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authorLi Zefan <lizf@cn.fujitsu.com>2009-04-09 11:40:27 +0800
committerIngo Molnar <mingo@elte.hu>2009-04-09 07:28:10 +0200
commit66bb74888eb4bef4ba7c87c931ecb7ecca3a240c (patch)
tree567e333523e16ba7285deac89efb645f72a97529 /Documentation/trace
parent9eb85125ce218a8b8d9a7c982510388e227adbec (diff)
downloadlinux-66bb74888eb4bef4ba7c87c931ecb7ecca3a240c.tar.gz
linux-66bb74888eb4bef4ba7c87c931ecb7ecca3a240c.tar.xz
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>
Diffstat (limited to 'Documentation/trace')
-rw-r--r--Documentation/trace/ftrace.txt1828
-rw-r--r--Documentation/trace/kmemtrace.txt126
-rw-r--r--Documentation/trace/mmiotrace.txt163
-rw-r--r--Documentation/trace/tracepoints.txt116
4 files changed, 2233 insertions, 0 deletions
diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt
new file mode 100644
index 000000000000..fd9a3e693813
--- /dev/null
+++ b/Documentation/trace/ftrace.txt
@@ -0,0 +1,1828 @@
+ ftrace - Function Tracer
+ ========================
+
+Copyright 2008 Red Hat Inc.
+ Author: Steven Rostedt <srostedt@redhat.com>
+ License: The GNU Free Documentation License, Version 1.2
+ (dual licensed under the GPL v2)
+Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton,
+ John Kacur, and David Teigland.
+
+Written for: 2.6.28-rc2
+
+Introduction
+------------
+
+Ftrace is an internal tracer designed to help out developers and
+designers of systems to find what is going on inside the kernel.
+It can be used for debugging or analyzing latencies and
+performance issues that take place outside of user-space.
+
+Although ftrace is the function tracer, it also includes an
+infrastructure that allows for other types of tracing. Some of
+the tracers that are currently in ftrace include a tracer to
+trace context switches, the time it takes for a high priority
+task to run after it was woken up, the time interrupts are
+disabled, and more (ftrace allows for tracer plugins, which
+means that the list of tracers can always grow).
+
+
+The File System
+---------------
+
+Ftrace uses the debugfs file system to hold the control files as
+well as the files to display output.
+
+To mount the debugfs system:
+
+ # mkdir /debug
+ # mount -t debugfs nodev /debug
+
+( Note: it is more common to mount at /sys/kernel/debug, but for
+ simplicity this document will use /debug)
+
+That's it! (assuming that you have ftrace configured into your kernel)
+
+After mounting the debugfs, you can see a directory called
+"tracing". This directory contains the control and output files
+of ftrace. Here is a list of some of the key files:
+
+
+ Note: all time values are in microseconds.
+
+ current_tracer:
+
+ This is used to set or display the current tracer
+ that is configured.
+
+ available_tracers:
+
+ This holds the different types of tracers that
+ have been compiled into the kernel. The
+ tracers listed here can be configured by
+ echoing their name into current_tracer.
+
+ tracing_enabled:
+
+ This sets or displays whether the current_tracer
+ is activated and tracing or not. Echo 0 into this
+ file to disable the tracer or 1 to enable it.
+
+ trace:
+
+ This file holds the output of the trace in a human
+ readable format (described below).
+
+ latency_trace:
+
+ This file shows the same trace but the information
+ is organized more to display possible latencies
+ in the system (described below).
+
+ trace_pipe:
+
+ The output is the same as the "trace" file but this
+ file is meant to be streamed with live tracing.
+ Reads from this file will block until new data
+ is retrieved. Unlike the "trace" and "latency_trace"
+ files, this file is a consumer. This means reading
+ from this file causes sequential reads to display
+ more current data. Once data is read from this
+ file, it is consumed, and will not be read
+ again with a sequential read. The "trace" and
+ "latency_trace" files are static, and if the
+ tracer is not adding more data, they will display
+ the same information every time they are read.
+
+ trace_options:
+
+ This file lets the user control the amount of data
+ that is displayed in one of the above output
+ files.
+
+ tracing_max_latency:
+
+ Some of the tracers record the max latency.
+ For example, the time interrupts are disabled.
+ This time is saved in this file. The max trace
+ will also be stored, and displayed by either
+ "trace" or "latency_trace". A new max trace will
+ only be recorded if the latency is greater than
+ the value in this file. (in microseconds)
+
+ buffer_size_kb:
+
+ This sets or displays the number of kilobytes each CPU
+ buffer can hold. The tracer buffers are the same size
+ for each CPU. The displayed number is the size of the
+ CPU buffer and not total size of all buffers. The
+ trace buffers are allocated in pages (blocks of memory
+ that the kernel uses for allocation, usually 4 KB in size).
+ If the last page allocated has room for more bytes
+ than requested, the rest of the page will be used,
+ making the actual allocation bigger than requested.
+ ( Note, the size may not be a multiple of the page size
+ due to buffer managment overhead. )
+
+ This can only be updated when the current_tracer
+ is set to "nop".
+
+ tracing_cpumask:
+
+ This is a mask that lets the user only trace
+ on specified CPUS. The format is a hex string
+ representing the CPUS.
+
+ set_ftrace_filter:
+
+ When dynamic ftrace is configured in (see the
+ section below "dynamic ftrace"), the code is dynamically
+ modified (code text rewrite) to disable calling of the
+ function profiler (mcount). This lets tracing be configured
+ in with practically no overhead in performance. This also
+ has a side effect of enabling or disabling specific functions
+ to be traced. Echoing names of functions into this file
+ will limit the trace to only those functions.
+
+ set_ftrace_notrace:
+
+ This has an effect opposite to that of
+ set_ftrace_filter. Any function that is added here will not
+ be traced. If a function exists in both set_ftrace_filter
+ and set_ftrace_notrace, the function will _not_ be traced.
+
+ set_ftrace_pid:
+
+ Have the function tracer only trace a single thread.
+
+ set_graph_function:
+
+ Set a "trigger" function where tracing should start
+ with the function graph tracer (See the section
+ "dynamic ftrace" for more details).
+
+ available_filter_functions:
+
+ This lists the functions that ftrace
+ has processed and can trace. These are the function
+ names that you can pass to "set_ftrace_filter" or
+ "set_ftrace_notrace". (See the section "dynamic ftrace"
+ below for more details.)
+
+
+The Tracers
+-----------
+
+Here is the list of current tracers that may be configured.
+
+ "function"
+
+ Function call tracer to trace all kernel functions.
+
+ "function_graph_tracer"
+
+ Similar to the function tracer except that the
+ function tracer probes the functions on their entry
+ whereas the function graph tracer traces on both entry
+ and exit of the functions. It then provides the ability
+ to draw a graph of function calls similar to C code
+ source.
+
+ "sched_switch"
+
+ Traces the context switches and wakeups between tasks.
+
+ "irqsoff"
+
+ Traces the areas that disable interrupts and saves
+ the trace with the longest max latency.
+ See tracing_max_latency. When a new max is recorded,
+ it replaces the old trace. It is best to view this
+ trace via the latency_trace file.
+
+ "preemptoff"
+
+ Similar to irqsoff but traces and records the amount of
+ time for which preemption is disabled.
+
+ "preemptirqsoff"
+
+ Similar to irqsoff and preemptoff, but traces and
+ records the largest time for which irqs and/or preemption
+ is disabled.
+
+ "wakeup"
+
+ Traces and records the max latency that it takes for
+ the highest priority task to get scheduled after
+ it has been woken up.
+
+ "hw-branch-tracer"
+
+ Uses the BTS CPU feature on x86 CPUs to traces all
+ branches executed.
+
+ "nop"
+
+ This is the "trace nothing" tracer. To remove all
+ tracers from tracing simply echo "nop" into
+ current_tracer.
+
+
+Examples of using the tracer
+----------------------------
+
+Here are typical examples of using the tracers when controlling
+them only with the debugfs interface (without using any
+user-land utilities).
+
+Output format:
+--------------
+
+Here is an example of the output format of the file "trace"
+
+ --------
+# tracer: function
+#
+# TASK-PID CPU# TIMESTAMP FUNCTION
+# | | | | |
+ bash-4251 [01] 10152.583854: path_put <-path_walk
+ bash-4251 [01] 10152.583855: dput <-path_put
+ bash-4251 [01] 10152.583855: _atomic_dec_and_lock <-dput
+ --------
+
+A header is printed with the tracer name that is represented by
+the trace. In this case the tracer is "function". Then a header
+showing the format. Task name "bash", the task PID "4251", the
+CPU that it was running on "01", the timestamp in <secs>.<usecs>
+format, the function name that was traced "path_put" and the
+parent function that called this function "path_walk". The
+timestamp is the time at which the function was entered.
+
+The sched_switch tracer also includes tracing of task wakeups
+and context switches.
+
+ ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 2916:115:S
+ ksoftirqd/1-7 [01] 1453.070013: 7:115:R + 10:115:S
+ ksoftirqd/1-7 [01] 1453.070013: 7:115:R ==> 10:115:R
+ events/1-10 [01] 1453.070013: 10:115:S ==> 2916:115:R
+ kondemand/1-2916 [01] 1453.070013: 2916:115:S ==> 7:115:R
+ ksoftirqd/1-7 [01] 1453.070013: 7:115:S ==> 0:140:R
+
+Wake ups are represented by a "+" and the context switches are
+shown as "==>". The format is:
+
+ Context switches:
+
+ Previous task Next Task
+
+ <pid>:<prio>:<state> ==> <pid>:<prio>:<state>
+
+ Wake ups:
+
+ Current task Task waking up
+
+ <pid>:<prio>:<state> + <pid>:<prio>:<state>
+
+The prio is the internal kernel priority, which is the inverse
+of the priority that is usually displayed by user-space tools.
+Zero represents the highest priority (99). Prio 100 starts the
+"nice" priorities with 100 being equal to nice -20 and 139 being
+nice 19. The prio "140" is reserved for the idle task which is
+the lowest priority thread (pid 0).
+
+
+Latency trace format
+--------------------
+
+For traces that display latency times, the latency_trace file
+gives somewhat more information to see why a latency happened.
+Here is a typical trace.
+
+# tracer: irqsoff
+#
+irqsoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 97 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: swapper-0 (uid:0 nice:0 policy:0 rt_prio:0)
+ -----------------
+ => started at: apic_timer_interrupt
+ => ended at: do_softirq
+
+# _------=> CPU#
+# / _-----=> irqs-off
+# | / _----=> need-resched
+# || / _---=> hardirq/softirq
+# ||| / _--=> preempt-depth
+# |||| /
+# ||||| delay
+# cmd pid ||||| time | caller
+# \ / ||||| \ | /
+ <idle>-0 0d..1 0us+: trace_hardirqs_off_thunk (apic_timer_interrupt)
+ <idle>-0 0d.s. 97us : __do_softirq (do_softirq)
+ <idle>-0 0d.s1 98us : trace_hardirqs_on (do_softirq)
+
+
+This shows that the current tracer is "irqsoff" tracing the time
+for which interrupts were disabled. It gives the trace version
+and the version of the kernel upon which this was executed on
+(2.6.26-rc8). Then it displays the max latency in microsecs (97
+us). The number of trace entries displayed and the total number
+recorded (both are three: #3/3). The type of preemption that was
+used (PREEMPT). VP, KP, SP, and HP are always zero and are
+reserved for later use. #P is the number of online CPUS (#P:2).
