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diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt new file mode 100644 index 0000000..fd9a3e6 --- /dev/null +++ b/Documentation/trace/ftrace.txt @@ -0,0 +1,1828 @@ + 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. + + 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 either + "trace" or "latency_trace". A new max trace will + only be recorded if the latency is greater than + the value in this file. (in microseconds) + + buffer_size_kb: + + This sets or displays the number of kilobytes each CPU + buffer can hold. The tracer buffers are the same size + for each CPU. The displayed number is the size of the + CPU buffer and not total size of all buffers. The + trace buffers are allocated in pages (blocks of memory + that the kernel uses for allocation, usually 4 KB in size). + If the last page allocated has room for more bytes + than requested, the rest of the page will be used, + making the actual allocation bigger than requested. + ( Note, the size may not be a multiple of the page size + due to buffer managment overhead. ) + + This can only be updated when the current_tracer + is set to "nop". + + tracing_cpumask: + + This is a mask that lets the user only trace + on specified CPUS. The format is a hex string + representing the CPUS. + + set_ftrace_filter: + + When dynamic ftrace is configured in (see the + section below "dynamic ftrace"), the code is dynamically + modified (code text rewrite) to disable calling of the + function profiler (mcount). This lets tracing be configured + in with practically no overhead in performance. This also + has a side effect of enabling or disabling specific functions + to be traced. Echoing names of functions into this file + will limit the trace to only those functions. + + set_ftrace_notrace: + + This has an effect opposite to that of + set_ftrace_filter. Any function that is added here will not + be traced. If a function exists in both set_ftrace_filter + and set_ftrace_notrace, the function will _not_ be traced. + + set_ftrace_pid: + + Have the function tracer only trace a single thread. + + 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_tracer" + + 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 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. + + "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 +-------------------- + +For traces that display latency times, the latency_trace file +gives somewhat more information to see why a latency happened. +Here is a typical trace. + +# tracer: irqsoff +# +irqsoff latency trace v1.1.5 on 2.6.26-rc8 +-------------------------------------------------------------------- + latency: 97 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) + ----------------- + | task: swapper-0 (uid:0 nice:0 policy:0 rt_prio:0) + ----------------- + => started at: apic_timer_interrupt + => ended at: do_softirq + +# _------=> CPU# +# / _-----=> irqs-off +# | / _----=> need-resched +# || / _---=> hardirq/softirq +# ||| / _--=> preempt-depth +# |||| / +# ||||| delay +# cmd pid ||||| time | caller +# \ / ||||| \ | / + <idle>-0 0d..1 0us+: trace_hardirqs_off_thunk (apic_timer_interrupt) + <idle>-0 0d.s. 97us : __do_softirq (do_softirq) + <idle>-0 0d.s1 98us : trace_hardirqs_on (do_softirq) + + +This shows that the current tracer is "irqsoff" tracing the time +for which interrupts were disabled. It gives the trace version +and the version of the kernel upon which this was executed on +(2.6.26-rc8). Then it displays the max latency in microsecs (97 +us). The number of trace entries displayed and the total number +recorded (both are three: #3/3). The type of preemption that was +used (PREEMPT). VP, KP, SP, and HP are always zero and are +reserved for later use. #P is the number of online CPUS (#P:2). + +The task is the process that was running when the latency +occurred. (swapper pid: 0). + +The start and stop (the functions in which the interrupts were +disabled and enabled respectively) that caused the latencies: + + apic_timer_interrupt is where the interrupts were disabled. + do_softirq is where they were enabled again. + +The next lines after the header are the trace itself. The header +explains which is which. + + cmd: The name of the process in the trace. + + pid: The PID of that process. + + CPU#: The CPU which the process was running on. + + irqs-off: 'd' interrupts are disabled. '.' otherwise. + Note: If the architecture does not support a way to + read the irq flags variable, an 'X' will always + be printed here. + + need-resched: 'N' task need_resched is set, '.' otherwise. + + hardirq/softirq: + 'H' - hard irq occurred inside a softirq. + 'h' - hard irq is running + 's' - soft irq is running + '.' - normal context. + + preempt-depth: The level of preempt_disabled + +The above is mostly meaningful for kernel developers. + + time: This differs from the trace file output. The trace file output + includes an absolute timestamp. The timestamp used by the + latency_trace file is relative to the start of the trace. + + delay: This is just to help catch your eye a bit better. And + needs to be fixed to be only relative to the same CPU. + The marks are determined by the difference between this + current trace and the next