (+0.000ms): simple_strtoul (strict_strtoul) |
| |
| raw - This will display raw numbers. This option is best for |
| use with user applications that can translate the raw |
| numbers better than having it done in the kernel. |
| |
| hex - Similar to raw, but the numbers will be in a hexadecimal |
| format. |
| |
| bin - This will print out the formats in raw binary. |
| |
| block - TBD (needs update) |
| |
| stacktrace - This is one of the options that changes the trace |
| itself. When a trace is recorded, so is the stack |
| of functions. This allows for back traces of |
| trace sites. |
| |
| userstacktrace - This option changes the trace. It records a |
| stacktrace of the current userspace thread. |
| |
| sym-userobj - when user stacktrace are enabled, look up which |
| object the address belongs to, and print a |
| relative address. This is especially useful when |
| ASLR is on, otherwise you don't get a chance to |
| resolve the address to object/file/line after |
| the app is no longer running |
| |
| The lookup is performed when you read |
| trace,trace_pipe. Example: |
| |
| a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0 |
| x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6] |
| |
| sched-tree - trace all tasks that are on the runqueue, at |
| every scheduling event. Will add overhead if |
| there's a lot of tasks running at once. |
| |
| latency-format - This option changes the trace. When |
| it is enabled, the trace displays |
| additional information about the |
| latencies, as described in "Latency |
| trace format". |
| |
| sched_switch |
| ------------ |
| |
| This tracer simply records schedule switches. Here is an example |
| of how to use it. |
| |
| # echo sched_switch > current_tracer |
| # echo 1 > tracing_enabled |
| # sleep 1 |
| # echo 0 > tracing_enabled |
| # cat 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 Space User Space |
| =============================================================== |
| 0(high) to 98(low) user RT priority 99(high) to 1(low) |
| with SCHED_RR or SCHED_FIFO |
| --------------------------------------------------------------- |
| 99 sched_priority is not used in scheduling |
| decisions(it must be specified as 0) |
| --------------------------------------------------------------- |
| 100(high) to 139(low) user nice -20(high) to 19(low) |
| --------------------------------------------------------------- |
| 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 > current_tracer |
| # echo latency-format > trace_options |
| # echo 0 > tracing_max_latency |
| # echo 1 > tracing_enabled |
| # ls -ltr |
| [...] |
| # echo 0 > tracing_enabled |
| # cat 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 > current_tracer |
| # echo latency-format > trace_options |
| # echo 0 > tracing_max_latency |
| # echo 1 > tracing_enabled |
| # ls -ltr |
| [...] |
| # echo 0 > tracing_enabled |
| # cat 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 > current_tracer |
| # echo latency-format > trace_options |
| # echo 0 > tracing_max_latency |
| # echo 1 > tracing_enabled |
| # ls -ltr |
| [...] |
| # echo 0 > tracing_enabled |
| # cat 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 > current_tracer |
| # echo latency-format > trace_options |
| # echo 0 > tracing_max_latency |
| # echo 1 > tracing_enabled |
| # chrt -f 5 sleep 1 |
| # echo 0 > tracing_enabled |
| # cat 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 > current_tracer |
| # echo 1 > tracing_enabled |
| # usleep 1 |
| # echo 0 > tracing_enabled |
| # cat 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(tracing_file("tracing_enabled"), O_WRONLY); |
| [...] |
| if (condition_hit()) { |
| write(trace_fd, "0", 1); |
| } |
| [...] |
| } |
| |
| |
| Single thread tracing |
| --------------------- |
| |
| By writing into set_ftrace_pid you can trace a |