+
+The task is the process that was running when the latency
+occurred. (swapper pid: 0).
+
+The start and stop (the functions in which the interrupts were
+disabled and enabled respectively) that caused the latencies:
+
+ apic_timer_interrupt is where the interrupts were disabled.
+ do_softirq is where they were enabled again.
+
+The next lines after the header are the trace itself. The header
+explains which is which.
+
+ cmd: The name of the process in the trace.
+
+ pid: The PID of that process.
+
+ CPU#: The CPU which the process was running on.
+
+ irqs-off: 'd' interrupts are disabled. '.' otherwise.
+ Note: If the architecture does not support a way to
+ read the irq flags variable, an 'X' will always
+ be printed here.
+
+ need-resched: 'N' task need_resched is set, '.' otherwise.
+
+ hardirq/softirq:
+ 'H' - hard irq occurred inside a softirq.
+ 'h' - hard irq is running
+ 's' - soft irq is running
+ '.' - normal context.
+
+ preempt-depth: The level of preempt_disabled
+
+The above is mostly meaningful for kernel developers.
+
+ time: This differs from the trace file output. The trace file output
+ includes an absolute timestamp. The timestamp used by the
+ latency_trace file is relative to the start of the trace.
+
+ delay: This is just to help catch your eye a bit better. And
+ needs to be fixed to be only relative to the same CPU.
+ The marks are determined by the difference between this
+ current trace and the next trace.
+ '!' - greater than preempt_mark_thresh (default 100)
+ '+' - greater than 1 microsecond
+ ' ' - less than or equal to 1 microsecond.
+
+ The rest is the same as the 'trace' file.
+
+
+trace_options
+-------------
+
+The trace_options file is used to control what gets printed in
+the trace output. To see what is available, simply cat the file:
+
+ cat /debug/tracing/trace_options
+ print-parent nosym-offset nosym-addr noverbose noraw nohex nobin \
+ noblock nostacktrace nosched-tree nouserstacktrace nosym-userobj
+
+To disable one of the options, echo in the option prepended with
+"no".
+
+ echo noprint-parent > /debug/tracing/trace_options
+
+To enable an option, leave off the "no".
+
+ echo sym-offset > /debug/tracing/trace_options
+
+Here are the available options:
+
+ print-parent - On function traces, display the calling (parent)
+ function as well as the function being traced.
+
+ print-parent:
+ bash-4000 [01] 1477.606694: simple_strtoul <-strict_strtoul
+
+ noprint-parent:
+ bash-4000 [01] 1477.606694: simple_strtoul
+
+
+ sym-offset - Display not only the function name, but also the
+ offset in the function. For example, instead of
+ seeing just "ktime_get", you will see
+ "ktime_get+0xb/0x20".
+
+ sym-offset:
+ bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0
+
+ sym-addr - this will also display the function address as well
+ as the function name.
+
+ sym-addr:
+ bash-4000 [01] 1477.606694: simple_strtoul <c0339346>
+
+ verbose - This deals with the latency_trace file.
+
+ bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
+ (+0.000ms): simple_strtoul (strict_strtoul)
+
+ raw - This will display raw numbers. This option is best for
+ use with user applications that can translate the raw
+ numbers better than having it done in the kernel.
+
+ hex - Similar to raw, but the numbers will be in a hexadecimal
+ format.
+
+ bin - This will print out the formats in raw binary.
+
+ block - TBD (needs update)
+
+ stacktrace - This is one of the options that changes the trace
+ itself. When a trace is recorded, so is the stack
+ of functions. This allows for back traces of
+ trace sites.
+
+ userstacktrace - This option changes the trace. It records a
+ stacktrace of the current userspace thread.
+
+ sym-userobj - when user stacktrace are enabled, look up which
+ object the address belongs to, and print a
+ relative address. This is especially useful when
+ ASLR is on, otherwise you don't get a chance to
+ resolve the address to object/file/line after
+ the app is no longer running
+
+ The lookup is performed when you read
+ trace,trace_pipe,latency_trace. Example:
+
+ a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
+x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
+
+ sched-tree - trace all tasks that are on the runqueue, at
+ every scheduling event. Will add overhead if
+ there's a lot of tasks running at once.
+
+
+sched_switch
+------------
+
+This tracer simply records schedule switches. Here is an example
+of how to use it.
+
+ # echo sched_switch > /debug/tracing/current_tracer
+ # echo 1 > /debug/tracing/tracing_enabled
+ # sleep 1
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/trace
+
+# tracer: sched_switch
+#
+# TASK-PID CPU# TIMESTAMP FUNCTION
+# | | | | |
+ bash-3997 [01] 240.132281: 3997:120:R + 4055:120:R
+ bash-3997 [01] 240.132284: 3997:120:R ==> 4055:120:R
+ sleep-4055 [01] 240.132371: 4055:120:S ==> 3997:120:R
+ bash-3997 [01] 240.132454: 3997:120:R + 4055:120:S
+ bash-3997 [01] 240.132457: 3997:120:R ==> 4055:120:R
+ sleep-4055 [01] 240.132460: 4055:120:D ==> 3997:120:R
+ bash-3997 [01] 240.132463: 3997:120:R + 4055:120:D
+ bash-3997 [01] 240.132465: 3997:120:R ==> 4055:120:R
+ <idle>-0 [00] 240.132589: 0:140:R + 4:115:S
+ <idle>-0 [00] 240.132591: 0:140:R ==> 4:115:R
+ ksoftirqd/0-4 [00] 240.132595: 4:115:S ==> 0:140:R
+ <idle>-0 [00] 240.132598: 0:140:R + 4:115:S
+ <idle>-0 [00] 240.132599: 0:140:R ==> 4:115:R
+ ksoftirqd/0-4 [00] 240.132603: 4:115:S ==> 0:140:R
+ sleep-4055 [01] 240.133058: 4055:120:S ==> 3997:120:R
+ [...]
+
+
+As we have discussed previously about this format, the header
+shows the name of the trace and points to the options. The
+"FUNCTION" is a misnomer since here it represents the wake ups
+and context switches.
+
+The sched_switch file only lists the wake ups (represented with
+'+') and context switches ('==>') with the previous task or
+current task first followed by the next task or task waking up.
+The format for both of these is PID:KERNEL-PRIO:TASK-STATE.
+Remember that the KERNEL-PRIO is the inverse of the actual
+priority with zero (0) being the highest priority and the nice
+values starting at 100 (nice -20). Below is a quick chart to map
+the kernel priority to user land priorities.
+
+ Kernel priority: 0 to 99 ==> user RT priority 99 to 0
+ Kernel priority: 100 to 139 ==> user nice -20 to 19
+ Kernel priority: 140 ==> idle task priority
+
+The task states are:
+
+ R - running : wants to run, may not actually be running
+ S - sleep : process is waiting to be woken up (handles signals)
+ D - disk sleep (uninterruptible sleep) : process must be woken up
+ (ignores signals)
+ T - stopped : process suspended
+ t - traced : process is being traced (with something like gdb)
+ Z - zombie : process waiting to be cleaned up
+ X - unknown
+
+
+ftrace_enabled
+--------------
+
+The following tracers (listed below) give different output
+depending on whether or not the sysctl ftrace_enabled is set. To
+set ftrace_enabled, one can either use the sysctl function or
+set it via the proc file system interface.
+
+ sysctl kernel.ftrace_enabled=1
+
+ or
+
+ echo 1 > /proc/sys/kernel/ftrace_enabled
+
+To disable ftrace_enabled simply replace the '1' with '0' in the
+above commands.
+
+When ftrace_enabled is set the tracers will also record the
+functions that are within the trace. The descriptions of the
+tracers will also show an example with ftrace enabled.
+
+
+irqsoff
+-------
+
+When interrupts are disabled, the CPU can not react to any other
+external event (besides NMIs and SMIs). This prevents the timer
+interrupt from triggering or the mouse interrupt from letting
+the kernel know of a new mouse event. The result is a latency
+with the reaction time.
+
+The irqsoff tracer tracks the time for which interrupts are
+disabled. When a new maximum latency is hit, the tracer saves
+the trace leading up to that latency point so that every time a
+new maximum is reached, the old saved trace is discarded and the
+new trace is saved.
+
+To reset the maximum, echo 0 into tracing_max_latency. Here is
+an example:
+
+ # echo irqsoff > /debug/tracing/current_tracer
+ # echo 0 > /debug/tracing/tracing_max_latency
+ # echo 1 > /debug/tracing/tracing_enabled
+ # ls -ltr
+ [...]
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/latency_trace
+# tracer: irqsoff
+#
+irqsoff latency trace v1.1.5 on 2.6.26
+--------------------------------------------------------------------
+ latency: 12 us, #3/3, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: bash-3730 (uid:0 nice:0 policy:0 rt_prio:0)
+ -----------------
+ => started at: sys_setpgid
+ => ended at: sys_setpgid
+
+# _------=> CPU#
+# / _-----=> irqs-off
+# | / _----=> need-resched
+# || / _---=> hardirq/softirq
+# ||| / _--=> preempt-depth
+# |||| /
+# ||||| delay
+# cmd pid ||||| time | caller
+# \ / ||||| \ | /
+ bash-3730 1d... 0us : _write_lock_irq (sys_setpgid)
+ bash-3730 1d..1 1us+: _write_unlock_irq (sys_setpgid)
+ bash-3730 1d..2 14us : trace_hardirqs_on (sys_setpgid)
+
+
+Here we see that that we had a latency of 12 microsecs (which is
+very good). The _write_lock_irq in sys_setpgid disabled
+interrupts. The difference between the 12 and the displayed
+timestamp 14us occurred because the clock was incremented
+between the time of recording the max latency and the time of
+recording the function that had that latency.