trace. + '!' - greater than preempt_mark_thresh (default 100) + '+' - greater than 1 microsecond + ' ' - less than or equal to 1 microsecond. + + The rest is the same as the 'trace' file. + + +trace_options +------------- + +The trace_options file is used to control what gets printed in +the trace output. To see what is available, simply cat the file: + + cat /debug/tracing/trace_options + print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \ + noblock nostacktrace nosched-tree nouserstacktrace nosym-userobj + +To disable one of the options, echo in the option prepended with +"no". + + echo noprint-parent > /debug/tracing/trace_options + +To enable an option, leave off the "no". + + echo sym-offset > /debug/tracing/trace_options + +Here are the available options: + + print-parent - On function traces, display the calling (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 latency_trace file. + + bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \ + (+0.000ms): simple_strtoul (strict_strtoul) + + raw - This will display raw numbers. This option is best for + use with user applications that can translate the raw + numbers better than having it done in the kernel. + + hex - Similar to raw, but the numbers will be in a hexadecimal + format. + + bin - This will print out the formats in raw binary. + + block - TBD (needs update) + + stacktrace - This is one of the options that changes the trace + itself. When a trace is recorded, so is the stack + of functions. This allows for back traces of + trace sites. + + userstacktrace - This option changes the trace. It records a + stacktrace of the current userspace thread. + + sym-userobj - when user stacktrace are enabled, look up which + object the address belongs to, and print a + relative address. This is especially useful when + ASLR is on, otherwise you don't get a chance to + resolve the address to object/file/line after + the app is no longer running + + The lookup is performed when you read + trace,trace_pipe,latency_trace. Example: + + a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0 +x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6] + + sched-tree - 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. + + +sched_switch +------------ + +This tracer simply records schedule switches. Here is an example +of how to use it. + + # echo sched_switch > /debug/tracing/current_tracer + # echo 1 > /debug/tracing/tracing_enabled + # sleep 1 + # echo 0 > /debug/tracing/tracing_enabled + # cat /debug/tracing/trace + +# tracer: sched_switch +# +# TASK-PID CPU# TIMESTAMP FUNCTION +# | | | | | + bash-3997 [01] 240.132281: 3997:120:R + 4055:120:R + bash-3997 [01] 240.132284: 3997:120:R ==> 4055:120:R + sleep-4055 [01] 240.132371: 4055:120:S ==> 3997:120:R + bash-3997 [01] 240.132454: 3997:120:R + 4055:120:S + bash-3997 [01] 240.132457: 3997:120:R ==> 4055:120:R + sleep-4055 [01] 240.132460: 4055:120:D ==> 3997:120:R + bash-3997 [01] 240.132463: 3997:120:R + 4055:120:D + bash-3997 [01] 240.132465: 3997:120:R ==> 4055:120:R + <idle>-0 [00] 240.132589: 0:140:R + 4:115:S + <idle>-0 [00] 240.132591: 0:140:R ==> 4:115:R + ksoftirqd/0-4 [00] 240.132595: 4:115:S ==> 0:140:R + <idle>-0 [00] 240.132598: 0:140:R + 4:115:S + <idle>-0 [00] 240.132599: 0:140:R ==> 4:115:R + ksoftirqd/0-4 [00] 240.132603: 4:115:S ==> 0:140:R + sleep-4055 [01] 240.133058: 4055:120:S ==> 3997:120:R + [...] + + +As we have discussed previously about this format, the header +shows the name of the trace and points to the options. The +"FUNCTION" is a misnomer since here it represents the wake ups +and context switches. + +The sched_switch file only lists the wake ups (represented with +'+') and context switches ('==>') with the previous task or +current task first followed by the next task or task waking up. +The format for both of these is PID:KERNEL-PRIO:TASK-STATE. +Remember that the KERNEL-PRIO is the inverse of the actual +priority with zero (0) being the highest priority and the nice +values starting at 100 (nice -20). Below is a quick chart to map +the kernel priority to user land priorities. + + Kernel priority: 0 to 99 ==> user RT priority 99 to 0 + Kernel priority: 100 to 139 ==> user nice -20 to 19 + Kernel priority: 140 ==> idle task priority + +The task states are: + + R - running : wants to run, may not actually be running + S - sleep : process is waiting to be woken up (handles signals) + D - disk sleep (uninterruptible sleep) : process must be woken up + (ignores signals) + T - stopped : process suspended + t - traced : process is being traced (with something like gdb) + Z - zombie : process waiting to be cleaned up + X - unknown + + +ftrace_enabled +-------------- + +The following tracers (listed below) give different output +depending on whether or not the sysctl ftrace_enabled is set. To +set ftrace_enabled, one can either use the sysctl function or +set it via the proc file system interface. + + sysctl kernel.ftrace_enabled=1 + + or + + echo 1 > /proc/sys/kernel/ftrace_enabled + +To disable ftrace_enabled simply replace the '1' with '0' in the +above commands. + +When ftrace_enabled is set the tracers will also record the +functions that are within the trace. The descriptions of the +tracers will also show an example with ftrace