| single thread. For example: |
| |
| # cat set_ftrace_pid |
| no pid |
| # echo 3111 > set_ftrace_pid |
| # cat set_ftrace_pid |
| 3111 |
| # echo function > current_tracer |
| # cat trace | head |
| # tracer: function |
| # |
| # TASK-PID CPU# TIMESTAMP FUNCTION |
| # | | | | | |
| yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return |
| yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range |
| yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel |
| yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel |
| yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll |
| yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll |
| # echo -1 > set_ftrace_pid |
| # cat trace |head |
| # tracer: function |
| # |
| # TASK-PID CPU# TIMESTAMP FUNCTION |
| # | | | | | |
| ##### CPU 3 buffer started #### |
| yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait |
| yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry |
| yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry |
| yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit |
| yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit |
| |
| If you want to trace a function when executing, you could use |
| something like this simple program: |
| |
| #include <stdio.h> |
| #include <stdlib.h> |
| #include <sys/types.h> |
| #include <sys/stat.h> |
| #include <fcntl.h> |
| #include <unistd.h> |
| |
| #define _STR(x) #x |
| #define STR(x) _STR(x) |
| #define MAX_PATH 256 |
| |
| const char *find_debugfs(void) |
| { |
| static char debugfs[MAX_PATH+1]; |
| static int debugfs_found; |
| char type[100]; |
| FILE *fp; |
| |
| if (debugfs_found) |
| return debugfs; |
| |
| if ((fp = fopen("/proc/mounts","r")) == NULL) { |
| perror("/proc/mounts"); |
| return NULL; |
| } |
| |
| while (fscanf(fp, "%*s %" |
| STR(MAX_PATH) |
| "s %99s %*s %*d %*d\n", |
| debugfs, type) == 2) { |
| if (strcmp(type, "debugfs") == 0) |
| break; |
| } |
| fclose(fp); |
| |
| if (strcmp(type, "debugfs") != 0) { |
| fprintf(stderr, "debugfs not mounted"); |
| return NULL; |
| } |
| |
| debugfs_found = 1; |
| |
| return debugfs; |
| } |
| |
| const char *tracing_file(const char *file_name) |
| { |
| static char trace_file[MAX_PATH+1]; |
| snprintf(trace_file, MAX_PATH, "%s/%s", find_debugfs(), file_name); |
| return trace_file; |
| } |
| |
| int main (int argc, char **argv) |
| { |
| if (argc < 1) |
| exit(-1); |
| |
| if (fork() > 0) { |
| int fd, ffd; |
| char line[64]; |
| int s; |
| |
| ffd = open(tracing_file("current_tracer"), O_WRONLY); |
| if (ffd < 0) |
| exit(-1); |
| write(ffd, "nop", 3); |
| |
| fd = open(tracing_file("set_ftrace_pid"), O_WRONLY); |
| s = sprintf(line, "%d\n", getpid()); |
| write(fd, line, s); |
| |
| write(ffd, "function", 8); |
| |
| close(fd); |
| close(ffd); |
| |
| execvp(argv[1], argv+1); |
| } |
| |
| return 0; |
| } |
| |
| |
| hw-branch-tracer (x86 only) |
| --------------------------- |
| |
| This tracer uses the x86 last branch tracing hardware feature to |
| collect a branch trace on all cpus with relatively low overhead. |
| |
| The tracer uses a fixed-size circular buffer per cpu and only |
| traces ring 0 branches. The trace file dumps that buffer in the |
| following format: |
| |
| # tracer: hw-branch-tracer |
| # |
| # CPU# TO <- FROM |
| 0 scheduler_tick+0xb5/0x1bf <- task_tick_idle+0x5/0x6 |
| 2 run_posix_cpu_timers+0x2b/0x72a <- run_posix_cpu_timers+0x25/0x72a |
| 0 scheduler_tick+0x139/0x1bf <- scheduler_tick+0xed/0x1bf |
| 0 scheduler_tick+0x17c/0x1bf <- scheduler_tick+0x148/0x1bf |
| 2 run_posix_cpu_timers+0x9e/0x72a <- run_posix_cpu_timers+0x5e/0x72a |
| 0 scheduler_tick+0x1b6/0x1bf <- scheduler_tick+0x1aa/0x1bf |
| |
| |
| The tracer may be used to dump the trace for the oops'ing cpu on |
| a kernel oops into the system log. To enable this, |