+
+Note the above example had ftrace_enabled not set. If we set the
+ftrace_enabled, we get a much larger output:
+
+# tracer: irqsoff
+#
+irqsoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 50 us, #101/101, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: ls-4339 (uid:0 nice:0 policy:0 rt_prio:0)
+ -----------------
+ => started at: __alloc_pages_internal
+ => ended at: __alloc_pages_internal
+
+# _------=> CPU#
+# / _-----=> irqs-off
+# | / _----=> need-resched
+# || / _---=> hardirq/softirq
+# ||| / _--=> preempt-depth
+# |||| /
+# ||||| delay
+# cmd pid ||||| time | caller
+# \ / ||||| \ | /
+ ls-4339 0...1 0us+: get_page_from_freelist (__alloc_pages_internal)
+ ls-4339 0d..1 3us : rmqueue_bulk (get_page_from_freelist)
+ ls-4339 0d..1 3us : _spin_lock (rmqueue_bulk)
+ ls-4339 0d..1 4us : add_preempt_count (_spin_lock)
+ ls-4339 0d..2 4us : __rmqueue (rmqueue_bulk)
+ ls-4339 0d..2 5us : __rmqueue_smallest (__rmqueue)
+ ls-4339 0d..2 5us : __mod_zone_page_state (__rmqueue_smallest)
+ ls-4339 0d..2 6us : __rmqueue (rmqueue_bulk)
+ ls-4339 0d..2 6us : __rmqueue_smallest (__rmqueue)
+ ls-4339 0d..2 7us : __mod_zone_page_state (__rmqueue_smallest)
+ ls-4339 0d..2 7us : __rmqueue (rmqueue_bulk)
+ ls-4339 0d..2 8us : __rmqueue_smallest (__rmqueue)
+[...]
+ ls-4339 0d..2 46us : __rmqueue_smallest (__rmqueue)
+ ls-4339 0d..2 47us : __mod_zone_page_state (__rmqueue_smallest)
+ ls-4339 0d..2 47us : __rmqueue (rmqueue_bulk)
+ ls-4339 0d..2 48us : __rmqueue_smallest (__rmqueue)
+ ls-4339 0d..2 48us : __mod_zone_page_state (__rmqueue_smallest)
+ ls-4339 0d..2 49us : _spin_unlock (rmqueue_bulk)
+ ls-4339 0d..2 49us : sub_preempt_count (_spin_unlock)
+ ls-4339 0d..1 50us : get_page_from_freelist (__alloc_pages_internal)
+ ls-4339 0d..2 51us : trace_hardirqs_on (__alloc_pages_internal)
+
+
+
+Here we traced a 50 microsecond latency. But we also see all the
+functions that were called during that time. Note that by
+enabling function tracing, we incur an added overhead. This
+overhead may extend the latency times. But nevertheless, this
+trace has provided some very helpful debugging information.
+
+
+preemptoff
+----------
+
+When preemption is disabled, we may be able to receive
+interrupts but the task cannot be preempted and a higher
+priority task must wait for preemption to be enabled again
+before it can preempt a lower priority task.
+
+The preemptoff tracer traces the places that disable preemption.
+Like the irqsoff tracer, it records the maximum latency for
+which preemption was disabled. The control of preemptoff tracer
+is much like the irqsoff tracer.
+
+ # echo preemptoff > /debug/tracing/current_tracer
+ # echo 0 > /debug/tracing/tracing_max_latency
+ # echo 1 > /debug/tracing/tracing_enabled
+ # ls -ltr
+ [...]
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/latency_trace
+# tracer: preemptoff
+#
+preemptoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 29 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
+ -----------------
+ => started at: do_IRQ
+ => ended at: __do_softirq
+
+# _------=> CPU#
+# / _-----=> irqs-off
+# | / _----=> need-resched
+# || / _---=> hardirq/softirq
+# ||| / _--=> preempt-depth
+# |||| /
+# ||||| delay
+# cmd pid ||||| time | caller
+# \ / ||||| \ | /
+ sshd-4261 0d.h. 0us+: irq_enter (do_IRQ)
+ sshd-4261 0d.s. 29us : _local_bh_enable (__do_softirq)
+ sshd-4261 0d.s1 30us : trace_preempt_on (__do_softirq)
+
+
+This has some more changes. Preemption was disabled when an
+interrupt came in (notice the 'h'), and was enabled while doing
+a softirq. (notice the 's'). But we also see that interrupts
+have been disabled when entering the preempt off section and
+leaving it (the 'd'). We do not know if interrupts were enabled
+in the mean time.
+
+# tracer: preemptoff
+#
+preemptoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 63 us, #87/87, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
+ -----------------
+ => started at: remove_wait_queue
+ => ended at: __do_softirq
+
+# _------=> CPU#
+# / _-----=> irqs-off
+# | / _----=> need-resched
+# || / _---=> hardirq/softirq
+# ||| / _--=> preempt-depth
+# |||| /
+# ||||| delay
+# cmd pid ||||| time | caller
+# \ / ||||| \ | /
+ sshd-4261 0d..1 0us : _spin_lock_irqsave (remove_wait_queue)
+ sshd-4261 0d..1 1us : _spin_unlock_irqrestore (remove_wait_queue)
+ sshd-4261 0d..1 2us : do_IRQ (common_interrupt)
+ sshd-4261 0d..1 2us : irq_enter (do_IRQ)
+ sshd-4261 0d..1 2us : idle_cpu (irq_enter)
+ sshd-4261 0d..1 3us : add_preempt_count (irq_enter)
+ sshd-4261 0d.h1 3us : idle_cpu (irq_enter)
+ sshd-4261 0d.h. 4us : handle_fasteoi_irq (do_IRQ)
+[...]
+ sshd-4261 0d.h. 12us : add_preempt_count (_spin_lock)
+ sshd-4261 0d.h1 12us : ack_ioapic_quirk_irq (handle_fasteoi_irq)
+ sshd-4261 0d.h1 13us : move_native_irq (ack_ioapic_quirk_irq)
+ sshd-4261 0d.h1 13us : _spin_unlock (handle_fasteoi_irq)
+ sshd-4261 0d.h1 14us : sub_preempt_count (_spin_unlock)
+ sshd-4261 0d.h1 14us : irq_exit (do_IRQ)
+ sshd-4261 0d.h1 15us : sub_preempt_count (irq_exit)
+ sshd-4261 0d..2 15us : do_softirq (irq_exit)
+ sshd-4261 0d... 15us : __do_softirq (do_softirq)
+ sshd-4261 0d... 16us : __local_bh_disable (__do_softirq)
+ sshd-4261 0d... 16us+: add_preempt_count (__local_bh_disable)
+ sshd-4261 0d.s4 20us : add_preempt_count (__local_bh_disable)
+ sshd-4261 0d.s4 21us : sub_preempt_count (local_bh_enable)
+ sshd-4261 0d.s5 21us : sub_preempt_count (local_bh_enable)
+[...]
+ sshd-4261 0d.s6 41us : add_preempt_count (__local_bh_disable)
+ sshd-4261 0d.s6 42us : sub_preempt_count (local_bh_enable)
+ sshd-4261 0d.s7 42us : sub_preempt_count (local_bh_enable)
+ sshd-4261 0d.s5 43us : add_preempt_count (__local_bh_disable)
+ sshd-4261 0d.s5 43us : sub_preempt_count (local_bh_enable_ip)
+ sshd-4261 0d.s6 44us : sub_preempt_count (local_bh_enable_ip)
+ sshd-4261 0d.s5 44us : add_preempt_count (__local_bh_disable)
+ sshd-4261 0d.s5 45us : sub_preempt_count (local_bh_enable)
+[...]
+ sshd-4261 0d.s. 63us : _local_bh_enable (__do_softirq)
+ sshd-4261 0d.s1 64us : trace_preempt_on (__do_softirq)
+
+
+The above is an example of the preemptoff trace with
+ftrace_enabled set. Here we see that interrupts were disabled
+the entire time. The irq_enter code lets us know that we entered
+an interrupt 'h'. Before that, the functions being traced still
+show that it is not in an interrupt, but we can see from the
+functions themselves that this is not the case.
+
+Notice that __do_softirq when called does not have a
+preempt_count. It may seem that we missed a preempt enabling.
+What really happened is that the preempt count is held on the
+thread's stack and we switched to the softirq stack (4K stacks
+in effect). The code does not copy the preempt count, but
+because interrupts are disabled, we do not need to worry about
+it. Having a tracer like this is good for letting people know
+what really happens inside the kernel.
+
+
+preemptirqsoff
+--------------
+
+Knowing the locations that have interrupts disabled or
+preemption disabled for the longest times is helpful. But
+sometimes we would like to know when either preemption and/or
+interrupts are disabled.
+
+Consider the following code:
+
+ local_irq_disable();
+ call_function_with_irqs_off();
+ preempt_disable();
+ call_function_with_irqs_and_preemption_off();
+ local_irq_enable();
+ call_function_with_preemption_off();
+ preempt_enable();
+
+The irqsoff tracer will record the total length of
+call_function_with_irqs_off() and
+call_function_with_irqs_and_preemption_off().
+
+The preemptoff tracer will record the total length of
+call_function_with_irqs_and_preemption_off() and
+call_function_with_preemption_off().
+
+But neither will trace the time that interrupts and/or
+preemption is disabled. This total time is the time that we can
+not schedule. To record this time, use the preemptirqsoff
+tracer.
+
+Again, using this trace is much like the irqsoff and preemptoff
+tracers.
+
+ # echo preemptirqsoff > /debug/tracing/current_tracer
+ # echo 0 > /debug/tracing/tracing_max_latency
+ # echo 1 > /debug/tracing/tracing_enabled
+ # ls -ltr
+ [...]
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/latency_trace
+# tracer: preemptirqsoff
+#
+preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 293 us, #3/3, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: ls-4860 (uid:0 nice:0 policy:0 rt_prio:0)
+ -----------------
+ => started at: apic_timer_interrupt
+ => ended at: __do_softirq
+
+# _------=> CPU#
+# / _-----=> irqs-off
+# | / _----=> need-resched
+# || / _---=> hardirq/softirq
+# ||| / _--=> preempt-depth
+# |||| /
+# ||||| delay
+# cmd pid ||||| time | caller
+# \ / ||||| \ | /
+ ls-4860 0d... 0us!: trace_hardirqs_off_thunk (apic_timer_interrupt)
+ ls-4860 0d.s. 294us : _local_bh_enable (__do_softirq)
+ ls-4860 0d.s1 294us : trace_preempt_on (__do_softirq)
+
+
+
+The trace_hardirqs_off_thunk is called from assembly on x86 when
+interrupts are disabled in the assembly code. Without the
+function tracing, we do not know if interrupts were enabled
+within the preemption points. We do see that it started with
+preemption enabled.
+
+Here is a trace with ftrace_enabled set:
+
+
+# tracer: preemptirqsoff
+#
+preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 105 us, #183/183, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: sshd-4261 (uid:0 nice:0 policy:0 rt_prio:0)
+ -----------------
+ => started at: write_chan
+ => ended at: __do_softirq
+
+# _------=> CPU#
+# / _-----=> irqs-off
+# | / _----=> need-resched
+# || / _---=> hardirq/softirq
+# ||| / _--=> preempt-depth
+# |||| /
+# ||||| delay
+# cmd pid ||||| time | caller
+# \ / ||||| \ | /
+ ls-4473 0.N.. 0us : preempt_schedule (write_chan)
+ ls-4473 0dN.1 1us : _spin_lock (schedule)
+ ls-4473 0dN.1 2us : add_preempt_count (_spin_lock)
+ ls-4473 0d..2 2us : put_prev_task_fair (schedule)
+[...]