enabled. + + +irqsoff +------- + +When interrupts are disabled, the CPU can not react to any other +external event (besides NMIs and SMIs). This prevents the timer +interrupt from triggering or the mouse interrupt from letting +the kernel know of a new mouse event. The result is a latency +with the reaction time. + +The irqsoff tracer tracks the time for which interrupts are +disabled. When a new maximum latency is hit, the tracer saves +the trace leading up to that latency point so that every time a +new maximum is reached, the old saved trace is discarded and the +new trace is saved. + +To reset the maximum, echo 0 into tracing_max_latency. Here is +an example: + + # echo irqsoff > /debug/tracing/current_tracer + # echo 0 > /debug/tracing/tracing_max_latency + # echo 1 > /debug/tracing/tracing_enabled + # ls -ltr + [...] + # echo 0 > /debug/tracing/tracing_enabled + # cat /debug/tracing/latency_trace +# tracer: irqsoff +# +irqsoff latency trace v1.1.5 on 2.6.26 +-------------------------------------------------------------------- + latency: 12 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) + ----------------- + | task: bash-3730 (uid:0 nice:0 policy:0 rt_prio:0) + ----------------- + => started at: sys_setpgid + => ended at: sys_setpgid + +# _------=> CPU# +# / _-----=> irqs-off +# | / _----=> need-resched +# || / _---=> hardirq/softirq +# ||| / _--=> preempt-depth +# |||| / +# ||||| delay +# cmd pid ||||| time | caller +# \ / ||||| \ | / + bash-3730 1d... 0us : _write_lock_irq (sys_setpgid) + bash-3730 1d..1 1us+: _write_unlock_irq (sys_setpgid) + bash-3730 1d..2 14us : trace_hardirqs_on (sys_setpgid) + + +Here we see that that we had a latency of 12 microsecs (which is +very good). The _write_lock_irq in sys_setpgid disabled +interrupts. The difference between the 12 and the displayed +timestamp 14us occurred because the clock was incremented +between the time of recording the max latency and the time of +recording the function that had that latency. + +Note the above example had ftrace_enabled not set. If we set the +ftrace_enabled, we get a much larger output: + +# tracer: irqsoff +# +irqsoff latency trace v1.1.5 on 2.6.26-rc8 +-------------------------------------------------------------------- + latency: 50 us, #101/101, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) + ----------------- + | task: ls-4339 (uid:0 nice:0 policy:0 rt_prio:0) + ----------------- + => started at: __alloc_pages_internal + => ended at: __alloc_pages_internal + +# _------=> CPU# +# / _-----=> irqs-off +# | / _----=> need-resched +# || / _---=> hardirq/softirq +# ||| / _--=> preempt-depth +# |||| / +# ||||| delay +# cmd pid ||||| time | caller +# \ / ||||| \ | / + ls-4339 0...1 0us+: get_page_from_freelist (__alloc_pages_internal) + ls-4339 0d..1 3us : rmqueue_bulk (get_page_from_freelist) + ls-4339 0d..1 3us : _spin_lock (rmqueue_bulk) + ls-4339 0d..1 4us : add_preempt_count (_spin_lock) + ls-4339 0d..2 4us : __rmqueue (rmqueue_bulk) + ls-4339 0d..2 5us : __rmqueue_smallest (__rmqueue) + ls-4339 0d..2 5us : __mod_zone_page_state (__rmqueue_smallest) + ls-4339 0d..2 6us : __rmqueue (rmqueue_bulk) + ls-4339 0d..2 6us : __rmqueue_smallest (__rmqueue) + ls-4339 0d..2 7us : __mod_zone_page_state (__rmqueue_smallest) + ls-4339 0d..2 7us : __rmqueue (rmqueue_bulk) + ls-4339 0d..2 8us : __rmqueue_smallest (__rmqueue) +[...] + ls-4339 0d..2 46us : __rmqueue_smallest (__rmqueue) + ls-4339 0d..2 47us : __mod_zone_page_state (__rmqueue_smallest) + ls-4339 0d..2 47us : __rmqueue (rmqueue_bulk) + ls-4339 0d..2 48us : __rmqueue_smallest (__rmqueue) + ls-4339 0d..2 48us : __mod_zone_page_state (__rmqueue_smallest) + ls-4339 0d..2 49us : _spin_unlock (rmqueue_bulk) + ls-4339 0d..2 49us : sub_preempt_count (_spin_unlock) + ls-4339 0d..1 50us : get_page_from_freelist (__alloc_pages_internal) + ls-4339 0d..2 51us : trace_hardirqs_on (__alloc_pages_internal) + + + +Here we traced a 50 microsecond latency. But we also see all the +functions that were called during that time. Note that by +enabling function tracing, we incur an added overhead. This +overhead may extend the latency times. But nevertheless, this +trace has provided some very helpful debugging information. + + +preemptoff +---------- + +When preemption is disabled, we may be able to receive +interrupts but the task cannot be preempted and a higher +priority task must wait for preemption to be enabled again +before it can preempt a lower priority task. + +The preemptoff tracer traces the places that disable preemption. +Like the irqsoff tracer, it records the maximum latency for +which preemption was disabled. The control of preemptoff tracer +is much like the irqsoff tracer. + + # echo preemptoff > /debug/tracing/current_tracer + # echo 0 > /debug/tracing/tracing_max_latency + # echo 1 > /debug/tracing/tracing_enabled + # ls -ltr + [...] + # echo 0 > /debug/tracing/tracing_enabled + # cat /debug/tracing/latency_trace +# tracer: preemptoff +# +preemptoff latency trace v1.1.5 on 2.6.26-rc8 +-------------------------------------------------------------------- + latency: 29 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) + ----------------- + | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) + ----------------- + => started at: