tracing: consolidate documents

Move kmemtrace.txt, tracepoints.txt, ftrace.txt and mmiotrace.txt to
the new trace/ directory.

I didnt find any references to those documents in both source
files and documents, so no extra work needs to be done.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Pekka Paalanen <pq@iki.fi>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
LKML-Reference: <49DD6E2B.6090200@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 0 insertions, 1828 deletions

diff --git a/Documentation/ftrace.txt b/Documentation/ftrace.txt
deleted file mode 100644
index fd9a3e6..0000000
--- a/Documentation/ftrace.txt
+++ /dev/null
@@ -1,1828 +0,0 @@
-		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.