+ ls-4473 0d..2 13us : set_normalized_timespec (ktime_get_ts)
+ ls-4473 0d..2 13us : __switch_to (schedule)
+ sshd-4261 0d..2 14us : finish_task_switch (schedule)
+ sshd-4261 0d..2 14us : _spin_unlock_irq (finish_task_switch)
+ sshd-4261 0d..1 15us : add_preempt_count (_spin_lock_irqsave)
+ sshd-4261 0d..2 16us : _spin_unlock_irqrestore (hrtick_set)
+ sshd-4261 0d..2 16us : do_IRQ (common_interrupt)
+ sshd-4261 0d..2 17us : irq_enter (do_IRQ)
+ sshd-4261 0d..2 17us : idle_cpu (irq_enter)
+ sshd-4261 0d..2 18us : add_preempt_count (irq_enter)
+ sshd-4261 0d.h2 18us : idle_cpu (irq_enter)
+ sshd-4261 0d.h. 18us : handle_fasteoi_irq (do_IRQ)
+ sshd-4261 0d.h. 19us : _spin_lock (handle_fasteoi_irq)
+ sshd-4261 0d.h. 19us : add_preempt_count (_spin_lock)
+ sshd-4261 0d.h1 20us : _spin_unlock (handle_fasteoi_irq)
+ sshd-4261 0d.h1 20us : sub_preempt_count (_spin_unlock)
+[...]
+ sshd-4261 0d.h1 28us : _spin_unlock (handle_fasteoi_irq)
+ sshd-4261 0d.h1 29us : sub_preempt_count (_spin_unlock)
+ sshd-4261 0d.h2 29us : irq_exit (do_IRQ)
+ sshd-4261 0d.h2 29us : sub_preempt_count (irq_exit)
+ sshd-4261 0d..3 30us : do_softirq (irq_exit)
+ sshd-4261 0d... 30us : __do_softirq (do_softirq)
+ sshd-4261 0d... 31us : __local_bh_disable (__do_softirq)
+ sshd-4261 0d... 31us+: add_preempt_count (__local_bh_disable)
+ sshd-4261 0d.s4 34us : add_preempt_count (__local_bh_disable)
+[...]
+ sshd-4261 0d.s3 43us : sub_preempt_count (local_bh_enable_ip)
+ sshd-4261 0d.s4 44us : sub_preempt_count (local_bh_enable_ip)
+ sshd-4261 0d.s3 44us : smp_apic_timer_interrupt (apic_timer_interrupt)
+ sshd-4261 0d.s3 45us : irq_enter (smp_apic_timer_interrupt)
+ sshd-4261 0d.s3 45us : idle_cpu (irq_enter)
+ sshd-4261 0d.s3 46us : add_preempt_count (irq_enter)
+ sshd-4261 0d.H3 46us : idle_cpu (irq_enter)
+ sshd-4261 0d.H3 47us : hrtimer_interrupt (smp_apic_timer_interrupt)
+ sshd-4261 0d.H3 47us : ktime_get (hrtimer_interrupt)
+[...]
+ sshd-4261 0d.H3 81us : tick_program_event (hrtimer_interrupt)
+ sshd-4261 0d.H3 82us : ktime_get (tick_program_event)
+ sshd-4261 0d.H3 82us : ktime_get_ts (ktime_get)
+ sshd-4261 0d.H3 83us : getnstimeofday (ktime_get_ts)
+ sshd-4261 0d.H3 83us : set_normalized_timespec (ktime_get_ts)
+ sshd-4261 0d.H3 84us : clockevents_program_event (tick_program_event)
+ sshd-4261 0d.H3 84us : lapic_next_event (clockevents_program_event)
+ sshd-4261 0d.H3 85us : irq_exit (smp_apic_timer_interrupt)
+ sshd-4261 0d.H3 85us : sub_preempt_count (irq_exit)
+ sshd-4261 0d.s4 86us : sub_preempt_count (irq_exit)
+ sshd-4261 0d.s3 86us : add_preempt_count (__local_bh_disable)
+[...]
+ sshd-4261 0d.s1 98us : sub_preempt_count (net_rx_action)
+ sshd-4261 0d.s. 99us : add_preempt_count (_spin_lock_irq)
+ sshd-4261 0d.s1 99us+: _spin_unlock_irq (run_timer_softirq)
+ sshd-4261 0d.s. 104us : _local_bh_enable (__do_softirq)
+ sshd-4261 0d.s. 104us : sub_preempt_count (_local_bh_enable)
+ sshd-4261 0d.s. 105us : _local_bh_enable (__do_softirq)
+ sshd-4261 0d.s1 105us : trace_preempt_on (__do_softirq)
+
+
+This is a very interesting trace. It started with the preemption
+of the ls task. We see that the task had the "need_resched" bit
+set via the 'N' in the trace. Interrupts were disabled before
+the spin_lock at the beginning of the trace. We see that a
+schedule took place to run sshd. When the interrupts were
+enabled, we took an interrupt. On return from the interrupt
+handler, the softirq ran. We took another interrupt while
+running the softirq as we see from the capital 'H'.
+
+
+wakeup
+------
+
+In a Real-Time environment it is very important to know the
+wakeup time it takes for the highest priority task that is woken
+up to the time that it executes. This is also known as "schedule
+latency". I stress the point that this is about RT tasks. It is
+also important to know the scheduling latency of non-RT tasks,
+but the average schedule latency is better for non-RT tasks.
+Tools like LatencyTop are more appropriate for such
+measurements.
+
+Real-Time environments are interested in the worst case latency.
+That is the longest latency it takes for something to happen,
+and not the average. We can have a very fast scheduler that may
+only have a large latency once in a while, but that would not
+work well with Real-Time tasks. The wakeup tracer was designed
+to record the worst case wakeups of RT tasks. Non-RT tasks are
+not recorded because the tracer only records one worst case and
+tracing non-RT tasks that are unpredictable will overwrite the
+worst case latency of RT tasks.
+
+Since this tracer only deals with RT tasks, we will run this
+slightly differently than we did with the previous tracers.
+Instead of performing an 'ls', we will run 'sleep 1' under
+'chrt' which changes the priority of the task.
+
+ # echo wakeup > /debug/tracing/current_tracer
+ # echo 0 > /debug/tracing/tracing_max_latency
+ # echo 1 > /debug/tracing/tracing_enabled
+ # chrt -f 5 sleep 1
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/latency_trace
+# tracer: wakeup
+#
+wakeup latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 4 us, #2/2, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: sleep-4901 (uid:0 nice:0 policy:1 rt_prio:5)
+ -----------------
+
+# _------=> CPU#
+# / _-----=> irqs-off
+# | / _----=> need-resched
+# || / _---=> hardirq/softirq
+# ||| / _--=> preempt-depth
+# |||| /
+# ||||| delay
+# cmd pid ||||| time | caller
+# \ / ||||| \ | /
+ <idle>-0 1d.h4 0us+: try_to_wake_up (wake_up_process)
+ <idle>-0 1d..4 4us : schedule (cpu_idle)
+
+
+Running this on an idle system, we see that it only took 4
+microseconds to perform the task switch. Note, since the trace
+marker in the schedule is before the actual "switch", we stop
+the tracing when the recorded task is about to schedule in. This
+may change if we add a new marker at the end of the scheduler.
+
+Notice that the recorded task is 'sleep' with the PID of 4901
+and it has an rt_prio of 5. This priority is user-space priority
+and not the internal kernel priority. The policy is 1 for
+SCHED_FIFO and 2 for SCHED_RR.
+
+Doing the same with chrt -r 5 and ftrace_enabled set.
+
+# tracer: wakeup
+#
+wakeup latency trace v1.1.5 on 2.6.26-rc8
+--------------------------------------------------------------------
+ latency: 50 us, #60/60, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
+ -----------------
+ | task: sleep-4068 (uid:0 nice:0 policy:2 rt_prio:5)
+ -----------------
+
+# _------=> CPU#
+# / _-----=> irqs-off
+# | / _----=> need-resched
+# || / _---=> hardirq/softirq
+# ||| / _--=> preempt-depth
+# |||| /
+# ||||| delay
+# cmd pid ||||| time | caller
+# \ / ||||| \ | /
+ksoftirq-7 1d.H3 0us : try_to_wake_up (wake_up_process)
+ksoftirq-7 1d.H4 1us : sub_preempt_count (marker_probe_cb)
+ksoftirq-7 1d.H3 2us : check_preempt_wakeup (try_to_wake_up)
+ksoftirq-7 1d.H3 3us : update_curr (check_preempt_wakeup)
+ksoftirq-7 1d.H3 4us : calc_delta_mine (update_curr)
+ksoftirq-7 1d.H3 5us : __resched_task (check_preempt_wakeup)
+ksoftirq-7 1d.H3 6us : task_wake_up_rt (try_to_wake_up)
+ksoftirq-7 1d.H3 7us : _spin_unlock_irqrestore (try_to_wake_up)
+[...]
+ksoftirq-7 1d.H2 17us : irq_exit (smp_apic_timer_interrupt)
+ksoftirq-7 1d.H2 18us : sub_preempt_count (irq_exit)
+ksoftirq-7 1d.s3 19us : sub_preempt_count (irq_exit)
+ksoftirq-7 1..s2 20us : rcu_process_callbacks (__do_softirq)
+[...]
+ksoftirq-7 1..s2 26us : __rcu_process_callbacks (rcu_process_callbacks)
+ksoftirq-7 1d.s2 27us : _local_bh_enable (__do_softirq)
+ksoftirq-7 1d.s2 28us : sub_preempt_count (_local_bh_enable)
+ksoftirq-7 1.N.3 29us : sub_preempt_count (ksoftirqd)
+ksoftirq-7 1.N.2 30us : _cond_resched (ksoftirqd)
+ksoftirq-7 1.N.2 31us : __cond_resched (_cond_resched)
+ksoftirq-7 1.N.2 32us : add_preempt_count (__cond_resched)
+ksoftirq-7 1.N.2 33us : schedule (__cond_resched)
+ksoftirq-7 1.N.2 33us : add_preempt_count (schedule)
+ksoftirq-7 1.N.3 34us : hrtick_clear (schedule)
+ksoftirq-7 1dN.3 35us : _spin_lock (schedule)
+ksoftirq-7 1dN.3 36us : add_preempt_count (_spin_lock)
+ksoftirq-7 1d..4 37us : put_prev_task_fair (schedule)
+ksoftirq-7 1d..4 38us : update_curr (put_prev_task_fair)
+[...]