do_IRQ + => ended at: __do_softirq + +# _------=> CPU# +# / _-----=> irqs-off +# | / _----=> need-resched +# || / _---=> hardirq/softirq +# ||| / _--=> preempt-depth +# |||| / +# ||||| delay +# cmd pid ||||| time | caller +# \ / ||||| \ | / + sshd-4261 0d.h. 0us+: irq_enter (do_IRQ) + sshd-4261 0d.s. 29us : _local_bh_enable (__do_softirq) + sshd-4261 0d.s1 30us : trace_preempt_on (__do_softirq) + + +This has some more changes. Preemption was disabled when an +interrupt came in (notice the 'h'), and was enabled while doing +a softirq. (notice the 's'). But we also see that interrupts +have been disabled when entering the preempt off section and +leaving it (the 'd'). We do not know if interrupts were enabled +in the mean time. + +# tracer: preemptoff +# +preemptoff latency trace v1.1.5 on 2.6.26-rc8 +-------------------------------------------------------------------- + latency: 63 us, #87/87, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) + ----------------- + | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) + ----------------- + => started at: remove_wait_queue + => ended at: __do_softirq + +# _------=> CPU# +# / _-----=> irqs-off +# | / _----=> need-resched +# || / _---=> hardirq/softirq +# ||| / _--=> preempt-depth +# |||| / +# ||||| delay +# cmd pid ||||| time | caller +# \ / ||||| \ | / + sshd-4261 0d..1 0us : _spin_lock_irqsave (remove_wait_queue) + sshd-4261 0d..1 1us : _spin_unlock_irqrestore (remove_wait_queue) + sshd-4261 0d..1 2us : do_IRQ (common_interrupt) + sshd-4261 0d..1 2us : irq_enter (do_IRQ) + sshd-4261 0d..1 2us : idle_cpu (irq_enter) + sshd-4261 0d..1 3us : add_preempt_count (irq_enter) + sshd-4261 0d.h1 3us : idle_cpu (irq_enter) + sshd-4261 0d.h. 4us : handle_fasteoi_irq (do_IRQ) +[...] + sshd-4261 0d.h. 12us : add_preempt_count (_spin_lock) + sshd-4261 0d.h1 12us : ack_ioapic_quirk_irq (handle_fasteoi_irq) + sshd-4261 0d.h1 13us : move_native_irq (ack_ioapic_quirk_irq) + sshd-4261 0d.h1 13us : _spin_unlock (handle_fasteoi_irq) + sshd-4261 0d.h1 14us : sub_preempt_count (_spin_unlock) + sshd-4261 0d.h1 14us : irq_exit (do_IRQ) + sshd-4261 0d.h1 15us : sub_preempt_count (irq_exit) + sshd-4261 0d..2 15us : do_softirq (irq_exit) + sshd-4261 0d... 15us : __do_softirq (do_softirq) + sshd-4261 0d... 16us : __local_bh_disable (__do_softirq) + sshd-4261 0d... 16us+: add_preempt_count (__local_bh_disable) + sshd-4261 0d.s4 20us : add_preempt_count (__local_bh_disable) + sshd-4261 0d.s4 21us : sub_preempt_count (local_bh_enable) + sshd-4261 0d.s5 21us : sub_preempt_count (local_bh_enable) +[...] + sshd-4261 0d.s6 41us : add_preempt_count (__local_bh_disable) + sshd-4261 0d.s6 42us : sub_preempt_count (local_bh_enable) + sshd-4261 0d.s7 42us : sub_preempt_count (local_bh_enable) + sshd-4261 0d.s5 43us : add_preempt_count (__local_bh_disable) + sshd-4261 0d.s5 43us : sub_preempt_count (local_bh_enable_ip) + sshd-4261 0d.s6 44us : sub_preempt_count (local_bh_enable_ip) + sshd-4261 0d.s5 44us : add_preempt_count (__local_bh_disable) + sshd-4261 0d.s5 45us : sub_preempt_count (local_bh_enable) +[...] + sshd-4261 0d.s. 63us : _local_bh_enable (__do_softirq) + sshd-4261 0d.s1 64us : trace_preempt_on (__do_softirq) + + +The above is an example of the preemptoff trace with +ftrace_enabled set. Here we see that interrupts were disabled +the entire time. The irq_enter code lets us know that we entered +an interrupt 'h'. Before that, the functions being traced still +show that it is not in an interrupt, but we can see from the +functions themselves that this is not the case. + +Notice that __do_softirq when called does not have a +preempt_count. It may seem that we missed a preempt enabling. +What really happened is that the preempt count is held on the +thread's stack and we switched to the softirq stack (4K stacks +in effect). The code does not copy the preempt count, but +because interrupts are disabled, we do not need to worry about +it. Having a tracer like this is good for letting people know +what really happens inside the kernel. + + +preemptirqsoff +-------------- + +Knowing the locations that have interrupts disabled or +preemption disabled for the longest times is helpful. But +sometimes we would like to know when either preemption and/or +interrupts are disabled. + +Consider the following code: + + local_irq_disable(); + call_function_with_irqs_off(); + preempt_disable(); + call_function_with_irqs_and_preemption_off(); + local_irq_enable(); + call_function_with_preemption_off(); + preempt_enable(); + +The irqsoff tracer will record the total length of +call_function_with_irqs_off() and +call_function_with_irqs_and_preemption_off(). + +The preemptoff tracer will record the total length of +call_function_with_irqs_and_preemption_off() and +call_function_with_preemption_off(). + +But neither will trace the time that interrupts and/or +preemption is disabled. This total time is the time that we can +not schedule. To record this time, use the preemptirqsoff +tracer. + +Again, using this trace is much like the irqsoff and preemptoff +tracers. + + # echo preemptirqsoff > /debug/tracing/current_tracer + # echo 0 > /debug/tracing/tracing_max_latency + # echo 1 > /debug/tracing/tracing_enabled + # ls -ltr + [...] + # echo 0 > /debug/tracing/tracing_enabled + # cat /debug/tracing/latency_trace +# tracer: preemptirqsoff +# +preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8 +-------------------------------------------------------------------- + latency: 293 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) + ----------------- + | task: ls-4860 (uid:0 nice:0 policy:0 rt_prio:0) + ----------------- + => started at: apic_timer_interrupt + => ended at: __do_softirq + +# _------=> CPU# +# / _-----=> irqs-off +# | / _----=> need-resched +# || / _---=> hardirq/softirq +# ||| / _--=> preempt-depth +# |||| / +# ||||| delay +# cmd pid ||||| time | caller +# \ / ||||| \ | / + ls-4860 0d... 0us!: trace_hardirqs_off_thunk (apic_timer_interrupt) + ls-4860 0d.s. 294us : _local_bh_enable (__do_softirq) + ls-4860 0d.s1 294us : trace_preempt_on (__do_softirq) + + + +The trace_hardirqs_off_thunk is called from assembly on x86 when +interrupts are disabled in the assembly code. Without the +function tracing, we do not know if interrupts were enabled +within the preemption points. We do see that it started with +preemption enabled. + +Here is a trace with ftrace_enabled set: + + +# tracer: preemptirqsoff +# +preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8 +-------------------------------------------------------------------- + latency: 105 us, #183/183, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) + ----------------- + | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0) + ----------------- + => started at: write_chan + => ended at: __do_softirq + +# _------=> CPU# +# / _-----=> irqs-off +# | / _----=> need-resched +# || / _---=> hardirq/softirq +# ||| / _--=> preempt-depth +# |||| / +# ||||| delay +# cmd pid ||||| time | caller +# \ / ||||| \ | / + ls-4473 0.N.. 0us : preempt_schedule (write_chan) + ls-4473 0dN.1 1us : _spin_lock (schedule) + ls-4473 0dN.1 2us : add_preempt_count (_spin_lock) + ls-4473 0d..2 2us : put_prev_task_fair (schedule) +[...] + ls-4473 0d..2 13us : set_normalized_timespec (ktime_get_ts) + ls-4473 0d..2 13us : __switch_to (schedule) + sshd-4261 0d..2 14us : finish_task_switch (schedule) + sshd-4261 0d..2 14us : _spin_unlock_irq (finish_task_switch) + sshd-4261 0d..1 15us : add_preempt_count (_spin_lock_irqsave) + sshd-4261 0d..2 16us : _spin_unlock_irqrestore (hrtick_set) + sshd-4261 0d..2 16us : do_IRQ (common_interrupt) + sshd-4261 0d..2 17us : irq_enter (do_IRQ) + sshd-4261 0d..2 17us : idle_cpu (irq_enter) + sshd-4261 0d..2 18us : add_preempt_count (irq_enter) + sshd-4261 0d.h2 18us : idle_cpu (irq_enter) + sshd-4261 0d.h. 18us : handle_fasteoi_irq (do_IRQ) + sshd-4261 0d.h. 19us : _spin_lock (handle_fasteoi_irq) + sshd-4261 0d.h. 19us : add_preempt_count (_spin_lock) + sshd-4261 0d.h1 20us : _spin_unlock (handle_fasteoi_irq) + sshd-4261 0d.h1 20us : sub_preempt_count (_spin_unlock) +[...] + sshd-4261 0d.h1 28us : _spin_unlock (handle_fasteoi_irq) + sshd-4261 0d.h1 29us : sub_preempt_count (_spin_unlock) + sshd-4261 0d.h2 29us : irq_exit (do_IRQ) + sshd-4261 0d.h2 29us : sub_preempt_count (irq_exit) + sshd-4261 0d..3 30us : do_softirq (irq_exit) + sshd-4261 0d... 30us : __do_softirq (do_softirq) + sshd-4261 0d... 31us : __local_bh_disable (__do_softirq) + sshd-4261 0d... 31us+: add_preempt_count (__local_bh_disable) + sshd-4261 0d.s4 34us : add_preempt_count (__local_bh_disable) +[...] + sshd-4261 0d.s3 43us : sub_preempt_count (local_bh_enable_ip) + sshd-4261 0d.s4 44us : sub_preempt_count (local_bh_enable_ip) + sshd-4261 0d.s3 44us : smp_apic_timer_interrupt (apic_timer_interrupt) + sshd-4261 0d.s3 45us : irq_enter (smp_apic_timer_interrupt) + sshd-4261 0d.s3 45us : idle_cpu (irq_enter) + sshd-4261 0d.s3 46us : add_preempt_count (irq_enter) + sshd-4261 0d.H3 46us : idle_cpu (irq_enter) + sshd-4261 0d.H3 47us : hrtimer_interrupt (smp_apic_timer_interrupt) + sshd-4261 0d.H3 47us : ktime_get (hrtimer_interrupt) +[...] + sshd-4261 0d.H3 81us : tick_program_event (hrtimer_interrupt) + sshd-4261 0d.H3 82us : ktime_get (tick_program_event) + sshd-4261 0d.H3 82us : ktime_get_ts (ktime_get) + sshd-4261 0d.H3 83us : getnstimeofday (ktime_get_ts) + sshd-4261 0d.H3 83us : set_normalized_timespec (ktime_get_ts) + sshd-4261 0d.H3 84us : clockevents_program_event (tick_program_event) + sshd-4261 0d.H3 84us : lapic_next_event (clockevents_program_event) + sshd-4261 0d.H3 85us : irq_exit (smp_apic_timer_interrupt) + sshd-4261 0d.H3 85us : sub_preempt_count (irq_exit) + sshd-4261 0d.s4 86us : sub_preempt_count (irq_exit) + sshd-4261 0d.s3 86us : add_preempt_count (__local_bh_disable) +[...] + sshd-4261 0d.s1 98us : sub_preempt_count (net_rx_action) + sshd-4261 0d.s. 99us : add_preempt_count (_spin_lock_irq) + sshd-4261 0d.s1 99us+: _spin_unlock_irq (run_timer_softirq) + sshd-4261 0d.s. 104us : _local_bh_enable (__do_softirq) + sshd-4261 0d.s. 104us : sub_preempt_count (_local_bh_enable) + sshd-4261 0d.s. 105us : _local_bh_enable (__do_softirq) + sshd-4261 0d.s1 105us : trace_preempt_on (__do_softirq) + + +This is a very interesting trace. It started with the preemption +of the ls task. We see that the task had the "need_resched" bit +set via the 'N' in the trace. Interrupts were disabled before +the spin_lock at the beginning of the trace. We see that a +schedule took place to run sshd. When