[patch 01/25] ftrace: remove unneeded documentation
From: Greg KH
Date: Mon Aug 04 2008 - 17:34:10 EST
2.6.26-stable review patch. If anyone has any objections, please let us know.
------------------
There is no ftrace in the 2.6.26 kernel release, so remove the
documentation as it isn't needed.
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
---
Documentation/ftrace.txt | 1353 -----------------------------------------------
1 file changed, 1353 deletions(-)
--- a/Documentation/ftrace.txt
+++ /dev/null
@@ -1,1353 +0,0 @@
- ftrace - Function Tracer
- ========================
-
-Copyright 2008 Red Hat Inc.
-Author: Steven Rostedt <srostedt@xxxxxxxxxx>
-
-
-Introduction
-------------
-
-Ftrace is an internal tracer designed to help out developers and
-designers of systems to find what is going on inside the kernel.
-It can be used for debugging or analyzing latencies and performance
-issues that take place outside of user-space.
-
-Although ftrace is the function tracer, it also includes an
-infrastructure that allows for other types of tracing. Some of the
-tracers that are currently in ftrace is a tracer to trace
-context switches, the time it takes for a high priority task to
-run after it was woken up, the time interrupts are disabled, and
-more.
-
-
-The File System
----------------
-
-Ftrace uses the debugfs file system to hold the control files as well
-as the files to display output.
-
-To mount the debugfs system:
-
- # mkdir /debug
- # mount -t debugfs nodev /debug
-
-
-That's it! (assuming that you have ftrace configured into your kernel)
-
-After mounting the debugfs, you can see a directory called
-"tracing". This directory contains the control and output files
-of ftrace. Here is a list of some of the key files:
-
-
- Note: all time values are in microseconds.
-
- current_tracer : This is used to set or display the current tracer
- that is configured.
-
- available_tracers : This holds the different types of tracers that
- has been compiled into the kernel. The tracers
- listed here can be configured by echoing in their
- name into current_tracer.
-
- tracing_enabled : This sets or displays whether the current_tracer
- is activated and tracing or not. Echo 0 into this
- file to disable the tracer or 1 (or non-zero) to
- enable it.
-
- trace : This file holds the output of the trace in a human readable
- format.
-
- latency_trace : This file shows the same trace but the information
- is organized more to display possible latencies
- in the system.
-
- trace_pipe : The output is the same as the "trace" file but this
- file is meant to be streamed with live tracing.
- Reads from this file will block until new data
- is retrieved. Unlike the "trace" and "latency_trace"
- files, this file is a consumer. This means reading
- from this file causes sequential reads to display
- more current data. Once data is read from this
- file, it is consumed, and will not be read
- again with a sequential read. The "trace" and
- "latency_trace" files are static, and if the
- tracer isn't adding more data, they will display
- the same information every time they are read.
-
- iter_ctrl : This file lets the user control the amount of data
- that is displayed in one of the above output
- files.
-
- trace_max_latency : Some of the tracers record the max latency.
- For example, the time interrupts are disabled.
- This time is saved in this file. The max trace
- will also be stored, and displayed by either
- "trace" or "latency_trace". A new max trace will
- only be recorded if the latency is greater than
- the value in this file. (in microseconds)
-
- trace_entries : This sets or displays the number of trace
- entries each CPU buffer can hold. The tracer buffers
- are the same size for each CPU, so care must be
- taken when modifying the trace_entries. The number
- of actually entries will be the number given
- times the number of possible CPUS. The buffers
- are saved as individual pages, and the actual entries
- will always be rounded up to entries per page.
-
- This can only be updated when the current_tracer
- is set to "none".
-
- NOTE: It is planned on changing the allocated buffers
- from being the number of possible CPUS to
- the number of online CPUS.
-
- tracing_cpumask : This is a mask that lets the user only trace
- on specified CPUS. The format is a hex string
- representing the CPUS.
-
- set_ftrace_filter : When dynamic ftrace is configured in, the
- code is dynamically modified to disable calling
- of the function profiler (mcount). This lets
- tracing be configured in with practically no overhead
- in performance. This also has a side effect of
- enabling or disabling specific functions to be
- traced. Echoing in names of functions into this
- file will limit the trace to only those files.
-
- set_ftrace_notrace: This has the opposite effect that
- set_ftrace_filter has. Any function that is added
- here will not be traced. If a function exists
- in both set_ftrace_filter and set_ftrace_notrace
- the function will _not_ bet traced.
-
- available_filter_functions : When a function is encountered the first
- time by the dynamic tracer, it is recorded and
- later the call is converted into a nop. This file
- lists the functions that have been recorded
- by the dynamic tracer and these functions can
- be used to set the ftrace filter by the above
- "set_ftrace_filter" file.
-
-
-The Tracers
------------
-
-Here are the list of current tracers that can be configured.
-
- ftrace - function tracer that uses mcount to trace all functions.
- It is possible to filter out which functions that are
- traced when dynamic ftrace is configured in.
-
- sched_switch - traces the context switches between tasks.
-
- irqsoff - traces the areas that disable interrupts and saves off
- the trace with the longest max latency.
- See tracing_max_latency. When a new max is recorded,
- it replaces the old trace. It is best to view this
- trace with the latency_trace file.
-
- preemptoff - Similar to irqsoff but traces and records the time
- preemption is disabled.
-
- preemptirqsoff - Similar to irqsoff and preemptoff, but traces and
- records the largest time irqs and/or preemption is
- disabled.
-
- wakeup - Traces and records the max latency that it takes for
- the highest priority task to get scheduled after
- it has been woken up.
-
- none - This is not a tracer. To remove all tracers from tracing
- simply echo "none" into current_tracer.
-
-
-Examples of using the tracer
-----------------------------
-
-Here are typical examples of using the tracers with only controlling
-them with the debugfs interface (without using any user-land utilities).
-
-Output format:
---------------
-
-Here's an example of the output format of the file "trace"
-
- --------
-# tracer: ftrace
-#
-# TASK-PID CPU# TIMESTAMP FUNCTION
-# | | | | |
- bash-4251 [01] 10152.583854: path_put <-path_walk
- bash-4251 [01] 10152.583855: dput <-path_put
- bash-4251 [01] 10152.583855: _atomic_dec_and_lock <-dput
- --------
-
-A header is printed with the trace that is represented. In this case
-the tracer is "ftrace". Then a header showing the format. Task name
-"bash", the task PID "4251", the CPU that it was running on
-"01", the timestamp in <secs>.<usecs> format, the function name that was
-traced "path_put" and the parent function that called this function
-"path_walk".