| ftrace_dump_on_oops must be set. To set ftrace_dump_on_oops, one |
| can either use the sysctl function or set it via the proc system |
| interface. |
| |
| sysctl kernel.ftrace_dump_on_oops=1 |
| |
| or |
| |
| echo 1 > /proc/sys/kernel/ftrace_dump_on_oops |
| |
| |
| Here's an example of such a dump after a null pointer |
| dereference in a kernel module: |
| |
| [57848.105921] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000 |
| [57848.106019] IP: [<ffffffffa0000006>] open+0x6/0x14 [oops] |
| [57848.106019] PGD 2354e9067 PUD 2375e7067 PMD 0 |
| [57848.106019] Oops: 0002 [#1] SMP |
| [57848.106019] last sysfs file: /sys/devices/pci0000:00/0000:00:1e.0/0000:20:05.0/local_cpus |
| [57848.106019] Dumping ftrace buffer: |
| [57848.106019] --------------------------------- |
| [...] |
| [57848.106019] 0 chrdev_open+0xe6/0x165 <- cdev_put+0x23/0x24 |
| [57848.106019] 0 chrdev_open+0x117/0x165 <- chrdev_open+0xfa/0x165 |
| [57848.106019] 0 chrdev_open+0x120/0x165 <- chrdev_open+0x11c/0x165 |
| [57848.106019] 0 chrdev_open+0x134/0x165 <- chrdev_open+0x12b/0x165 |
| [57848.106019] 0 open+0x0/0x14 [oops] <- chrdev_open+0x144/0x165 |
| [57848.106019] 0 page_fault+0x0/0x30 <- open+0x6/0x14 [oops] |
| [57848.106019] 0 error_entry+0x0/0x5b <- page_fault+0x4/0x30 |
| [57848.106019] 0 error_kernelspace+0x0/0x31 <- error_entry+0x59/0x5b |
| [57848.106019] 0 error_sti+0x0/0x1 <- error_kernelspace+0x2d/0x31 |
| [57848.106019] 0 page_fault+0x9/0x30 <- error_sti+0x0/0x1 |
| [57848.106019] 0 do_page_fault+0x0/0x881 <- page_fault+0x1a/0x30 |
| [...] |
| [57848.106019] 0 do_page_fault+0x66b/0x881 <- is_prefetch+0x1ee/0x1f2 |
| [57848.106019] 0 do_page_fault+0x6e0/0x881 <- do_page_fault+0x67a/0x881 |
| [57848.106019] 0 oops_begin+0x0/0x96 <- do_page_fault+0x6e0/0x881 |
| [57848.106019] 0 trace_hw_branch_oops+0x0/0x2d <- oops_begin+0x9/0x96 |
| [...] |
| [57848.106019] 0 ds_suspend_bts+0x2a/0xe3 <- ds_suspend_bts+0x1a/0xe3 |
| [57848.106019] --------------------------------- |
| [57848.106019] CPU 0 |
| [57848.106019] Modules linked in: oops |
| [57848.106019] Pid: 5542, comm: cat Tainted: G W 2.6.28 #23 |
| [57848.106019] RIP: 0010:[<ffffffffa0000006>] [<ffffffffa0000006>] open+0x6/0x14 [oops] |
| [57848.106019] RSP: 0018:ffff880235457d48 EFLAGS: 00010246 |
| [...] |
| |
| |
| function graph tracer |
| --------------------------- |
| |
| This tracer is similar to the function tracer except that it |
| probes a function on its entry and its exit. This is done by |
| using a dynamically allocated stack of return addresses in each |
| task_struct. On function entry the tracer overwrites the return |
| address of each function traced to set a custom probe. Thus the |
| original return address is stored on the stack of return address |
| in the task_struct. |
| |
| Probing on both ends of a function leads to special features |
| such as: |
| |
| - measure of a function's time execution |
| - having a reliable call stack to draw function calls graph |
| |
| This tracer is useful in several situations: |
| |
| - you want to find the reason of a strange kernel behavior and |
| need to see what happens in detail on any areas (or specific |
| ones). |
| |
| - you are experiencing weird latencies but it's difficult to |
| find its origin. |
| |
| - you want to find quickly which path is taken by a specific |
| function |
| |
| - you just want to peek inside a working kernel and want to see |
| what happens there. |
| |
| # tracer: function_graph |
| # |
| # CPU DURATION FUNCTION CALLS |
| # | | | | | | | |
| |
| 0) | sys_open() { |
| 0) | do_sys_open() { |
| 0) | getname() { |
| 0) | kmem_cache_alloc() { |
| 0) 1.382 us | __might_sleep(); |
| 0) 2.478 us | } |
| 0) | strncpy_from_user() { |
| 0) | might_fault() { |
| 0) 1.389 us | __might_sleep(); |
| 0) 2.553 us | } |
| 0) 3.807 us | } |
| 0) 7.876 us | } |
| 0) | alloc_fd() { |