the interrupts were +enabled, we took an interrupt. On return from the interrupt +handler, the softirq ran. We took another interrupt while +running the softirq as we see from the capital 'H'. + + +wakeup +------ + +In a Real-Time environment it is very important to know the +wakeup time it takes for the highest priority task that is woken +up to the time that it executes. This is also known as "schedule +latency". I stress the point that this is about RT tasks. It is +also important to know the scheduling latency of non-RT tasks, +but the average schedule latency is better for non-RT tasks. +Tools like LatencyTop are more appropriate for such +measurements. + +Real-Time environments are interested in the worst case latency. +That is the longest latency it takes for something to happen, +and not the average. We can have a very fast scheduler that may +only have a large latency once in a while, but that would not +work well with Real-Time tasks. The wakeup tracer was designed +to record the worst case wakeups of RT tasks. Non-RT tasks are +not recorded because the tracer only records one worst case and +tracing non-RT tasks that are unpredictable will overwrite the +worst case latency of RT tasks. + +Since this tracer only deals with RT tasks, we will run this +slightly differently than we did with the previous tracers. +Instead of performing an 'ls', we will run 'sleep 1' under +'chrt' which changes the priority of the task. + + # echo wakeup > /debug/tracing/current_tracer + # echo 0 > /debug/tracing/tracing_max_latency + # echo 1 > /debug/tracing/tracing_enabled + # chrt -f 5 sleep 1 + # echo 0 > /debug/tracing/tracing_enabled + # cat /debug/tracing/latency_trace +# tracer: wakeup +# +wakeup latency trace v1.1.5 on 2.6.26-rc8 +-------------------------------------------------------------------- + latency: 4 us, #2/2, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) + ----------------- + | task: sleep-4901 (uid:0 nice:0 policy:1 rt_prio:5) + ----------------- + +# _------=> CPU# +# / _-----=> irqs-off +# | / _----=> need-resched +# || / _---=> hardirq/softirq +# ||| / _--=> preempt-depth +# |||| / +# ||||| delay +# cmd pid ||||| time | caller +# \ / ||||| \ | / + <idle>-0 1d.h4 0us+: try_to_wake_up (wake_up_process) + <idle>-0 1d..4 4us : schedule (cpu_idle) + + +Running this on an idle system, we see that it only took 4 +microseconds to perform the task switch. Note, since the trace +marker in the schedule is before the actual "switch", we stop +the tracing when the recorded task is about to schedule in. This +may change if we add a new marker at the end of the scheduler. + +Notice that the recorded task is 'sleep' with the PID of 4901 +and it has an rt_prio of 5. This priority is user-space priority +and not the internal kernel priority. The policy is 1 for +SCHED_FIFO and 2 for SCHED_RR. + +Doing the same with chrt -r 5 and ftrace_enabled set. + +# tracer: wakeup +# +wakeup latency trace v1.1.5 on 2.6.26-rc8 +-------------------------------------------------------------------- + latency: 50 us, #60/60, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2) + ----------------- + | task: sleep-4068 (uid:0 nice:0 policy:2 rt_prio:5) + ----------------- + +# _------=> CPU# +# / _-----=> irqs-off +# | / _----=> need-resched +# || / _---=> hardirq/softirq +# ||| / _--=> preempt-depth +# |||| / +# ||||| delay +# cmd pid ||||| time | caller +# \ / ||||| \ | / +ksoftirq-7 1d.H3 0us : try_to_wake_up (wake_up_process) +ksoftirq-7 1d.H4 1us : sub_preempt_count (marker_probe_cb) +ksoftirq-7 1d.H3 2us : check_preempt_wakeup (try_to_wake_up) +ksoftirq-7 1d.H3 3us : update_curr (check_preempt_wakeup) +ksoftirq-7 1d.H3 4us : calc_delta_mine (update_curr) +ksoftirq-7 1d.H3 5us : __resched_task (check_preempt_wakeup) +ksoftirq-7 1d.H3 6us : task_wake_up_rt (try_to_wake_up) +ksoftirq-7 1d.H3 7us : _spin_unlock_irqrestore (try_to_wake_up) +[...] +ksoftirq-7 1d.H2 17us : irq_exit (smp_apic_timer_interrupt) +ksoftirq-7 1d.H2 18us : sub_preempt_count (irq_exit) +ksoftirq-7 1d.s3 19us : sub_preempt_count (irq_exit) +ksoftirq-7 1..s2 20us : rcu_process_callbacks (__do_softirq) +[...] +ksoftirq-7 1..s2 26us : __rcu_process_callbacks (rcu_process_callbacks) +ksoftirq-7 1d.s2 27us : _local_bh_enable (__do_softirq) +ksoftirq-7 1d.s2 28us : sub_preempt_count (_local_bh_enable) +ksoftirq-7 1.N.3 29us : sub_preempt_count (ksoftirqd) +ksoftirq-7 1.N.2 30us : _cond_resched (ksoftirqd) +ksoftirq-7 1.N.2 31us : __cond_resched (_cond_resched) +ksoftirq-7 1.N.2 32us : add_preempt_count (__cond_resched) +ksoftirq-7 1.N.2 33us : schedule (__cond_resched) +ksoftirq-7 1.N.2 33us : add_preempt_count (schedule) +ksoftirq-7 1.N.3 34us : hrtick_clear (schedule) +ksoftirq-7 1dN.3 35us : _spin_lock (schedule) +ksoftirq-7 1dN.3 36us : add_preempt_count (_spin_lock) +ksoftirq-7 1d..4 37us : put_prev_task_fair (schedule) +ksoftirq-7 1d..4 38us : update_curr (put_prev_task_fair) +[...] +ksoftirq-7 1d..5 47us : _spin_trylock (tracing_record_cmdline) +ksoftirq-7 1d..5 48us : add_preempt_count (_spin_trylock) +ksoftirq-7 1d..6 49us : _spin_unlock (tracing_record_cmdline) +ksoftirq-7 1d..6 49us : sub_preempt_count (_spin_unlock) +ksoftirq-7 1d..4 50us : schedule (__cond_resched) + +The interrupt went off while running