-
-The sched_switch tracer also includes tracing of task wake ups and
-context switches.
-
- ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 2916:115:S
- ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 10:115:S
- ksoftirqd/1-7 [01] 1453.070013: 7:115:R ==> 10:115:R
- events/1-10 [01] 1453.070013: 10:115:S ==> 2916:115:R
- kondemand/1-2916 [01] 1453.070013: 2916:115:S ==> 7:115:R
- ksoftirqd/1-7 [01] 1453.070013: 7:115:S ==> 0:140:R
-
-Wake ups are represented by a "+" and the context switches show
-"==>". The format is:
-
- Context switches:
-
- Previous task Next Task
-
- <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 inverse to the
-priority that is usually displayed by user-space tools. Zero represents
-the highest priority (99). Prio 100 starts the "nice" priorities with
-100 being equal to nice -20 and 139 being nice 19. The prio "140" is
-reserved for the idle task which is the lowest priority thread (pid 0).
-
-
-Latency trace format
---------------------
-
-For traces that display latency times, the latency_trace file gives
-a bit more information to see why a latency happened. Here's a typical
-trace.
-
-# tracer: irqsoff
-#
-irqsoff latency trace v1.1.5 on 2.6.26-rc8
---------------------------------------------------------------------
- latency: 97 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
- -----------------
- | task: swapper-0 (uid:0 nice:0 policy:0 rt_prio:0)
- -----------------
- => started at: apic_timer_interrupt
- => ended at: do_softirq
-
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| /
-# ||||| delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- <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)
-
-
-vim:ft=help
-
-
-This shows that the current tracer is "irqsoff" tracing the time
-interrupts are disabled. It gives the trace version and the kernel
-this was executed on (2.6.26-rc8). Then it displays the max latency
-in microsecs (97 us). The number of trace entries displayed
-by the total number recorded (both are three: #3/3). The type of
-preemption that was used (PREEMPT). VP, KP, SP, and HP are always zero
-and reserved for later use. #P is the number of online CPUS (#P:2).
-
-The task is the process that was running when the latency happened.
-(swapper pid: 0).
-
-The start and stop that caused the latencies:
-
- apic_timer_interrupt is where the interrupts were disabled.
- do_softirq is where they were enabled again.
-
-The next lines after the header are the trace itself. The header
-explains which is which.
-
- cmd: The name of the process in the trace.
-
- pid: The PID of that process.
-
- CPU#: The CPU that the process was running on.
-
- irqs-off: 'd' interrupts are disabled. '.' otherwise.
-
- need-resched: 'N' task need_resched is set, '.' otherwise.
-
- hardirq/softirq:
- 'H' - hard irq happened inside a softirq.
- 'h' - hard irq is running
- 's' - soft irq is running
- '.' - normal context.
-
- preempt-depth: The level of preempt_disabled
-
-The above is mostly meaningful for kernel developers.
-
- time: This differs from the trace output where as the trace output
- contained a absolute timestamp. This timestamp is relative
- to the start of the first entry in the the trace.
-
- delay: This is just to help catch your eye a bit better. And
- needs to be fixed to be only relative to the same CPU.
- The marks is determined by the difference between this
- current trace and the next trace.
- '!' - greater than preempt_mark_thresh (default 100)
- '+' - greater than 1 microsecond
- ' ' - less than or equal to 1 microsecond.
-
- The rest is the same as the 'trace' file.
-
-
-iter_ctrl
----------
-
-The iter_ctrl file is used to control what gets printed in the trace
-output. To see what is available, simply cat the file:
-
- cat /debug/tracing/iter_ctrl
- print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \
- noblock nostacktrace nosched-tree
-
-To disable one of the options, echo in the option appended with "no".
-
- echo noprint-parent > /debug/tracing/iter_ctrl
-
-To enable an option, leave off the "no".
-
- echo sym-offest > /debug/tracing/iter_ctrl
-
-Here are the available options:
-
- print-parent - On function traces, display the calling function
- as well as the function being traced.
-
- print-parent:
- bash-4000 [01] 1477.606694: simple_strtoul <-strict_strtoul
-
- noprint-parent:
- bash-4000 [01] 1477.606694: simple_strtoul
-
-
- sym-offset - Display not only the function name, but also the offset
- in the function. For example, instead of seeing just
- "ktime_get" you will see "ktime_get+0xb/0x20"
-
- sym-offset:
- bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0
-
- sym-addr - this will also display the function address as well as
- the function name.
-
- sym-addr:
- bash-4000 [01] 1477.606694: simple_strtoul <c0339346>
-
- verbose - This deals with the latency_trace file.
-
- bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
- (+0.000ms): simple_strtoul (strict_strtoul)
-
- raw - This will display raw numbers. This option is best for use with
- user applications that can translate the raw numbers better than
- having it done in the kernel.
-
- hex - similar to raw, but the numbers will be in a hexadecimal format.
-
- bin - This will print out the formats in raw binary.
-
- block - TBD (needs update)
-
- stacktrace - This is one of the options that changes the trace itself.
- When a trace is recorded, so is the stack of functions.
- This allows for back traces of trace sites.
-
- sched-tree - TBD (any users??)
-
-
-sched_switch
-------------
-
-This tracer simply records schedule switches. Here's an example
-on how to implement it.
-
- # echo sched_switch > /debug/tracing/current_tracer
- # echo 1 > /debug/tracing/tracing_enabled
- # sleep 1
- # echo 0 > /debug/tracing/tracing_enabled
- # cat /debug/tracing/trace
-
-# tracer: sched_switch
-#
-# TASK-PID CPU# TIMESTAMP FUNCTION
-# | | | | |
- bash-3997 [01] 240.132281: 3997:120:R + 4055:120:R
- bash-3997 [01] 240.132284: 3997:120:R ==> 4055:120:R
- sleep-4055 [01] 240.132371: 4055:120:S ==> 3997:120:R
- bash-3997 [01] 240.132454: 3997:120:R + 4055:120:S
- bash-3997 [01] 240.132457: 3997:120:R ==> 4055:120:R
- sleep-4055 [01] 240.132460: 4055:120:D ==> 3997:120:R
- bash-3997 [01] 240.132463: 3997:120:R + 4055:120:D
- bash-3997 [01] 240.132465: 3997:120:R ==> 4055:120:R
- <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 only lists the wake ups (represented with '+')
-and context switches ('==>') with the previous task or current
-first followed by the next task or task waking up. The format for both
-of these is PID:KERNEL-PRIO:TASK-STATE. Remember that the KERNEL-PRIO
-is the inverse of the actual priority with zero (0) being the highest
-priority and the nice values starting at 100 (nice -20). Below is
-a quick chart to map the kernel priority to user land priorities.