+ksoftirq-7 1d..5 47us : _spin_trylock (tracing_record_cmdline)
+ksoftirq-7 1d..5 48us : add_preempt_count (_spin_trylock)
+ksoftirq-7 1d..6 49us : _spin_unlock (tracing_record_cmdline)
+ksoftirq-7 1d..6 49us : sub_preempt_count (_spin_unlock)
+ksoftirq-7 1d..4 50us : schedule (__cond_resched)
+
+The interrupt went off while running ksoftirqd. This task runs
+at SCHED_OTHER. Why did not we see the 'N' set early? This may
+be a harmless bug with x86_32 and 4K stacks. On x86_32 with 4K
+stacks configured, the interrupt and softirq run with their own
+stack. Some information is held on the top of the task's stack
+(need_resched and preempt_count are both stored there). The
+setting of the NEED_RESCHED bit is done directly to the task's
+stack, but the reading of the NEED_RESCHED is done by looking at
+the current stack, which in this case is the stack for the hard
+interrupt. This hides the fact that NEED_RESCHED has been set.
+We do not see the 'N' until we switch back to the task's
+assigned stack.
+
+function
+--------
+
+This tracer is the function tracer. Enabling the function tracer
+can be done from the debug file system. Make sure the
+ftrace_enabled is set; otherwise this tracer is a nop.
+
+ # sysctl kernel.ftrace_enabled=1
+ # echo function > /debug/tracing/current_tracer
+ # echo 1 > /debug/tracing/tracing_enabled
+ # usleep 1
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/trace
+# tracer: function
+#
+# TASK-PID CPU# TIMESTAMP FUNCTION
+# | | | | |
+ bash-4003 [00] 123.638713: finish_task_switch <-schedule
+ bash-4003 [00] 123.638714: _spin_unlock_irq <-finish_task_switch
+ bash-4003 [00] 123.638714: sub_preempt_count <-_spin_unlock_irq
+ bash-4003 [00] 123.638715: hrtick_set <-schedule
+ bash-4003 [00] 123.638715: _spin_lock_irqsave <-hrtick_set
+ bash-4003 [00] 123.638716: add_preempt_count <-_spin_lock_irqsave
+ bash-4003 [00] 123.638716: _spin_unlock_irqrestore <-hrtick_set
+ bash-4003 [00] 123.638717: sub_preempt_count <-_spin_unlock_irqrestore
+ bash-4003 [00] 123.638717: hrtick_clear <-hrtick_set
+ bash-4003 [00] 123.638718: sub_preempt_count <-schedule
+ bash-4003 [00] 123.638718: sub_preempt_count <-preempt_schedule
+ bash-4003 [00] 123.638719: wait_for_completion <-__stop_machine_run
+ bash-4003 [00] 123.638719: wait_for_common <-wait_for_completion
+ bash-4003 [00] 123.638720: _spin_lock_irq <-wait_for_common
+ bash-4003 [00] 123.638720: add_preempt_count <-_spin_lock_irq
+[...]
+
+
+Note: function tracer uses ring buffers to store the above
+entries. The newest data may overwrite the oldest data.
+Sometimes using echo to stop the trace is not sufficient because
+the tracing could have overwritten the data that you wanted to
+record. For this reason, it is sometimes better to disable
+tracing directly from a program. This allows you to stop the
+tracing at the point that you hit the part that you are
+interested in. To disable the tracing directly from a C program,
+something like following code snippet can be used:
+
+int trace_fd;
+[...]
+int main(int argc, char *argv[]) {
+ [...]
+ trace_fd = open("/debug/tracing/tracing_enabled", O_WRONLY);
+ [...]
+ if (condition_hit()) {
+ write(trace_fd, "0", 1);
+ }
+ [...]
+}
+
+Note: Here we hard coded the path name. The debugfs mount is not
+guaranteed to be at /debug (and is more commonly at
+/sys/kernel/debug). For simple one time traces, the above is
+sufficent. For anything else, a search through /proc/mounts may
+be needed to find where the debugfs file-system is mounted.
+
+
+Single thread tracing
+---------------------
+
+By writing into /debug/tracing/set_ftrace_pid you can trace a
+single thread. For example:
+
+# cat /debug/tracing/set_ftrace_pid
+no pid
+# echo 3111 > /debug/tracing/set_ftrace_pid
+# cat /debug/tracing/set_ftrace_pid
+3111
+# echo function > /debug/tracing/current_tracer
+# cat /debug/tracing/trace | head
+ # tracer: function
+ #
+ # TASK-PID CPU# TIMESTAMP FUNCTION
+ # | | | | |
+ yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return
+ yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
+ yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
+ yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
+ yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll
+ yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll
+# echo -1 > /debug/tracing/set_ftrace_pid
+# cat /debug/tracing/trace |head
+ # tracer: function
+ #
+ # TASK-PID CPU# TIMESTAMP FUNCTION
+ # | | | | |
+ ##### CPU 3 buffer started ####
+ yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait
+ yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry
+ yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry
+ yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit
+ yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit
+
+If you want to trace a function when executing, you could use
+something like this simple program:
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+int main (int argc, char **argv)
+{
+ if (argc < 1)
+ exit(-1);
+
+ if (fork() > 0) {
+ int fd, ffd;
+ char line[64];
+ int s;
+
+ ffd = open("/debug/tracing/current_tracer", O_WRONLY);
+ if (ffd < 0)
+ exit(-1);
+ write(ffd, "nop", 3);
+
+ fd = open("/debug/tracing/set_ftrace_pid", O_WRONLY);
+ s = sprintf(line, "%d\n", getpid());
+ write(fd, line, s);
+
+ write(ffd, "function", 8);
+
+ close(fd);
+ close(ffd);
+
+ execvp(argv[1], argv+1);
+ }
+
+ return 0;
+}
+
+
+hw-branch-tracer (x86 only)
+---------------------------
+
+This tracer uses the x86 last branch tracing hardware feature to
+collect a branch trace on all cpus with relatively low overhead.
+
+The tracer uses a fixed-size circular buffer per cpu and only
+traces ring 0 branches. The trace file dumps that buffer in the
+following format:
+
+# tracer: hw-branch-tracer
+#
+# CPU# TO <- FROM
+ 0 scheduler_tick+0xb5/0x1bf <- task_tick_idle+0x5/0x6
+ 2 run_posix_cpu_timers+0x2b/0x72a <- run_posix_cpu_timers+0x25/0x72a
+ 0 scheduler_tick+0x139/0x1bf <- scheduler_tick+0xed/0x1bf
+ 0 scheduler_tick+0x17c/0x1bf <- scheduler_tick+0x148/0x1bf
+ 2 run_posix_cpu_timers+0x9e/0x72a <- run_posix_cpu_timers+0x5e/0x72a
+ 0 scheduler_tick+0x1b6/0x1bf <- scheduler_tick+0x1aa/0x1bf
+
+
+The tracer may be used to dump the trace for the oops'ing cpu on
+a kernel oops into the system log. To enable this,
+ftrace_dump_on_oops must be set. To set ftrace_dump_on_oops, one
+can either use the sysctl function or set it via the proc system
+interface.
+
+ sysctl kernel.ftrace_dump_on_oops=1
+
+or
+
+ echo 1 > /proc/sys/kernel/ftrace_dump_on_oops
+
+
+Here's an example of such a dump after a null pointer
+dereference in a kernel module:
+
+[57848.105921] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
+[57848.106019] IP: [<ffffffffa0000006>] open+0x6/0x14 [oops]
+[57848.106019] PGD 2354e9067 PUD 2375e7067 PMD 0
+[57848.106019] Oops: 0002 [#1] SMP
+[57848.106019] last sysfs file: /sys/devices/pci0000:00/0000:00:1e.0/0000:20:05.0/local_cpus
+[57848.106019] Dumping ftrace buffer:
+[57848.106019] ---------------------------------
+[...]
+[57848.106019] 0 chrdev_open+0xe6/0x165 <- cdev_put+0x23/0x24
+[57848.106019] 0 chrdev_open+0x117/0x165 <- chrdev_open+0xfa/0x165
+[57848.106019] 0 chrdev_open+0x120/0x165 <- chrdev_open+0x11c/0x165
+[57848.106019] 0 chrdev_open+0x134/0x165 <- chrdev_open+0x12b/0x165
+[57848.106019] 0 open+0x0/0x14 [oops] <- chrdev_open+0x144/0x165
+[57848.106019] 0 page_fault+0x0/0x30 <- open+0x6/0x14 [oops]
+[57848.106019] 0 error_entry+0x0/0x5b <- page_fault+0x4/0x30
+[57848.106019] 0 error_kernelspace+0x0/0x31 <- error_entry+0x59/0x5b
+[57848.106019] 0 error_sti+0x0/0x1 <- error_kernelspace+0x2d/0x31
+[57848.106019] 0 page_fault+0x9/0x30 <- error_sti+0x0/0x1
+[57848.106019] 0 do_page_fault+0x0/0x881 <- page_fault+0x1a/0x30
+[...]
+[57848.106019] 0 do_page_fault+0x66b/0x881 <- is_prefetch+0x1ee/0x1f2
+[57848.106019] 0 do_page_fault+0x6e0/0x881 <- do_page_fault+0x67a/0x881
+[57848.106019] 0 oops_begin+0x0/0x96 <- do_page_fault+0x6e0/0x881
+[57848.106019] 0 trace_hw_branch_oops+0x0/0x2d <- oops_begin+0x9/0x96
+[...]
+[57848.106019] 0 ds_suspend_bts+0x2a/0xe3 <- ds_suspend_bts+0x1a/0xe3
+[57848.106019] ---------------------------------
+[57848.106019] CPU 0
+[57848.106019] Modules linked in: oops
+[57848.106019] Pid: 5542, comm: cat Tainted: G W 2.6.28 #23
+[57848.106019] RIP: 0010:[<ffffffffa0000006>] [<ffffffffa0000006>] open+0x6/0x14 [oops]
+[57848.106019] RSP: 0018:ffff880235457d48 EFLAGS: 00010246
+[...]
+
+
+function graph tracer
+---------------------------
+
+This tracer is similar to the function tracer except that it
+probes a function on its entry and its exit. This is done by
+using a dynamically allocated stack of return addresses in each
+task_struct. On function entry the tracer overwrites the return
+address of each function traced to set a custom probe. Thus the
+original return address is stored on the stack of return address
+in the task_struct.
+
+Probing on both ends of a function leads to special features
+such as:
+
+- measure of a function's time execution
+- having a reliable call stack to draw function calls graph
+
+This tracer is useful in several situations:
+
+- you want to find the reason of a strange kernel behavior and
+ need to see what happens in detail on any areas (or specific
+ ones).
+
+- you are experiencing weird latencies but it's difficult to
+ find its origin.
+
+- you want to find quickly which path is taken by a specific
+ function
+
+- you just want to peek inside a working kernel and want to see
+ what happens there.
+
+# tracer: function_graph
+#
+# CPU DURATION FUNCTION CALLS
+# | | | | | | |
+
+ 0) | sys_open() {
+ 0) | do_sys_open() {
+ 0) | getname() {
+ 0) | kmem_cache_alloc() {
+ 0) 1.382 us | __might_sleep();
+ 0) 2.478 us | }
+ 0) | strncpy_from_user() {
+ 0) | might_fault() {
+ 0) 1.389 us | __might_sleep();
+ 0) 2.553 us | }
+ 0) 3.807 us | }
+ 0) 7.876 us | }
+ 0) | alloc_fd() {
+ 0) 0.668 us | _spin_lock();
+ 0) 0.570 us | expand_files();
+ 0) 0.586 us | _spin_unlock();
+
+
+There are several columns that can be dynamically
+enabled/disabled. You can use every combination of options you
+want, depending on your needs.