| 0) 0.668 us | _spin_lock(); |
| 0) 0.570 us | expand_files(); |
| 0) 0.586 us | _spin_unlock(); |
| |
| |
| There are several columns that can be dynamically |
| enabled/disabled. You can use every combination of options you |
| want, depending on your needs. |
| |
| - The cpu number on which the function executed is default |
| enabled. It is sometimes better to only trace one cpu (see |
| tracing_cpu_mask file) or you might sometimes see unordered |
| function calls while cpu tracing switch. |
| |
| hide: echo nofuncgraph-cpu > trace_options |
| show: echo funcgraph-cpu > trace_options |
| |
| - The duration (function's time of execution) is displayed on |
| the closing bracket line of a function or on the same line |
| than the current function in case of a leaf one. It is default |
| enabled. |
| |
| hide: echo nofuncgraph-duration > trace_options |
| show: echo funcgraph-duration > trace_options |
| |
| - The overhead field precedes the duration field in case of |
| reached duration thresholds. |
| |
| hide: echo nofuncgraph-overhead > trace_options |
| show: echo funcgraph-overhead > trace_options |
| depends on: funcgraph-duration |
| |
| ie: |
| |
| 0) | up_write() { |
| 0) 0.646 us | _spin_lock_irqsave(); |
| 0) 0.684 us | _spin_unlock_irqrestore(); |
| 0) 3.123 us | } |
| 0) 0.548 us | fput(); |
| 0) + 58.628 us | } |
| |
| [...] |
| |
| 0) | putname() { |
| 0) | kmem_cache_free() { |
| 0) 0.518 us | __phys_addr(); |
| 0) 1.757 us | } |
| 0) 2.861 us | } |
| 0) ! 115.305 us | } |
| 0) ! 116.402 us | } |
| |
| + means that the function exceeded 10 usecs. |
| ! means that the function exceeded 100 usecs. |
| |
| |
| - The task/pid field displays the thread cmdline and pid which |
| executed the function. It is default disabled. |
| |
| hide: echo nofuncgraph-proc > trace_options |
| show: echo funcgraph-proc > trace_options |
| |
| ie: |
| |
| # tracer: function_graph |
| # |
| # CPU TASK/PID DURATION FUNCTION CALLS |
| # | | | | | | | | | |
| 0) sh-4802 | | d_free() { |
| 0) sh-4802 | | call_rcu() { |
| 0) sh-4802 | | __call_rcu() { |
| 0) sh-4802 | 0.616 us | rcu_process_gp_end(); |
| 0) sh-4802 | 0.586 us | check_for_new_grace_period(); |
| 0) sh-4802 | 2.899 us | } |
| 0) sh-4802 | 4.040 us | } |
| 0) sh-4802 | 5.151 us | } |
| 0) sh-4802 | + 49.370 us | } |
| |
| |
| - The absolute time field is an absolute timestamp given by the |
| system clock since it started. A snapshot of this time is |
| given on each entry/exit of functions |
| |
| hide: echo nofuncgraph-abstime > trace_options |
| show: echo funcgraph-abstime > trace_options |
| |
| ie: |
| |
| # |
| # TIME CPU DURATION FUNCTION CALLS |
| # | | | | | | | | |
| 360.774522 | 1) 0.541 us | } |
| 360.774522 | 1) 4.663 us | } |
| 360.774523 | 1) 0.541 us | __wake_up_bit(); |
| 360.774524 | 1) 6.796 us | } |
| 360.774524 | 1) 7.952 us | } |
| 360.774525 | 1) 9.063 us | } |
| 360.774525 | 1) 0.615 us | journal_mark_dirty(); |
| 360.774527 | 1) 0.578 us | __brelse(); |
| 360.774528 | 1) | reiserfs_prepare_for_journal() { |
| 360.774528 | 1) | unlock_buffer() { |
| 360.774529 | 1) | wake_up_bit() { |
| 360.774529 | 1) | bit_waitqueue() { |
| 360.774530 | 1) 0.594 us | __phys_addr(); |
| |
| |
| You can put some comments on specific functions by using |
| trace_printk() For example, if you want to put a comment inside |
| the __might_sleep() function, you just have to include |
| <linux/ftrace.h> and call trace_printk() inside __might_sleep() |
| |
| trace_printk("I'm a comment!\n") |
| |
| will produce: |
| |
| 1) | __might_sleep() { |
| 1) | /* I'm a comment! */ |
| 1) 1.449 us | } |
| |
| |
| You might find other useful features for this tracer in the |
| following "dynamic ftrace" section such as tracing only specific |
| functions or tasks. |
| |
| 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 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 \ |