-
- Kernel priority: 0 to 99 ==> user RT priority 99 to 0
- Kernel priority: 100 to 139 ==> user nice -20 to 19
- Kernel priority: 140 ==> idle task priority
-
-The task states are:
-
- R - running : wants to run, may not actually be running
- S - sleep : process is waiting to be woken up (handles signals)
- D - deep sleep : process must be woken up (ignores signals)
- T - stopped : process suspended
- t - traced : process is being traced (with something like gdb)
- Z - zombie : process waiting to be cleaned up
- X - unknown
-
-
-ftrace_enabled
---------------
-
-The following tracers give different output depending on whether
-or not the sysctl ftrace_enabled is set. To set ftrace_enabled,
-one can either use the sysctl function or set it via the proc
-file system interface.
-
- sysctl kernel.ftrace_enabled=1
-
- or
-
- echo 1 > /proc/sys/kernel/ftrace_enabled
-
-To disable ftrace_enabled simply replace the '1' with '0' in
-the above commands.
-
-When ftrace_enabled is set the tracers will also record the functions
-that are within the trace. The descriptions of the tracers
-will also show an example with ftrace enabled.
-
-
-irqsoff
--------
-
-When interrupts are disabled, the CPU can not react to any other
-external event (besides NMIs and SMIs). This prevents the timer
-interrupt from triggering or the mouse interrupt from letting the
-kernel know of a new mouse event. The result is a latency with the
-reaction time.
-
-The irqsoff tracer tracks the time interrupts are disabled and when
-they are re-enabled. When a new maximum latency is hit, it saves off
-the trace so that it may be retrieved at a later time. Every time a
-new maximum in reached, the old saved trace is discarded and the new
-trace is saved.
-
-To reset the maximum, echo 0 into tracing_max_latency. Here's an
-example:
-
- # echo irqsoff > /debug/tracing/current_tracer
- # echo 0 > /debug/tracing/tracing_max_latency
- # echo 1 > /debug/tracing/tracing_enabled
- # ls -ltr
- [...]
- # echo 0 > /debug/tracing/tracing_enabled
- # cat /debug/tracing/latency_trace
-# tracer: irqsoff
-#
-irqsoff latency trace v1.1.5 on 2.6.26-rc8
---------------------------------------------------------------------
- latency: 6 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
- -----------------
- | task: bash-4269 (uid:0 nice:0 policy:0 rt_prio:0)
- -----------------
- => started at: copy_page_range
- => ended at: copy_page_range
-
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| /
-# ||||| delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- bash-4269 1...1 0us+: _spin_lock (copy_page_range)
- bash-4269 1...1 7us : _spin_unlock (copy_page_range)
- bash-4269 1...2 7us : trace_preempt_on (copy_page_range)
-
-
-vim:ft=help
-
-Here we see that that we had a latency of 6 microsecs (which is
-very good). The spin_lock in copy_page_range disabled interrupts.
-The difference between the 6 and the displayed timestamp 7us is
-because the clock must have incremented between the time of recording
-the max latency and recording the function that had that latency.
-
-Note the above had ftrace_enabled not set. If we set the ftrace_enabled
-we get a much larger output:
-
-# tracer: irqsoff
-#
-irqsoff latency trace v1.1.5 on 2.6.26-rc8
---------------------------------------------------------------------
- latency: 50 us, #101/101, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
- -----------------
- | task: ls-4339 (uid:0 nice:0 policy:0 rt_prio:0)
- -----------------
- => started at: __alloc_pages_internal
- => ended at: __alloc_pages_internal
-
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| /
-# ||||| delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- ls-4339 0...1 0us+: get_page_from_freelist (__alloc_pages_internal)
- ls-4339 0d..1 3us : rmqueue_bulk (get_page_from_freelist)
- ls-4339 0d..1 3us : _spin_lock (rmqueue_bulk)
- ls-4339 0d..1 4us : add_preempt_count (_spin_lock)
- ls-4339 0d..2 4us : __rmqueue (rmqueue_bulk)
- ls-4339 0d..2 5us : __rmqueue_smallest (__rmqueue)
- ls-4339 0d..2 5us : __mod_zone_page_state (__rmqueue_smallest)
- ls-4339 0d..2 6us : __rmqueue (rmqueue_bulk)
- ls-4339 0d..2 6us : __rmqueue_smallest (__rmqueue)
- ls-4339 0d..2 7us : __mod_zone_page_state (__rmqueue_smallest)
- ls-4339 0d..2 7us : __rmqueue (rmqueue_bulk)
- ls-4339 0d..2 8us : __rmqueue_smallest (__rmqueue)
-[...]
- ls-4339 0d..2 46us : __rmqueue_smallest (__rmqueue)
- ls-4339 0d..2 47us : __mod_zone_page_state (__rmqueue_smallest)
- ls-4339 0d..2 47us : __rmqueue (rmqueue_bulk)
- ls-4339 0d..2 48us : __rmqueue_smallest (__rmqueue)
- ls-4339 0d..2 48us : __mod_zone_page_state (__rmqueue_smallest)
- ls-4339 0d..2 49us : _spin_unlock (rmqueue_bulk)
- ls-4339 0d..2 49us : sub_preempt_count (_spin_unlock)
- ls-4339 0d..1 50us : get_page_from_freelist (__alloc_pages_internal)
- ls-4339 0d..2 51us : trace_hardirqs_on (__alloc_pages_internal)
-
-
-vim:ft=help
-
-
-Here we traced a 50 microsecond latency. But we also see all the
-functions that were called during that time. Note that enabling
-function tracing we endure an added overhead. This overhead may
-extend the latency times. But never the less, this trace has provided
-some very helpful debugging.
-
-
-preemptoff
-----------
-
-When preemption is disabled we may be able to receive interrupts but
-the task can not be preempted and a higher priority task must wait
-for preemption to be enabled again before it can preempt a lower
-priority task.
-
-The preemptoff tracer traces the places that disables preemption.
-Like the irqsoff, it records the maximum latency that preemption
-was disabled. The control of preemptoff is much like the irqsoff.
-
- # echo preemptoff > /debug/tracing/current_tracer
- # echo 0 > /debug/tracing/tracing_max_latency
- # echo 1 > /debug/tracing/tracing_enabled
- # ls -ltr
- [...]