+
+- The cpu number on which the function executed is default
+ enabled. It is sometimes better to only trace one cpu (see
+ tracing_cpu_mask file) or you might sometimes see unordered
+ function calls while cpu tracing switch.
+
+ hide: echo nofuncgraph-cpu > /debug/tracing/trace_options
+ show: echo funcgraph-cpu > /debug/tracing/trace_options
+
+- The duration (function's time of execution) is displayed on
+ the closing bracket line of a function or on the same line
+ than the current function in case of a leaf one. It is default
+ enabled.
+
+ hide: echo nofuncgraph-duration > /debug/tracing/trace_options
+ show: echo funcgraph-duration > /debug/tracing/trace_options
+
+- The overhead field precedes the duration field in case of
+ reached duration thresholds.
+
+ hide: echo nofuncgraph-overhead > /debug/tracing/trace_options
+ show: echo funcgraph-overhead > /debug/tracing/trace_options
+ depends on: funcgraph-duration
+
+ ie:
+
+ 0) | up_write() {
+ 0) 0.646 us | _spin_lock_irqsave();
+ 0) 0.684 us | _spin_unlock_irqrestore();
+ 0) 3.123 us | }
+ 0) 0.548 us | fput();
+ 0) + 58.628 us | }
+
+ [...]
+
+ 0) | putname() {
+ 0) | kmem_cache_free() {
+ 0) 0.518 us | __phys_addr();
+ 0) 1.757 us | }
+ 0) 2.861 us | }
+ 0) ! 115.305 us | }
+ 0) ! 116.402 us | }
+
+ + means that the function exceeded 10 usecs.
+ ! means that the function exceeded 100 usecs.
+
+
+- The task/pid field displays the thread cmdline and pid which
+ executed the function. It is default disabled.
+
+ hide: echo nofuncgraph-proc > /debug/tracing/trace_options
+ show: echo funcgraph-proc > /debug/tracing/trace_options
+
+ ie:
+
+ # tracer: function_graph
+ #
+ # CPU TASK/PID DURATION FUNCTION CALLS
+ # | | | | | | | | |
+ 0) sh-4802 | | d_free() {
+ 0) sh-4802 | | call_rcu() {
+ 0) sh-4802 | | __call_rcu() {
+ 0) sh-4802 | 0.616 us | rcu_process_gp_end();
+ 0) sh-4802 | 0.586 us | check_for_new_grace_period();
+ 0) sh-4802 | 2.899 us | }
+ 0) sh-4802 | 4.040 us | }
+ 0) sh-4802 | 5.151 us | }
+ 0) sh-4802 | + 49.370 us | }
+
+
+- The absolute time field is an absolute timestamp given by the
+ system clock since it started. A snapshot of this time is
+ given on each entry/exit of functions
+
+ hide: echo nofuncgraph-abstime > /debug/tracing/trace_options
+ show: echo funcgraph-abstime > /debug/tracing/trace_options
+
+ ie:
+
+ #
+ # TIME CPU DURATION FUNCTION CALLS
+ # | | | | | | | |
+ 360.774522 | 1) 0.541 us | }
+ 360.774522 | 1) 4.663 us | }
+ 360.774523 | 1) 0.541 us | __wake_up_bit();
+ 360.774524 | 1) 6.796 us | }
+ 360.774524 | 1) 7.952 us | }
+ 360.774525 | 1) 9.063 us | }
+ 360.774525 | 1) 0.615 us | journal_mark_dirty();
+ 360.774527 | 1) 0.578 us | __brelse();
+ 360.774528 | 1) | reiserfs_prepare_for_journal() {
+ 360.774528 | 1) | unlock_buffer() {
+ 360.774529 | 1) | wake_up_bit() {
+ 360.774529 | 1) | bit_waitqueue() {
+ 360.774530 | 1) 0.594 us | __phys_addr();
+
+
+You can put some comments on specific functions by using
+trace_printk() For example, if you want to put a comment inside
+the __might_sleep() function, you just have to include
+<linux/ftrace.h> and call trace_printk() inside __might_sleep()
+
+trace_printk("I'm a comment!\n")
+
+will produce:
+
+ 1) | __might_sleep() {
+ 1) | /* I'm a comment! */
+ 1) 1.449 us | }
+
+
+You might find other useful features for this tracer in the
+following "dynamic ftrace" section such as tracing only specific
+functions or tasks.
+
+dynamic ftrace
+--------------
+
+If CONFIG_DYNAMIC_FTRACE is set, the system will run with
+virtually no overhead when function tracing is disabled. The way
+this works is the mcount function call (placed at the start of
+every kernel function, produced by the -pg switch in gcc),
+starts of pointing to a simple return. (Enabling FTRACE will
+include the -pg switch in the compiling of the kernel.)
+
+At compile time every C file object is run through the
+recordmcount.pl script (located in the scripts directory). This
+script will process the C object using objdump to find all the
+locations in the .text section that call mcount. (Note, only the
+.text section is processed, since processing other sections like
+.init.text may cause races due to those sections being freed).
+
+A new section called "__mcount_loc" is created that holds
+references to all the mcount call sites in the .text section.
+This section is compiled back into the original object. The
+final linker will add all these references into a single table.
+
+On boot up, before SMP is initialized, the dynamic ftrace code
+scans this table and updates all the locations into nops. It
+also records the locations, which are added to the
+available_filter_functions list. Modules are processed as they
+are loaded and before they are executed. When a module is
+unloaded, it also removes its functions from the ftrace function
+list. This is automatic in the module unload code, and the
+module author does not need to worry about it.
+
+When tracing is enabled, kstop_machine is called to prevent
+races with the CPUS executing code being modified (which can
+cause the CPU to do undesireable things), and the nops are
+patched back to calls. But this time, they do not call mcount
+(which is just a function stub). They now call into the ftrace
+infrastructure.
+
+One special side-effect to the recording of the functions being
+traced is that we can now selectively choose which functions we
+wish to trace and which ones we want the mcount calls to remain
+as nops.
+
+Two files are used, one for enabling and one for disabling the
+tracing of specified functions. They are:
+
+ set_ftrace_filter
+
+and
+
+ set_ftrace_notrace
+
+A list of available functions that you can add to these files is
+listed in:
+
+ available_filter_functions
+
+ # cat /debug/tracing/available_filter_functions
+put_prev_task_idle
+kmem_cache_create
+pick_next_task_rt
+get_online_cpus
+pick_next_task_fair
+mutex_lock
+[...]
+
+If I am only interested in sys_nanosleep and hrtimer_interrupt:
+
+ # echo sys_nanosleep hrtimer_interrupt \
+ > /debug/tracing/set_ftrace_filter
+ # echo ftrace > /debug/tracing/current_tracer
+ # echo 1 > /debug/tracing/tracing_enabled
+ # usleep 1
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/trace
+# tracer: ftrace
+#
+# TASK-PID CPU# TIMESTAMP FUNCTION
+# | | | | |
+ usleep-4134 [00] 1317.070017: hrtimer_interrupt <-smp_apic_timer_interrupt
+ usleep-4134 [00] 1317.070111: sys_nanosleep <-syscall_call
+ <idle>-0 [00] 1317.070115: hrtimer_interrupt <-smp_apic_timer_interrupt
+
+To see which functions are being traced, you can cat the file:
+
+ # cat /debug/tracing/set_ftrace_filter
+hrtimer_interrupt
+sys_nanosleep
+
+
+Perhaps this is not enough. The filters also allow simple wild
+cards. Only the following are currently available
+
+ <match>* - will match functions that begin with <match>
+ *<match> - will match functions that end with <match>
+ *<match>* - will match functions that have <match> in it
+
+These are the only wild cards which are supported.
+
+ <match>*<match> will not work.
+
+Note: It is better to use quotes to enclose the wild cards,
+ otherwise the shell may expand the parameters into names
+ of files in the local directory.
+
+ # echo 'hrtimer_*' > /debug/tracing/set_ftrace_filter
+
+Produces:
+
+# tracer: ftrace
+#
+# TASK-PID CPU# TIMESTAMP FUNCTION
+# | | | | |
+ bash-4003 [00] 1480.611794: hrtimer_init <-copy_process
+ bash-4003 [00] 1480.611941: hrtimer_start <-hrtick_set
+ bash-4003 [00] 1480.611956: hrtimer_cancel <-hrtick_clear
+ bash-4003 [00] 1480.611956: hrtimer_try_to_cancel <-hrtimer_cancel
+ <idle>-0 [00] 1480.612019: hrtimer_get_next_event <-get_next_timer_interrupt
+ <idle>-0 [00] 1480.612025: hrtimer_get_next_event <-get_next_timer_interrupt
+ <idle>-0 [00] 1480.612032: hrtimer_get_next_event <-get_next_timer_interrupt
+ <idle>-0 [00] 1480.612037: hrtimer_get_next_event <-get_next_timer_interrupt
+ <idle>-0 [00] 1480.612382: hrtimer_get_next_event <-get_next_timer_interrupt
+
+
+Notice that we lost the sys_nanosleep.
+
+ # cat /debug/tracing/set_ftrace_filter
+hrtimer_run_queues
+hrtimer_run_pending
+hrtimer_init
+hrtimer_cancel
+hrtimer_try_to_cancel
+hrtimer_forward
+hrtimer_start
+hrtimer_reprogram
+hrtimer_force_reprogram
+hrtimer_get_next_event
+hrtimer_interrupt
+hrtimer_nanosleep
+hrtimer_wakeup
+hrtimer_get_remaining
+hrtimer_get_res
+hrtimer_init_sleeper
+
+
+This is because the '>' and '>>' act just like they do in bash.
+To rewrite the filters, use '>'
+To append to the filters, use '>>'
+
+To clear out a filter so that all functions will be recorded
+again:
+
+ # echo > /debug/tracing/set_ftrace_filter
+ # cat /debug/tracing/set_ftrace_filter
+ #
+
+Again, now we want to append.
+
+ # echo sys_nanosleep > /debug/tracing/set_ftrace_filter
+ # cat /debug/tracing/set_ftrace_filter
+sys_nanosleep
+ # echo 'hrtimer_*' >> /debug/tracing/set_ftrace_filter
+ # cat /debug/tracing/set_ftrace_filter
+hrtimer_run_queues
+hrtimer_run_pending
+hrtimer_init
+hrtimer_cancel
+hrtimer_try_to_cancel
+hrtimer_forward
+hrtimer_start
+hrtimer_reprogram
+hrtimer_force_reprogram
+hrtimer_get_next_event
+hrtimer_interrupt
+sys_nanosleep
+hrtimer_nanosleep
+hrtimer_wakeup
+hrtimer_get_remaining
+hrtimer_get_res
+hrtimer_init_sleeper
+
+
+The set_ftrace_notrace prevents those functions from being
+traced.