| > set_ftrace_filter |
| # echo ftrace > current_tracer |
| # echo 1 > tracing_enabled |
| # usleep 1 |
| # echo 0 > tracing_enabled |
| # cat 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 set_ftrace_filter |
| hrtimer_interrupt |
| sys_nanosleep |
| |
| |
| Perhaps this is not enough. The filters also allow simple wild |
| cards. Only the following are currently available |
| |
| <match>* - will match functions that begin with <match> |
| *<match> - will match functions that end with <match> |
| *<match>* - will match functions that have <match> in it |
| |
| These are the only wild cards which are supported. |
| |
| <match>*<match> will not work. |
| |
| Note: It is better to use quotes to enclose the wild cards, |
| otherwise the shell may expand the parameters into names |
| of files in the local directory. |
| |
| # echo 'hrtimer_*' > 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 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 > set_ftrace_filter |
| # cat set_ftrace_filter |
| # |
| |
| Again, now we want to append. |
| |
| # echo sys_nanosleep > set_ftrace_filter |
| # cat set_ftrace_filter |
| sys_nanosleep |
| # echo 'hrtimer_*' >> set_ftrace_filter |
| # cat 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*' > 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. |
| |
| |
| Dynamic ftrace with the function graph tracer |
| --------------------------------------------- |
| |
| Although what has been explained above concerns both the |
| function tracer and the function-graph-tracer, there are some |
| special features only available in the function-graph tracer. |
| |
| If you want to trace only one function and all of its children, |
| you just have to echo its name into set_graph_function: |
| |
| echo __do_fault > set_graph_function |
| |
| will produce the following "expanded" trace of the __do_fault() |
| function: |
| |
| 0) | __do_fault() { |
| 0) | filemap_fault() { |
| 0) | find_lock_page() { |
| 0) 0.804 us | find_get_page(); |
| 0) | __might_sleep() { |
| 0) 1.329 us | } |
| 0) 3.904 us | } |
| 0) 4.979 us | } |
| 0) 0.653 us | _spin_lock(); |
| 0) 0.578 us | page_add_file_rmap(); |
| 0) 0.525 us | native_set_pte_at(); |
| 0) 0.585 us | _spin_unlock(); |
| 0) | unlock_page() { |
| 0) 0.541 us | page_waitqueue(); |
| 0) 0.639 us | __wake_up_bit(); |
| 0) 2.786 us | } |
| 0) + 14.237 us | } |
| 0) | __do_fault() { |
| 0) | filemap_fault() { |
| 0) | find_lock_page() { |
| 0) 0.698 us | find_get_page(); |
| 0) | __might_sleep() { |
| 0) 1.412 us | } |
| 0) 3.950 us | } |
| 0) 5.098 us | } |
| 0) 0.631 us | _spin_lock(); |
| 0) 0.571 us | page_add_file_rmap(); |
| 0) 0.526 us | native_set_pte_at(); |
| 0) 0.586 us | _spin_unlock(); |
| 0) | unlock_page() { |
| 0) 0.533 us | page_waitqueue(); |
| 0) 0.638 us | __wake_up_bit(); |
| 0) 2.793 us | } |
| 0) + 14.012 us | } |
| |
| You can also expand several functions at once: |
| |
| echo sys_open > set_graph_function |
| echo sys_close >> set_graph_function |
| |
| Now if you want to go back to trace all functions you can clear |
| this special filter via: |
| |
| echo > set_graph_function |
| |
| |
| 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 > current_tracer |
| # cat trace_pipe > /tmp/trace.out & |
| [1] 4153 |
| # echo 1 > tracing_enabled |
| # usleep 1 |
| # echo 0 > tracing_enabled |
| # cat 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 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 > current_tracer |
| # echo 10000 > buffer_size_kb |
| # cat 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 > buffer_size_kb |
| -bash: echo: write error: Cannot allocate memory |
| # cat buffer_size_kb |
| 85 |
| |
| ----------- |
| |
| More details can be found in the source code, in the |
| kernel/trace/*.c files. |