- # echo 0 > /debug/tracing/tracing_enabled
- # cat /debug/tracing/latency_trace
-# tracer: preemptoff
-#
-preemptoff latency trace v1.1.5 on 2.6.26-rc8
---------------------------------------------------------------------
- latency: 29 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
- -----------------
- | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
- -----------------
- => started at: do_IRQ
- => ended at: __do_softirq
-
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| /
-# ||||| delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- sshd-4261 0d.h. 0us+: irq_enter (do_IRQ)
- sshd-4261 0d.s. 29us : _local_bh_enable (__do_softirq)
- sshd-4261 0d.s1 30us : trace_preempt_on (__do_softirq)
-
-
-vim:ft=help
-
-This has some more changes. Preemption was disabled when an interrupt
-came in (notice the 'h'), and was enabled while doing a softirq.
-(notice the 's'). But we also see that interrupts have been disabled
-when entering the preempt off section and leaving it (the 'd').
-We do not know if interrupts were enabled in the mean time.
-
-# tracer: preemptoff
-#
-preemptoff latency trace v1.1.5 on 2.6.26-rc8
---------------------------------------------------------------------
- latency: 63 us, #87/87, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
- -----------------
- | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
- -----------------
- => started at: remove_wait_queue
- => ended at: __do_softirq
-
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| /
-# ||||| delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- sshd-4261 0d..1 0us : _spin_lock_irqsave (remove_wait_queue)
- sshd-4261 0d..1 1us : _spin_unlock_irqrestore (remove_wait_queue)
- sshd-4261 0d..1 2us : do_IRQ (common_interrupt)
- sshd-4261 0d..1 2us : irq_enter (do_IRQ)
- sshd-4261 0d..1 2us : idle_cpu (irq_enter)
- sshd-4261 0d..1 3us : add_preempt_count (irq_enter)
- sshd-4261 0d.h1 3us : idle_cpu (irq_enter)
- sshd-4261 0d.h. 4us : handle_fasteoi_irq (do_IRQ)
-[...]
- sshd-4261 0d.h. 12us : add_preempt_count (_spin_lock)
- sshd-4261 0d.h1 12us : ack_ioapic_quirk_irq (handle_fasteoi_irq)
- sshd-4261 0d.h1 13us : move_native_irq (ack_ioapic_quirk_irq)
- sshd-4261 0d.h1 13us : _spin_unlock (handle_fasteoi_irq)
- sshd-4261 0d.h1 14us : sub_preempt_count (_spin_unlock)
- sshd-4261 0d.h1 14us : irq_exit (do_IRQ)
- sshd-4261 0d.h1 15us : sub_preempt_count (irq_exit)
- sshd-4261 0d..2 15us : do_softirq (irq_exit)
- sshd-4261 0d... 15us : __do_softirq (do_softirq)
- sshd-4261 0d... 16us : __local_bh_disable (__do_softirq)
- sshd-4261 0d... 16us+: add_preempt_count (__local_bh_disable)
- sshd-4261 0d.s4 20us : add_preempt_count (__local_bh_disable)
- sshd-4261 0d.s4 21us : sub_preempt_count (local_bh_enable)
- sshd-4261 0d.s5 21us : sub_preempt_count (local_bh_enable)
-[...]
- sshd-4261 0d.s6 41us : add_preempt_count (__local_bh_disable)
- sshd-4261 0d.s6 42us : sub_preempt_count (local_bh_enable)
- sshd-4261 0d.s7 42us : sub_preempt_count (local_bh_enable)
- sshd-4261 0d.s5 43us : add_preempt_count (__local_bh_disable)
- sshd-4261 0d.s5 43us : sub_preempt_count (local_bh_enable_ip)
- sshd-4261 0d.s6 44us : sub_preempt_count (local_bh_enable_ip)
- sshd-4261 0d.s5 44us : add_preempt_count (__local_bh_disable)
- sshd-4261 0d.s5 45us : sub_preempt_count (local_bh_enable)
-[...]
- sshd-4261 0d.s. 63us : _local_bh_enable (__do_softirq)
- sshd-4261 0d.s1 64us : trace_preempt_on (__do_softirq)
-
-
-The above is an example of the preemptoff trace with ftrace_enabled
-set. Here we see that interrupts were disabled the entire time.
-The irq_enter code lets us know that we entered an interrupt 'h'.
-Before that, the functions being traced still show that it is not
-in an interrupt, but we can see by the functions themselves that
-this is not the case.
-
-Notice that the __do_softirq when called doesn't have a preempt_count.
-It may seem that we missed a preempt enabled. What really happened
-is that the preempt count is held on the threads stack and we
-switched to the softirq stack (4K stacks in effect). The code
-does not copy the preempt count, but because interrupts are disabled
-we don't need to worry about it. Having a tracer like this is good
-to let people know what really happens inside the kernel.
-
-
-preemptirqsoff
---------------
-
-Knowing the locations that have interrupts disabled or preemption
-disabled for the longest times is helpful. But sometimes we would
-like to know when either preemption and/or interrupts are disabled.
-
-The following code:
-
- local_irq_disable();
- call_function_with_irqs_off();
- preempt_disable();
- call_function_with_irqs_and_preemption_off();
- local_irq_enable();
- call_function_with_preemption_off();
- preempt_enable();
-
-The irqsoff tracer will record the total length of
-call_function_with_irqs_off() and
-call_function_with_irqs_and_preemption_off().
-
-The preemptoff tracer will record the total length of
-call_function_with_irqs_and_preemption_off() and
-call_function_with_preemption_off().
-
-But neither will trace the time that interrupts and/or preemption
-is disabled. This total time is the time that we can not schedule.
-To record this time, use the preemptirqsoff tracer.
-
-Again, using this trace is much like the irqsoff and preemptoff tracers.
-
- # echo preemptoff > /debug/tracing/current_tracer
- # echo 0 > /debug/tracing/tracing_max_latency
- # echo 1 > /debug/tracing/tracing_enabled
- # ls -ltr
- [...]
- # echo 0 > /debug/tracing/tracing_enabled
- # cat /debug/tracing/latency_trace
-# tracer: preemptirqsoff
-#
-preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
---------------------------------------------------------------------
- latency: 293 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
- -----------------
- | task: ls-4860 (uid:0 nice:0 policy:0 rt_prio:0)
- -----------------
- => started at: apic_timer_interrupt
- => ended at: __do_softirq
-
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| /
-# ||||| delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- ls-4860 0d... 0us!: trace_hardirqs_off_thunk (apic_timer_interrupt)
- ls-4860 0d.s. 294us : _local_bh_enable (__do_softirq)
- ls-4860 0d.s1 294us : trace_preempt_on (__do_softirq)
-
-
-vim:ft=help
-
-
-The trace_hardirqs_off_thunk is called from assembly on x86 when
-interrupts are disabled in the assembly code. Without the function
-tracing, we don't know if interrupts were enabled within the preemption
-points. We do see that it started with preemption enabled.