ksoftirqd. This task runs +at SCHED_OTHER. Why did not we see the 'N' set early? This may +be a harmless bug with x86_32 and 4K stacks. On x86_32 with 4K +stacks configured, the interrupt and softirq run with their own +stack. Some information is held on the top of the task's stack +(need_resched and preempt_count are both stored there). The +setting of the NEED_RESCHED bit is done directly to the task's +stack, but the reading of the NEED_RESCHED is done by looking at +the current stack, which in this case is the stack for the hard +interrupt. This hides the fact that NEED_RESCHED has been set. +We do not see the 'N' until we switch back to the task's +assigned stack. + +function +-------- + +This tracer is the function tracer. Enabling the function tracer +can be done from the debug file system. Make sure the +ftrace_enabled is set; otherwise this tracer is a nop. + + # sysctl kernel.ftrace_enabled=1 + # echo function > /debug/tracing/current_tracer + # echo 1 > /debug/tracing/tracing_enabled + # usleep 1 + # echo 0 > /debug/tracing/tracing_enabled + # cat /debug/tracing/trace +# tracer: function +# +# TASK-PID CPU# TIMESTAMP FUNCTION +# | | | | | + bash-4003 [00] 123.638713: finish_task_switch <-schedule + bash-4003 [00] 123.638714: _spin_unlock_irq <-finish_task_switch + bash-4003 [00] 123.638714: sub_preempt_count <-_spin_unlock_irq + bash-4003 [00] 123.638715: hrtick_set <-schedule + bash-4003 [00] 123.638715: _spin_lock_irqsave <-hrtick_set + bash-4003 [00] 123.638716: add_preempt_count <-_spin_lock_irqsave + bash-4003 [00] 123.638716: _spin_unlock_irqrestore <-hrtick_set + bash-4003 [00] 123.638717: sub_preempt_count <-_spin_unlock_irqrestore + bash-4003 [00] 123.638717: hrtick_clear <-hrtick_set + bash-4003 [00] 123.638718: sub_preempt_count <-schedule + bash-4003 [00] 123.638718: sub_preempt_count <-preempt_schedule + bash-4003 [00] 123.638719: wait_for_completion <-__stop_machine_run + bash-4003 [00] 123.638719: wait_for_common <-wait_for_completion + bash-4003 [00] 123.638720: _spin_lock_irq <-wait_for_common + bash-4003 [00] 123.638720: add_preempt_count <-_spin_lock_irq +[...] + + +Note: function tracer uses ring buffers to store the above +entries. The newest data may overwrite the oldest data. +Sometimes using echo to stop the trace is not sufficient because +the tracing could have overwritten the data that you wanted to +record. For this reason, it is sometimes better to disable +tracing directly from a program. This allows you to stop the +tracing at the point that you hit the part that you are +interested in. To disable the tracing directly from a C program, +something like following code snippet can be used: + +int trace_fd; +[...] +int main(int argc, char *argv[]) { + [...] + trace_fd = open("/debug/tracing/tracing_enabled", O_WRONLY); + [...] + if (condition_hit()) { + write(trace_fd, "0", 1); + } + [...] +} + +Note: Here we hard coded the path name. The debugfs mount is not +guaranteed to be at /debug (and is more commonly at +/sys/kernel/debug). For simple one time traces, the above is +sufficent. For anything else, a search through /proc/mounts may +be needed to find where the debugfs file-system is mounted. + + +Single thread tracing +--------------------- + +By writing into /debug/tracing/set_ftrace_pid you can trace a +single thread. For example: + +# cat /debug/tracing/set_ftrace_pid +no pid +# echo 3111 > /debug/tracing/set_ftrace_pid +# cat /debug/tracing/set_ftrace_pid +3111 +# echo function > /debug/tracing/current_tracer +# cat /debug/tracing/trace | head + # tracer: function + # + # TASK-PID CPU# TIMESTAMP FUNCTION + # | | | | | + yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return + yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range + yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel + yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel + yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll + yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll +# echo -1 > /debug/tracing/set_ftrace_pid +# cat /debug/tracing/trace |head + # tracer: function + # + # TASK-PID CPU# TIMESTAMP FUNCTION + # | | | | | + ##### CPU 3 buffer started #### + yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait + yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry + yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry + yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit + yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit + +If you want to trace a function when executing, you could use +something like this simple program: + +#include <stdio.h> +#include <stdlib.h> +#include <sys/types.h> +#include <sys/stat.h> +#include <fcntl.h> +#include <unistd.h> + +int main (int argc, char **argv) +{ + if (argc < 1) + exit(-1); + + if (fork() > 0) { + int fd, ffd; + char line[64]; + int s; + + ffd = open("/debug/tracing/current_tracer", O_WRONLY); + if (ffd < 0) + exit(-1); + write(ffd, "nop", 3); + + fd = open("/debug/tracing/set_ftrace_pid", O_WRONLY); + s = sprintf(line, "%d\n", getpid()); + write(fd, line, s); + + write(ffd, "function", 8); + + close(fd); + close(ffd); + + execvp(argv[1], argv+1); + } + + return 0; +} + + +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 > /debug/tracing/trace_options + show: echo funcgraph-cpu > /debug/tracing/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 > /debug/tracing/trace_options + show: echo funcgraph-duration > /debug/tracing/trace_options + +- The overhead field precedes the duration field in case of + reached duration thresholds. + + hide: echo nofuncgraph-overhead > /debug/tracing/trace_options + show: echo funcgraph-overhead > /debug/tracing/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 > /debug/tracing/trace_options + show: echo funcgraph-proc > /debug/tracing/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 > /debug/tracing/trace_options + show: echo funcgraph-abstime > /debug/tracing/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 /debug/tracing/available_filter_functions +put_prev_task_idle +kmem_cache_create +pick_next_task_rt +get_online_cpus +pick_next_task_fair +mutex_lock +[...] + +If I am only interested in sys_nanosleep and hrtimer_interrupt: + + # echo sys_nanosleep hrtimer_interrupt \ + > /debug/tracing/set_ftrace_filter + # echo ftrace > /debug/tracing/current_tracer + # echo 1 > /debug/tracing/tracing_enabled + # usleep 1 + # echo 0 > /debug/tracing/tracing_enabled + # cat /debug/tracing/trace +# tracer: ftrace +# +# TASK-PID CPU# TIMESTAMP FUNCTION +# | | | | | + usleep-4134 [00] 1317.070017: hrtimer_interrupt <-smp_apic_timer_interrupt + usleep-4134 [00] 1317.070111: sys_nanosleep <-syscall_call + <idle>-0 [00] 1317.070115: hrtimer_interrupt <-smp_apic_timer_interrupt + +To see which functions are being traced, you can cat the file: + + # cat /debug/tracing/set_ftrace_filter +hrtimer_interrupt +sys_nanosleep + + +Perhaps this is not enough. The filters also allow simple wild +cards. Only the following are currently available + + <match>* - will match functions that begin with <match> + *<match> - will match functions that end with <match> + *<match>* - will match functions that have <match> in it + +These are the only wild cards which are supported. + + <match>*<match> will not work. + +Note: It is better to use quotes to enclose the wild cards, + otherwise the shell may expand the parameters into names + of files in the local directory. + + # echo 'hrtimer_*' > /debug/tracing/set_ftrace_filter + +Produces: + +# tracer: ftrace +# +# TASK-PID CPU# TIMESTAMP FUNCTION +# | | | | | + bash-4003 [00] 1480.611794: hrtimer_init <-copy_process + bash-4003 [00] 1480.611941: hrtimer_start <-hrtick_set + bash-4003 [00] 1480.611956: hrtimer_cancel <-hrtick_clear + bash-4003 [00] 1480.611956: hrtimer_try_to_cancel <-hrtimer_cancel + <idle>-0 [00] 1480.612019: hrtimer_get_next_event <-get_next_timer_interrupt + <idle>-0 [00] 1480.612025: hrtimer_get_next_event <-get_next_timer_interrupt + <idle>-0 [00] 1480.612032: hrtimer_get_next_event <-get_next_timer_interrupt + <idle>-0 [00] 1480.612037: hrtimer_get_next_event <-get_next_timer_interrupt + <idle>-0 [00] 1480.612382: hrtimer_get_next_event <-get_next_timer_interrupt + + +Notice that we lost the sys_nanosleep. + + # cat /debug/tracing/set_ftrace_filter +hrtimer_run_queues +hrtimer_run_pending +hrtimer_init +hrtimer_cancel +hrtimer_try_to_cancel +hrtimer_forward +hrtimer_start +hrtimer_reprogram +hrtimer_force_reprogram +hrtimer_get_next_event +hrtimer_interrupt +hrtimer_nanosleep +hrtimer_wakeup +hrtimer_get_remaining +hrtimer_get_res +hrtimer_init_sleeper + + +This is because the '>' and '>>' act just like they do in bash. +To rewrite the filters, use '>' +To append to the filters, use '>>' + +To clear out a filter so that all functions will be recorded +again: + + # echo > /debug/tracing/set_ftrace_filter + # cat /debug/tracing/set_ftrace_filter + # + +Again, now we want to append. + + # echo sys_nanosleep > /debug/tracing/set_ftrace_filter + # cat /debug/tracing/set_ftrace_filter +sys_nanosleep + # echo 'hrtimer_*' >> /debug/tracing/set_ftrace_filter + # cat /debug/tracing/set_ftrace_filter +hrtimer_run_queues +hrtimer_run_pending +hrtimer_init +hrtimer_cancel +hrtimer_try_to_cancel +hrtimer_forward +hrtimer_start +hrtimer_reprogram +hrtimer_force_reprogram +hrtimer_get_next_event +hrtimer_interrupt +sys_nanosleep +hrtimer_nanosleep +hrtimer_wakeup +hrtimer_get_remaining +hrtimer_get_res +hrtimer_init_sleeper + + +The set_ftrace_notrace prevents those functions from being +traced. + + # echo '*preempt*' '*lock*' > /debug/tracing/set_ftrace_notrace + +Produces: + +# tracer: ftrace +# +# TASK-PID CPU# TIMESTAMP FUNCTION +# | | | | | + bash-4043 [01] 115.281644: finish_task_switch <-schedule + bash-4043 [01] 115.281645: hrtick_set <-schedule + bash-4043 [01] 115.281645: hrtick_clear <-hrtick_set + bash-4043 [01] 115.281646: wait_for_completion <-__stop_machine_run + bash-4043 [01] 115.281647: wait_for_common <-wait_for_completion + bash-4043 [01] 115.281647: kthread_stop <-stop_machine_run + bash-4043 [01] 115.281648: init_waitqueue_head <-kthread_stop + bash-4043 [01] 115.281648: wake_up_process <-kthread_stop + bash-4043 [01] 115.281649: try_to_wake_up <-wake_up_process + +We can see that there's no more lock or preempt tracing. + + +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 > /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 + +----------- + +More details can be found in the source code, in the +kernel/tracing/*.c files. |