+
+ # echo '*preempt*' '*lock*' > /debug/tracing/set_ftrace_notrace
+
+Produces:
+
+# tracer: ftrace
+#
+# TASK-PID CPU# TIMESTAMP FUNCTION
+# | | | | |
+ bash-4043 [01] 115.281644: finish_task_switch <-schedule
+ bash-4043 [01] 115.281645: hrtick_set <-schedule
+ bash-4043 [01] 115.281645: hrtick_clear <-hrtick_set
+ bash-4043 [01] 115.281646: wait_for_completion <-__stop_machine_run
+ bash-4043 [01] 115.281647: wait_for_common <-wait_for_completion
+ bash-4043 [01] 115.281647: kthread_stop <-stop_machine_run
+ bash-4043 [01] 115.281648: init_waitqueue_head <-kthread_stop
+ bash-4043 [01] 115.281648: wake_up_process <-kthread_stop
+ bash-4043 [01] 115.281649: try_to_wake_up <-wake_up_process
+
+We can see that there's no more lock or preempt tracing.
+
+
+Dynamic ftrace with the function graph tracer
+---------------------------------------------
+
+Although what has been explained above concerns both the
+function tracer and the function-graph-tracer, there are some
+special features only available in the function-graph tracer.
+
+If you want to trace only one function and all of its children,
+you just have to echo its name into set_graph_function:
+
+ echo __do_fault > set_graph_function
+
+will produce the following "expanded" trace of the __do_fault()
+function:
+
+ 0) | __do_fault() {
+ 0) | filemap_fault() {
+ 0) | find_lock_page() {
+ 0) 0.804 us | find_get_page();
+ 0) | __might_sleep() {
+ 0) 1.329 us | }
+ 0) 3.904 us | }
+ 0) 4.979 us | }
+ 0) 0.653 us | _spin_lock();
+ 0) 0.578 us | page_add_file_rmap();
+ 0) 0.525 us | native_set_pte_at();
+ 0) 0.585 us | _spin_unlock();
+ 0) | unlock_page() {
+ 0) 0.541 us | page_waitqueue();
+ 0) 0.639 us | __wake_up_bit();
+ 0) 2.786 us | }
+ 0) + 14.237 us | }
+ 0) | __do_fault() {
+ 0) | filemap_fault() {
+ 0) | find_lock_page() {
+ 0) 0.698 us | find_get_page();
+ 0) | __might_sleep() {
+ 0) 1.412 us | }
+ 0) 3.950 us | }
+ 0) 5.098 us | }
+ 0) 0.631 us | _spin_lock();
+ 0) 0.571 us | page_add_file_rmap();
+ 0) 0.526 us | native_set_pte_at();
+ 0) 0.586 us | _spin_unlock();
+ 0) | unlock_page() {
+ 0) 0.533 us | page_waitqueue();
+ 0) 0.638 us | __wake_up_bit();
+ 0) 2.793 us | }
+ 0) + 14.012 us | }
+
+You can also expand several functions at once:
+
+ echo sys_open > set_graph_function
+ echo sys_close >> set_graph_function
+
+Now if you want to go back to trace all functions you can clear
+this special filter via:
+
+ echo > set_graph_function
+
+
+trace_pipe
+----------
+
+The trace_pipe outputs the same content as the trace file, but
+the effect on the tracing is different. Every read from
+trace_pipe is consumed. This means that subsequent reads will be
+different. The trace is live.
+
+ # echo function > /debug/tracing/current_tracer
+ # cat /debug/tracing/trace_pipe > /tmp/trace.out &
+[1] 4153
+ # echo 1 > /debug/tracing/tracing_enabled
+ # usleep 1
+ # echo 0 > /debug/tracing/tracing_enabled
+ # cat /debug/tracing/trace
+# tracer: function
+#
+# TASK-PID CPU# TIMESTAMP FUNCTION
+# | | | | |
+
+ #
+ # cat /tmp/trace.out
+ bash-4043 [00] 41.267106: finish_task_switch <-schedule
+ bash-4043 [00] 41.267106: hrtick_set <-schedule
+ bash-4043 [00] 41.267107: hrtick_clear <-hrtick_set
+ bash-4043 [00] 41.267108: wait_for_completion <-__stop_machine_run
+ bash-4043 [00] 41.267108: wait_for_common <-wait_for_completion
+ bash-4043 [00] 41.267109: kthread_stop <-stop_machine_run
+ bash-4043 [00] 41.267109: init_waitqueue_head <-kthread_stop
+ bash-4043 [00] 41.267110: wake_up_process <-kthread_stop
+ bash-4043 [00] 41.267110: try_to_wake_up <-wake_up_process
+ bash-4043 [00] 41.267111: select_task_rq_rt <-try_to_wake_up
+
+
+Note, reading the trace_pipe file will block until more input is
+added. By changing the tracer, trace_pipe will issue an EOF. We
+needed to set the function tracer _before_ we "cat" the
+trace_pipe file.
+
+
+trace entries
+-------------
+
+Having too much or not enough data can be troublesome in
+diagnosing an issue in the kernel. The file buffer_size_kb is
+used to modify the size of the internal trace buffers. The
+number listed is the number of entries that can be recorded per
+CPU. To know the full size, multiply the number of possible CPUS
+with the number of entries.
+
+ # cat /debug/tracing/buffer_size_kb
+1408 (units kilobytes)
+
+Note, to modify this, you must have tracing completely disabled.
+To do that, echo "nop" into the current_tracer. If the
+current_tracer is not set to "nop", an EINVAL error will be
+returned.
+
+ # echo nop > /debug/tracing/current_tracer
+ # echo 10000 > /debug/tracing/buffer_size_kb
+ # cat /debug/tracing/buffer_size_kb
+10000 (units kilobytes)
+
+The number of pages which will be allocated is limited to a
+percentage of available memory. Allocating too much will produce
+an error.
+
+ # echo 1000000000000 > /debug/tracing/buffer_size_kb
+-bash: echo: write error: Cannot allocate memory
+ # cat /debug/tracing/buffer_size_kb
+85
+
+-----------
+
+More details can be found in the source code, in the
+kernel/tracing/*.c files.
diff --git a/Documentation/trace/kmemtrace.txt b/Documentation/trace/kmemtrace.txt
new file mode 100644
index 000000000000..a956d9b7f943
--- /dev/null
+++ b/Documentation/trace/kmemtrace.txt
@@ -0,0 +1,126 @@
+ kmemtrace - Kernel Memory Tracer
+
+ by Eduard - Gabriel Munteanu
+ <eduard.munteanu@linux360.ro>
+
+I. Introduction
+===============
+
+kmemtrace helps kernel developers figure out two things:
+1) how different allocators (SLAB, SLUB etc.) perform
+2) how kernel code allocates memory and how much
+
+To do this, we trace every allocation and export information to the userspace
+through the relay interface. We export things such as the number of requested
+bytes, the number of bytes actually allocated (i.e. including internal
+fragmentation), whether this is a slab allocation or a plain kmalloc() and so
+on.
+
+The actual analysis is performed by a userspace tool (see section III for
+details on where to get it from). It logs the data exported by the kernel,
+processes it and (as of writing this) can provide the following information:
+- the total amount of memory allocated and fragmentation per call-site
+- the amount of memory allocated and fragmentation per allocation
+- total memory allocated and fragmentation in the collected dataset
+- number of cross-CPU allocation and frees (makes sense in NUMA environments)
+
+Moreover, it can potentially find inconsistent and erroneous behavior in
+kernel code, such as using slab free functions on kmalloc'ed memory or
+allocating less memory than requested (but not truly failed allocations).
+
+kmemtrace also makes provisions for tracing on some arch and analysing the
+data on another.
+
+II. Design and goals
+====================
+
+kmemtrace was designed to handle rather large amounts of data. Thus, it uses
+the relay interface to export whatever is logged to userspace, which then
+stores it. Analysis and reporting is done asynchronously, that is, after the
+data is collected and stored. By design, it allows one to log and analyse
+on different machines and different arches.
+
+As of writing this, the ABI is not considered stable, though it might not
+change much. However, no guarantees are made about compatibility yet. When
+deemed stable, the ABI should still allow easy extension while maintaining
+backward compatibility. This is described further in Documentation/ABI.
+
+Summary of design goals:
+ - allow logging and analysis to be done across different machines
+ - be fast and anticipate usage in high-load environments (*)
+ - be reasonably extensible
+ - make it possible for GNU/Linux distributions to have kmemtrace
+ included in their repositories
+
+(*) - one of the reasons Pekka Enberg's original userspace data analysis
+ tool's code was rewritten from Perl to C (although this is more than a
+ simple conversion)
+
+
+III. Quick usage guide
+======================
+
+1) Get a kernel that supports kmemtrace and build it accordingly (i.e. enable
+CONFIG_KMEMTRACE).
+
+2) Get the userspace tool and build it:
+$ git-clone git://repo.or.cz/kmemtrace-user.git # current repository
+$ cd kmemtrace-user/
+$ ./autogen.sh
+$ ./configure
+$ make
+
+3) Boot the kmemtrace-enabled kernel if you haven't, preferably in the
+'single' runlevel (so that relay buffers don't fill up easily), and run
+kmemtrace:
+# '$' does not mean user, but root here.
+$ mount -t debugfs none /sys/kernel/debug
+$ mount -t proc none /proc
+$ cd path/to/kmemtrace-user/
+$ ./kmemtraced
+Wait a bit, then stop it with CTRL+C.
+$ cat /sys/kernel/debug/kmemtrace/total_overruns # Check if we didn't
+ # overrun, should
+ # be zero.
+$ (Optionally) [Run kmemtrace_check separately on each cpu[0-9]*.out file to
+ check its correctness]
+$ ./kmemtrace-report
+
+Now you should have a nice and short summary of how the allocator performs.
+
+IV. FAQ and known issues
+========================
+
+Q: 'cat /sys/kernel/debug/kmemtrace/total_overruns' is non-zero, how do I fix
+this? Should I worry?
+A: If it's non-zero, this affects kmemtrace's accuracy, depending on how
+large the number is. You can fix it by supplying a higher
+'kmemtrace.subbufs=N' kernel parameter.
+---
+
+Q: kmemtrace_check reports errors, how do I fix this? Should I worry?
+A: This is a bug and should be reported. It can occur for a variety of
+reasons:
+ - possible bugs in relay code
+ - possible misuse of relay by kmemtrace
+ - timestamps being collected unorderly
+Or you may fix it yourself and send us a patch.
+---
+
+Q: kmemtrace_report shows many errors, how do I fix this? Should I worry?