-
-Here is a trace with ftrace_enabled set:
-
-
-# tracer: preemptirqsoff
-#
-preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
---------------------------------------------------------------------
- latency: 105 us, #183/183, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
- -----------------
- | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
- -----------------
- => started at: write_chan
- => ended at: __do_softirq
-
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| /
-# ||||| delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- ls-4473 0.N.. 0us : preempt_schedule (write_chan)
- ls-4473 0dN.1 1us : _spin_lock (schedule)
- ls-4473 0dN.1 2us : add_preempt_count (_spin_lock)
- ls-4473 0d..2 2us : put_prev_task_fair (schedule)
-[...]
- ls-4473 0d..2 13us : set_normalized_timespec (ktime_get_ts)
- ls-4473 0d..2 13us : __switch_to (schedule)
- sshd-4261 0d..2 14us : finish_task_switch (schedule)
- sshd-4261 0d..2 14us : _spin_unlock_irq (finish_task_switch)
- sshd-4261 0d..1 15us : add_preempt_count (_spin_lock_irqsave)
- sshd-4261 0d..2 16us : _spin_unlock_irqrestore (hrtick_set)
- sshd-4261 0d..2 16us : do_IRQ (common_interrupt)
- sshd-4261 0d..2 17us : irq_enter (do_IRQ)
- sshd-4261 0d..2 17us : idle_cpu (irq_enter)
- sshd-4261 0d..2 18us : add_preempt_count (irq_enter)
- sshd-4261 0d.h2 18us : idle_cpu (irq_enter)
- sshd-4261 0d.h. 18us : handle_fasteoi_irq (do_IRQ)
- sshd-4261 0d.h. 19us : _spin_lock (handle_fasteoi_irq)
- sshd-4261 0d.h. 19us : add_preempt_count (_spin_lock)
- sshd-4261 0d.h1 20us : _spin_unlock (handle_fasteoi_irq)
- sshd-4261 0d.h1 20us : sub_preempt_count (_spin_unlock)
-[...]
- sshd-4261 0d.h1 28us : _spin_unlock (handle_fasteoi_irq)
- sshd-4261 0d.h1 29us : sub_preempt_count (_spin_unlock)
- sshd-4261 0d.h2 29us : irq_exit (do_IRQ)
- sshd-4261 0d.h2 29us : sub_preempt_count (irq_exit)
- sshd-4261 0d..3 30us : do_softirq (irq_exit)
- sshd-4261 0d... 30us : __do_softirq (do_softirq)
- sshd-4261 0d... 31us : __local_bh_disable (__do_softirq)
- sshd-4261 0d... 31us+: add_preempt_count (__local_bh_disable)
- sshd-4261 0d.s4 34us : add_preempt_count (__local_bh_disable)
-[...]
- sshd-4261 0d.s3 43us : sub_preempt_count (local_bh_enable_ip)
- sshd-4261 0d.s4 44us : sub_preempt_count (local_bh_enable_ip)
- sshd-4261 0d.s3 44us : smp_apic_timer_interrupt (apic_timer_interrupt)
- sshd-4261 0d.s3 45us : irq_enter (smp_apic_timer_interrupt)
- sshd-4261 0d.s3 45us : idle_cpu (irq_enter)
- sshd-4261 0d.s3 46us : add_preempt_count (irq_enter)
- sshd-4261 0d.H3 46us : idle_cpu (irq_enter)
- sshd-4261 0d.H3 47us : hrtimer_interrupt (smp_apic_timer_interrupt)
- sshd-4261 0d.H3 47us : ktime_get (hrtimer_interrupt)
-[...]
- sshd-4261 0d.H3 81us : tick_program_event (hrtimer_interrupt)
- sshd-4261 0d.H3 82us : ktime_get (tick_program_event)
- sshd-4261 0d.H3 82us : ktime_get_ts (ktime_get)
- sshd-4261 0d.H3 83us : getnstimeofday (ktime_get_ts)
- sshd-4261 0d.H3 83us : set_normalized_timespec (ktime_get_ts)
- sshd-4261 0d.H3 84us : clockevents_program_event (tick_program_event)
- sshd-4261 0d.H3 84us : lapic_next_event (clockevents_program_event)
- sshd-4261 0d.H3 85us : irq_exit (smp_apic_timer_interrupt)
- sshd-4261 0d.H3 85us : sub_preempt_count (irq_exit)
- sshd-4261 0d.s4 86us : sub_preempt_count (irq_exit)
- sshd-4261 0d.s3 86us : add_preempt_count (__local_bh_disable)
-[...]
- sshd-4261 0d.s1 98us : sub_preempt_count (net_rx_action)
- sshd-4261 0d.s. 99us : add_preempt_count (_spin_lock_irq)
- sshd-4261 0d.s1 99us+: _spin_unlock_irq (run_timer_softirq)
- sshd-4261 0d.s. 104us : _local_bh_enable (__do_softirq)
- sshd-4261 0d.s. 104us : sub_preempt_count (_local_bh_enable)
- sshd-4261 0d.s. 105us : _local_bh_enable (__do_softirq)
- sshd-4261 0d.s1 105us : trace_preempt_on (__do_softirq)
-
-
-This is a very interesting trace. It started with the preemption of
-the ls task. We see that the task had the "need_resched" bit set
-with the 'N' in the trace. Interrupts are disabled in the spin_lock
-and the trace started. We see that a schedule took place to run
-sshd. When the interrupts were enabled we took an interrupt.
-On return of the interrupt the softirq ran. We took another interrupt
-while running the softirq as we see with the capital 'H'.
-
-
-wakeup
-------
-
-In Real-Time environment it is very important to know the wakeup
-time it takes for the highest priority task that wakes up to the
-time it executes. This is also known as "schedule latency".
-I stress the point that this is about RT tasks. It is also important
-to know the scheduling latency of non-RT tasks, but the average
-schedule latency is better for non-RT tasks. Tools like
-LatencyTop is more appropriate for such measurements.
-
-Real-Time environments is interested in the worst case latency.
-That is the longest latency it takes for something to happen, and
-not the average. We can have a very fast scheduler that may only
-have a large latency once in a while, but that would not work well
-with Real-Time tasks. The wakeup tracer was designed to record
-the worst case wakeups of RT tasks. Non-RT tasks are not recorded
-because the tracer only records one worst case and tracing non-RT
-tasks that are unpredictable will overwrite the worst case latency
-of RT tasks.
-
-Since this tracer only deals with RT tasks, we will run this slightly
-different than we did with the previous tracers. Instead of performing
-an 'ls' we will run 'sleep 1' under 'chrt' which changes the
-priority of the task.
-
- # echo wakeup > /debug/tracing/current_tracer
- # echo 0 > /debug/tracing/tracing_max_latency
- # echo 1 > /debug/tracing/tracing_enabled
- # chrt -f 5 sleep 1
- # echo 0 > /debug/tracing/tracing_enabled
- # cat /debug/tracing/latency_trace
-# tracer: wakeup
-#
-wakeup latency trace v1.1.5 on 2.6.26-rc8
---------------------------------------------------------------------
- latency: 4 us, #2/2, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
- -----------------
- | task: sleep-4901 (uid:0 nice:0 policy:1 rt_prio:5)
- -----------------
-
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| /
-# ||||| delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
- <idle>-0 1d.h4 0us+: try_to_wake_up (wake_up_process)
- <idle>-0 1d..4 4us : schedule (cpu_idle)
-
-
-vim:ft=help
-
-
-Running this on an idle system we see that it only took 4 microseconds
-to perform the task switch. Note, since the trace marker in the
-schedule is before the actual "switch" we stop the tracing when
-the recorded task is about to schedule in. This may change if
-we add a new marker at the end of the scheduler.
-
-Notice that the recorded task is 'sleep' with the PID of 4901 and it
-has an rt_prio of 5. This priority is user-space priority and not
-the internal kernel priority. The policy is 1 for SCHED_FIFO and 2
-for SCHED_RR.
-
-Doing the same with chrt -r 5 and ftrace_enabled set.
-
-# tracer: wakeup
-#
-wakeup latency trace v1.1.5 on 2.6.26-rc8
---------------------------------------------------------------------
- latency: 50 us, #60/60, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
- -----------------
- | task: sleep-4068 (uid:0 nice:0 policy:2 rt_prio:5)
- -----------------
-
-# _------=> CPU#
-# / _-----=> irqs-off
-# | / _----=> need-resched
-# || / _---=> hardirq/softirq
-# ||| / _--=> preempt-depth
-# |||| /
-# ||||| delay
-# cmd pid ||||| time | caller
-# \ / ||||| \ | /
-ksoftirq-7 1d.H3 0us : try_to_wake_up (wake_up_process)
-ksoftirq-7 1d.H4 1us : sub_preempt_count (marker_probe_cb)
-ksoftirq-7 1d.H3 2us : check_preempt_wakeup (try_to_wake_up)
-ksoftirq-7 1d.H3 3us : update_curr (check_preempt_wakeup)
-ksoftirq-7 1d.H3 4us : calc_delta_mine (update_curr)
-ksoftirq-7 1d.H3 5us : __resched_task (check_preempt_wakeup)
-ksoftirq-7 1d.H3 6us : task_wake_up_rt (try_to_wake_up)
-ksoftirq-7 1d.H3 7us : _spin_unlock_irqrestore (try_to_wake_up)
-[...]
-ksoftirq-7 1d.H2 17us : irq_exit (smp_apic_timer_interrupt)
-ksoftirq-7 1d.H2 18us : sub_preempt_count (irq_exit)
-ksoftirq-7 1d.s3 19us : sub_preempt_count (irq_exit)
-ksoftirq-7 1..s2 20us : rcu_process_callbacks (__do_softirq)
-[...]
-ksoftirq-7 1..s2 26us : __rcu_process_callbacks (rcu_process_callbacks)
-ksoftirq-7 1d.s2 27us : _local_bh_enable (__do_softirq)
-ksoftirq-7 1d.s2 28us : sub_preempt_count (_local_bh_enable)
-ksoftirq-7 1.N.3 29us : sub_preempt_count (ksoftirqd)
-ksoftirq-7 1.N.2 30us : _cond_resched (ksoftirqd)
-ksoftirq-7 1.N.2 31us : __cond_resched (_cond_resched)
-ksoftirq-7 1.N.2 32us : add_preempt_count (__cond_resched)
-ksoftirq-7 1.N.2 33us : schedule (__cond_resched)
-ksoftirq-7 1.N.2 33us : add_preempt_count (schedule)
-ksoftirq-7 1.N.3 34us : hrtick_clear (schedule)
-ksoftirq-7 1dN.3 35us : _spin_lock (schedule)
-ksoftirq-7 1dN.3 36us : add_preempt_count (_spin_lock)
-ksoftirq-7 1d..4 37us : put_prev_task_fair (schedule)
-ksoftirq-7 1d..4 38us : update_curr (put_prev_task_fair)
-[...]
-ksoftirq-7 1d..5 47us : _spin_trylock (tracing_record_cmdline)
-ksoftirq-7 1d..5 48us : add_preempt_count (_spin_trylock)
-ksoftirq-7 1d..6 49us : _spin_unlock (tracing_record_cmdline)
-ksoftirq-7 1d..6 49us : sub_preempt_count (_spin_unlock)
-ksoftirq-7 1d..4 50us : schedule (__cond_resched)
-
-The interrupt went off while running ksoftirqd. This task runs at
-SCHED_OTHER. Why didn't we see the 'N' set early? This may be
-a harmless bug with x86_32 and 4K stacks. The need_reched() function
-that tests if we need to reschedule looks on the actual stack.
-Where as the setting of the NEED_RESCHED bit happens on the
-task's stack. But because we are in a hard interrupt, the test
-is with the interrupts stack which has that to be false. We don't
-see the 'N' until we switch back to the task's stack.
-
-ftrace
-------
-
-ftrace is not only the name of the tracing infrastructure, but it
-is also a name of one of the tracers. The tracer is the function
-tracer. Enabling the function tracer can be done from the
-debug file system. Make sure the ftrace_enabled is set otherwise
-this tracer is a nop.
-
- # sysctl kernel.ftrace_enabled=1
- # echo ftrace > /debug/tracing/current_tracer
- # echo 1 > /debug/tracing/tracing_enabled
- # usleep 1
- # echo 0 > /debug/tracing/tracing_enabled
- # cat /debug/tracing/trace
-# tracer: ftrace
-#
-# TASK-PID CPU# TIMESTAMP FUNCTION
-# | | | | |
- bash-4003 [00] 123.638713: finish_task_switch <-schedule
- bash-4003 [00] 123.638714: _spin_unlock_irq <-finish_task_switch
- bash-4003 [00] 123.638714: sub_preempt_count <-_spin_unlock_irq
- bash-4003 [00] 123.638715: hrtick_set <-schedule
- bash-4003 [00] 123.638715: _spin_lock_irqsave <-hrtick_set
- bash-4003 [00] 123.638716: add_preempt_count <-_spin_lock_irqsave
- bash-4003 [00] 123.638716: _spin_unlock_irqrestore <-hrtick_set
- bash-4003 [00] 123.638717: sub_preempt_count <-_spin_unlock_irqrestore
- bash-4003 [00] 123.638717: hrtick_clear <-hrtick_set
- bash-4003 [00] 123.638718: sub_preempt_count <-schedule
- bash-4003 [00] 123.638718: sub_preempt_count <-preempt_schedule
- bash-4003 [00] 123.638719: wait_for_completion <-__stop_machine_run
- bash-4003 [00] 123.638719: wait_for_common <-wait_for_completion
- bash-4003 [00] 123.638720: _spin_lock_irq <-wait_for_common
- bash-4003 [00] 123.638720: add_preempt_count <-_spin_lock_irq
-[...]
-
-
-Note: It is sometimes better to enable or disable tracing directly from
-a program, because the buffer may be overflowed by the echo commands
-before you get to the point you want to trace. It is also easier to
-stop the tracing at the point that you hit the part that you are
-interested in. Since the ftrace buffer is a ring buffer with the
-oldest data being overwritten, usually it is sufficient to start the
-tracer with an echo command but have you code stop it. Something
-like the following is usually appropriate for this.
-
-int trace_fd;
-[...]
-int main(int argc, char *argv[]) {
- [...]
- trace_fd = open("/debug/tracing/tracing_enabled", O_WRONLY);
- [...]
- if (condition_hit()) {
- write(trace_fd, "0", 1);
- }
- [...]
-}
-
-
-dynamic ftrace
---------------
-
-If CONFIG_DYNAMIC_FTRACE is set, then the system will run with
-virtually no overhead when function tracing is disabled. The way
-this works is the mcount function call (placed at the start of
-every kernel function, produced by the -pg switch in gcc), starts
-of pointing to a simple return.
-
-When dynamic ftrace is initialized, it calls kstop_machine to make it
-act like a uniprocessor so that it can freely modify code without
-worrying about other processors executing that same code. At
-initialization, the mcount calls are change to call a "record_ip"
-function. After this, the first time a kernel function is called,
-it has the calling address saved in a hash table.
-
-Later on the ftraced kernel thread is awoken and will again call
-kstop_machine if new functions have been recorded. The ftraced thread
-will change all calls to mcount to "nop". Just calling mcount
-and having mcount return has shown a 10% overhead. By converting
-it to a nop, there is no recordable overhead to the system.
-
-One special side-effect to the recording of the functions being
-traced, is that we can now selectively choose which functions we
-want to trace and which ones we want the mcount calls to remain as
-nops.
-
-Two files that contain to the enabling and disabling of recorded
-functions are:
-
- set_ftrace_filter
-
-and
-
- set_ftrace_notrace
-
-A list of available functions that you can add to this files is listed
-in:
-
- available_filter_functions
-
- # cat /debug/tracing/available_filter_functions
-put_prev_task_idle
-kmem_cache_create
-pick_next_task_rt
-get_online_cpus
-pick_next_task_fair
-mutex_lock
-[...]
-
-If I'm only interested in sys_nanosleep and hrtimer_interrupt:
-
- # echo sys_nanosleep hrtimer_interrupt \
- > /debug/tracing/set_ftrace_filter
- # echo ftrace > /debug/tracing/current_tracer
- # echo 1 > /debug/tracing/tracing_enabled
- # usleep 1
- # echo 0 > /debug/tracing/tracing_enabled
- # cat /debug/tracing/trace
-# tracer: ftrace
-#
-# TASK-PID CPU# TIMESTAMP FUNCTION
-# | | | | |
- usleep-4134 [00] 1317.070017: hrtimer_interrupt <-smp_apic_timer_interrupt
- usleep-4134 [00] 1317.070111: sys_nanosleep <-syscall_call
- <idle>-0 [00] 1317.070115: hrtimer_interrupt <-smp_apic_timer_interrupt
-
-To see what functions are being traced, you can cat the file:
-
- # cat /debug/tracing/set_ftrace_filter
-hrtimer_interrupt
-sys_nanosleep
-
-
-Perhaps this isn't enough. The filters also allow simple wild cards.
-Only the following is currently available
-
- <match>* - will match functions that begins with <match>
- *<match> - will match functions that end with <match>
- *<match>* - will match functions that have <match> in it
-
-Thats all the wild cards that are allowed.
-
- <match>*<match> will not work.
-
- # echo hrtimer_* > /debug/tracing/set_ftrace_filter
-
-Produces:
-
-# tracer: ftrace
-#
-# TASK-PID CPU# TIMESTAMP FUNCTION
-# | | | | |
- bash-4003 [00] 1480.611794: hrtimer_init <-copy_process
- bash-4003 [00] 1480.611941: hrtimer_start <-hrtick_set
- bash-4003 [00] 1480.611956: hrtimer_cancel <-hrtick_clear
- bash-4003 [00] 1480.611956: hrtimer_try_to_cancel <-hrtimer_cancel
- <idle>-0 [00] 1480.612019: hrtimer_get_next_event <-get_next_timer_interrupt
- <idle>-0 [00] 1480.612025: hrtimer_get_next_event <-get_next_timer_interrupt
- <idle>-0 [00] 1480.612032: hrtimer_get_next_event <-get_next_timer_interrupt
- <idle>-0 [00] 1480.612037: hrtimer_get_next_event <-get_next_timer_interrupt
- <idle>-0 [00] 1480.612382: hrtimer_get_next_event <-get_next_timer_interrupt
-
-
-Notice that we lost the sys_nanosleep.
-
- # cat /debug/tracing/set_ftrace_filter
-hrtimer_run_queues
-hrtimer_run_pending
-hrtimer_init
-hrtimer_cancel
-hrtimer_try_to_cancel
-hrtimer_forward
-hrtimer_start
-hrtimer_reprogram
-hrtimer_force_reprogram
-hrtimer_get_next_event
-hrtimer_interrupt
-hrtimer_nanosleep
-hrtimer_wakeup
-hrtimer_get_remaining
-hrtimer_get_res
-hrtimer_init_sleeper
-
-
-This is because the '>' and '>>' act just like they do in bash.
-To rewrite the filters, use '>'
-To append to the filters, use '>>'
-
-To clear out a filter so that all functions will be recorded again.
-
- # echo > /debug/tracing/set_ftrace_filter
- # cat /debug/tracing/set_ftrace_filter
- #
-
-Again, now we want to append.
-
- # echo sys_nanosleep > /debug/tracing/set_ftrace_filter
- # cat /debug/tracing/set_ftrace_filter
-sys_nanosleep
- # echo hrtimer_* >> /debug/tracing/set_ftrace_filter
- # cat /debug/tracing/set_ftrace_filter
-hrtimer_run_queues
-hrtimer_run_pending
-hrtimer_init
-hrtimer_cancel
-hrtimer_try_to_cancel
-hrtimer_forward
-hrtimer_start
-hrtimer_reprogram
-hrtimer_force_reprogram
-hrtimer_get_next_event
-hrtimer_interrupt
-sys_nanosleep
-hrtimer_nanosleep
-hrtimer_wakeup
-hrtimer_get_remaining
-hrtimer_get_res
-hrtimer_init_sleeper
-
-
-The set_ftrace_notrace prevents those functions from being traced.
-
- # echo '*preempt*' '*lock*' > /debug/tracing/set_ftrace_notrace
-
-Produces:
-
-# tracer: ftrace
-#
-# TASK-PID CPU# TIMESTAMP FUNCTION
-# | | | | |
- bash-4043 [01] 115.281644: finish_task_switch <-schedule
- bash-4043 [01] 115.281645: hrtick_set <-schedule
- bash-4043 [01] 115.281645: hrtick_clear <-hrtick_set
- bash-4043 [01] 115.281646: wait_for_completion <-__stop_machine_run
- bash-4043 [01] 115.281647: wait_for_common <-wait_for_completion
- bash-4043 [01] 115.281647: kthread_stop <-stop_machine_run
- bash-4043 [01] 115.281648: init_waitqueue_head <-kthread_stop
- bash-4043 [01] 115.281648: wake_up_process <-kthread_stop
- bash-4043 [01] 115.281649: try_to_wake_up <-wake_up_process
-
-We can see that there's no more lock or preempt tracing.
-
-ftraced
--------
-
-As mentioned above, when dynamic ftrace is configured in, a kernel
-thread wakes up once a second and checks to see if there are mcount
-calls that need to be converted into nops. If there is not, then
-it simply goes back to sleep. But if there is, it will call
-kstop_machine to convert the calls to nops.
-
-There may be a case that you do not want this added latency.
-Perhaps you are doing some audio recording and this activity might
-cause skips in the playback. There is an interface to disable
-and enable the ftraced kernel thread.
-
- # echo 0 > /debug/tracing/ftraced_enabled
-
-This will disable the calling of the kstop_machine to update the
-mcount calls to nops. Remember that there's a large overhead
-to calling mcount. Without this kernel thread, that overhead will
-exist.
-
-Any write to the ftraced_enabled file will cause the kstop_machine
-to run if there are recorded calls to mcount. This means that a
-user can manually perform the updates when they want to by simply
-echoing a '0' into the ftraced_enabled file.
-
-The updates are also done at the beginning of enabling a tracer
-that uses ftrace function recording.
-
-
-trace_pipe
-----------
-
-The trace_pipe outputs the same as trace, but the effect on the
-tracing is different. Every read from trace_pipe is consumed.
-This means that subsequent reads will be different. The trace
-is live.
-
- # echo ftrace > /debug/tracing/current_tracer
- # cat /debug/tracing/trace_pipe > /tmp/trace.out &
-[1] 4153
- # echo 1 > /debug/tracing/tracing_enabled
- # usleep 1
- # echo 0 > /debug/tracing/tracing_enabled
- # cat /debug/tracing/trace
-# tracer: ftrace
-#
-# TASK-PID CPU# TIMESTAMP FUNCTION
-# | | | | |
-
- #
- # cat /tmp/trace.out
- bash-4043 [00] 41.267106: finish_task_switch <-schedule
- bash-4043 [00] 41.267106: hrtick_set <-schedule
- bash-4043 [00] 41.267107: hrtick_clear <-hrtick_set
- bash-4043 [00] 41.267108: wait_for_completion <-__stop_machine_run
- bash-4043 [00] 41.267108: wait_for_common <-wait_for_completion
- bash-4043 [00] 41.267109: kthread_stop <-stop_machine_run
- bash-4043 [00] 41.267109: init_waitqueue_head <-kthread_stop
- bash-4043 [00] 41.267110: wake_up_process <-kthread_stop
- bash-4043 [00] 41.267110: try_to_wake_up <-wake_up_process
- bash-4043 [00] 41.267111: select_task_rq_rt <-try_to_wake_up
-
-
-Note, reading the trace_pipe will block until more input is added.
-By changing the tracer, trace_pipe will issue an EOF. We needed
-to set the ftrace tracer _before_ cating the trace_pipe file.
-
-
-trace entries
--------------
-
-Having too much or not enough data can be troublesome in diagnosing
-some issue in the kernel. The file trace_entries is used to modify
-the size of the internal trace buffers. The numbers listed
-is the number of entries that can be recorded per CPU. To know
-the full size, multiply the number of possible CPUS with the
-number of entries.
-
- # cat /debug/tracing/trace_entries
-65620
-
-Note, to modify this you must have tracing fulling disabled. To do that,
-echo "none" into the current_tracer.
-
- # echo none > /debug/tracing/current_tracer
- # echo 100000 > /debug/tracing/trace_entries
- # cat /debug/tracing/trace_entries
-100045
-
-
-Notice that we echoed in 100,000 but the size is 100,045. The entries
-are held by individual pages. It allocates the number of pages it takes
-to fulfill the request. If more entries may fit on the last page
-it will add them.
-
- # echo 1 > /debug/tracing/trace_entries
- # cat /debug/tracing/trace_entries
-85
-
-This shows us that 85 entries can fit on a single page.
-
-The number of pages that will be allocated is a percentage of available
-memory. Allocating too much will produces an error.
-
- # echo 1000000000000 > /debug/tracing/trace_entries
--bash: echo: write error: Cannot allocate memory
- # cat /debug/tracing/trace_entries
-85
-
--
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