+A: This is a known issue and I'm working on it. These might be true errors
+in kernel code, which may have inconsistent behavior (e.g. allocating memory
+with kmem_cache_alloc() and freeing it with kfree()). Pekka Enberg pointed
+out this behavior may work with SLAB, but may fail with other allocators.
+
+It may also be due to lack of tracing in some unusual allocator functions.
+
+We don't want bug reports regarding this issue yet.
+---
+
+V. See also
+===========
+
+Documentation/kernel-parameters.txt
+Documentation/ABI/testing/debugfs-kmemtrace
+
diff --git a/Documentation/trace/mmiotrace.txt b/Documentation/trace/mmiotrace.txt
new file mode 100644
index 000000000000..5731c67abc55
--- /dev/null
+++ b/Documentation/trace/mmiotrace.txt
@@ -0,0 +1,163 @@
+ In-kernel memory-mapped I/O tracing
+
+
+Home page and links to optional user space tools:
+
+ http://nouveau.freedesktop.org/wiki/MmioTrace
+
+MMIO tracing was originally developed by Intel around 2003 for their Fault
+Injection Test Harness. In Dec 2006 - Jan 2007, using the code from Intel,
+Jeff Muizelaar created a tool for tracing MMIO accesses with the Nouveau
+project in mind. Since then many people have contributed.
+
+Mmiotrace was built for reverse engineering any memory-mapped IO device with
+the Nouveau project as the first real user. Only x86 and x86_64 architectures
+are supported.
+
+Out-of-tree mmiotrace was originally modified for mainline inclusion and
+ftrace framework by Pekka Paalanen <pq@iki.fi>.
+
+
+Preparation
+-----------
+
+Mmiotrace feature is compiled in by the CONFIG_MMIOTRACE option. Tracing is
+disabled by default, so it is safe to have this set to yes. SMP systems are
+supported, but tracing is unreliable and may miss events if more than one CPU
+is on-line, therefore mmiotrace takes all but one CPU off-line during run-time
+activation. You can re-enable CPUs by hand, but you have been warned, there
+is no way to automatically detect if you are losing events due to CPUs racing.
+
+
+Usage Quick Reference
+---------------------
+
+$ mount -t debugfs debugfs /debug
+$ echo mmiotrace > /debug/tracing/current_tracer
+$ cat /debug/tracing/trace_pipe > mydump.txt &
+Start X or whatever.
+$ echo "X is up" > /debug/tracing/trace_marker
+$ echo nop > /debug/tracing/current_tracer
+Check for lost events.
+
+
+Usage
+-----
+
+Make sure debugfs is mounted to /debug. If not, (requires root privileges)
+$ mount -t debugfs debugfs /debug
+
+Check that the driver you are about to trace is not loaded.
+
+Activate mmiotrace (requires root privileges):
+$ echo mmiotrace > /debug/tracing/current_tracer
+
+Start storing the trace:
+$ cat /debug/tracing/trace_pipe > mydump.txt &
+The 'cat' process should stay running (sleeping) in the background.
+
+Load the driver you want to trace and use it. Mmiotrace will only catch MMIO
+accesses to areas that are ioremapped while mmiotrace is active.
+
+During tracing you can place comments (markers) into the trace by
+$ echo "X is up" > /debug/tracing/trace_marker
+This makes it easier to see which part of the (huge) trace corresponds to
+which action. It is recommended to place descriptive markers about what you
+do.
+
+Shut down mmiotrace (requires root privileges):
+$ echo nop > /debug/tracing/current_tracer
+The 'cat' process exits. If it does not, kill it by issuing 'fg' command and
+pressing ctrl+c.
+
+Check that mmiotrace did not lose events due to a buffer filling up. Either
+$ grep -i lost mydump.txt
+which tells you exactly how many events were lost, or use
+$ dmesg
+to view your kernel log and look for "mmiotrace has lost events" warning. If
+events were lost, the trace is incomplete. You should enlarge the buffers and
+try again. Buffers are enlarged by first seeing how large the current buffers
+are:
+$ cat /debug/tracing/buffer_size_kb
+gives you a number. Approximately double this number and write it back, for
+instance:
+$ echo 128000 > /debug/tracing/buffer_size_kb
+Then start again from the top.
+
+If you are doing a trace for a driver project, e.g. Nouveau, you should also
+do the following before sending your results:
+$ lspci -vvv > lspci.txt
+$ dmesg > dmesg.txt
+$ tar zcf pciid-nick-mmiotrace.tar.gz mydump.txt lspci.txt dmesg.txt
+and then send the .tar.gz file. The trace compresses considerably. Replace
+"pciid" and "nick" with the PCI ID or model name of your piece of hardware
+under investigation and your nick name.
+
+
+How Mmiotrace Works
+-------------------
+
+Access to hardware IO-memory is gained by mapping addresses from PCI bus by
+calling one of the ioremap_*() functions. Mmiotrace is hooked into the
+__ioremap() function and gets called whenever a mapping is created. Mapping is
+an event that is recorded into the trace log. Note, that ISA range mappings
+are not caught, since the mapping always exists and is returned directly.
+
+MMIO accesses are recorded via page faults. Just before __ioremap() returns,
+the mapped pages are marked as not present. Any access to the pages causes a
+fault. The page fault handler calls mmiotrace to handle the fault. Mmiotrace
+marks the page present, sets TF flag to achieve single stepping and exits the
+fault handler. The instruction that faulted is executed and debug trap is
+entered. Here mmiotrace again marks the page as not present. The instruction
+is decoded to get the type of operation (read/write), data width and the value
+read or written. These are stored to the trace log.
+
+Setting the page present in the page fault handler has a race condition on SMP
+machines. During the single stepping other CPUs may run freely on that page
+and events can be missed without a notice. Re-enabling other CPUs during
+tracing is discouraged.
+
+
+Trace Log Format
+----------------
+
+The raw log is text and easily filtered with e.g. grep and awk. One record is
+one line in the log. A record starts with a keyword, followed by keyword
+dependant arguments. Arguments are separated by a space, or continue until the
+end of line. The format for version 20070824 is as follows:
+
+Explanation Keyword Space separated arguments
+---------------------------------------------------------------------------
+
+read event R width, timestamp, map id, physical, value, PC, PID
+write event W width, timestamp, map id, physical, value, PC, PID
+ioremap event MAP timestamp, map id, physical, virtual, length, PC, PID
+iounmap event UNMAP timestamp, map id, PC, PID
+marker MARK timestamp, text
+version VERSION the string "20070824"
+info for reader LSPCI one line from lspci -v
+PCI address map PCIDEV space separated /proc/bus/pci/devices data
+unk. opcode UNKNOWN timestamp, map id, physical, data, PC, PID
+
+Timestamp is in seconds with decimals. Physical is a PCI bus address, virtual
+is a kernel virtual address. Width is the data width in bytes and value is the
+data value. Map id is an arbitrary id number identifying the mapping that was
+used in an operation. PC is the program counter and PID is process id. PC is
+zero if it is not recorded. PID is always zero as tracing MMIO accesses
+originating in user space memory is not yet supported.
+
+For instance, the following awk filter will pass all 32-bit writes that target
+physical addresses in the range [0xfb73ce40, 0xfb800000[
+
+$ awk '/W 4 / { adr=strtonum($5); if (adr >= 0xfb73ce40 &&
+adr < 0xfb800000) print; }'
+
+
+Tools for Developers
+--------------------
+
+The user space tools include utilities for:
+- replacing numeric addresses and values with hardware register names
+- replaying MMIO logs, i.e., re-executing the recorded writes
+
+
diff --git a/Documentation/trace/tracepoints.txt b/Documentation/trace/tracepoints.txt
new file mode 100644
index 000000000000..c0e1ceed75a4
--- /dev/null
+++ b/Documentation/trace/tracepoints.txt
@@ -0,0 +1,116 @@
+ Using the Linux Kernel Tracepoints
+
+ Mathieu Desnoyers
+
+
+This document introduces Linux Kernel Tracepoints and their use. It
+provides examples of how to insert tracepoints in the kernel and
+connect probe functions to them and provides some examples of probe
+functions.
+
+
+* Purpose of tracepoints
+
+A tracepoint placed in code provides a hook to call a function (probe)
+that you can provide at runtime. A tracepoint can be "on" (a probe is
+connected to it) or "off" (no probe is attached). When a tracepoint is
+"off" it has no effect, except for adding a tiny time penalty
+(checking a condition for a branch) and space penalty (adding a few
+bytes for the function call at the end of the instrumented function
+and adds a data structure in a separate section). When a tracepoint
+is "on", the function you provide is called each time the tracepoint
+is executed, in the execution context of the caller. When the function
+provided ends its execution, it returns to the caller (continuing from
+the tracepoint site).
+
+You can put tracepoints at important locations in the code. They are
+lightweight hooks that can pass an arbitrary number of parameters,
+which prototypes are described in a tracepoint declaration placed in a
+header file.
+
+They can be used for tracing and performance accounting.
+
+
+* Usage
+
+Two elements are required for tracepoints :
+
+- A tracepoint definition, placed in a header file.
+- The tracepoint statement, in C code.
+
+In order to use tracepoints, you should include linux/tracepoint.h.
+
+In include/trace/subsys.h :
+
+#include <linux/tracepoint.h>
+
+DECLARE_TRACE(subsys_eventname,
+ TP_PROTO(int firstarg, struct task_struct *p),
+ TP_ARGS(firstarg, p));
+
+In subsys/file.c (where the tracing statement must be added) :
+
+#include <trace/subsys.h>
+
+DEFINE_TRACE(subsys_eventname);
+
+void somefct(void)
+{
+ ...
+ trace_subsys_eventname(arg, task);
+ ...
+}
+
+Where :
+- subsys_eventname is an identifier unique to your event
+ - subsys is the name of your subsystem.
+ - eventname is the name of the event to trace.
+
+- TP_PROTO(int firstarg, struct task_struct *p) is the prototype of the
+ function called by this tracepoint.
+
+- TP_ARGS(firstarg, p) are the parameters names, same as found in the
+ prototype.
+
+Connecting a function (probe) to a tracepoint is done by providing a
+probe (function to call) for the specific tracepoint through
+register_trace_subsys_eventname(). Removing a probe is done through
+unregister_trace_subsys_eventname(); it will remove the probe.
+
+tracepoint_synchronize_unregister() must be called before the end of
+the module exit function to make sure there is no caller left using
+the probe. This, and the fact that preemption is disabled around the
+probe call, make sure that probe removal and module unload are safe.
+See the "Probe example" section below for a sample probe module.
+
+The tracepoint mechanism supports inserting multiple instances of the
+same tracepoint, but a single definition must be made of a given
+tracepoint name over all the kernel to make sure no type conflict will
+occur. Name mangling of the tracepoints is done using the prototypes
+to make sure typing is correct. Verification of probe type correctness
+is done at the registration site by the compiler. Tracepoints can be
+put in inline functions, inlined static functions, and unrolled loops
+as well as regular functions.
+
+The naming scheme "subsys_event" is suggested here as a convention
+intended to limit collisions. Tracepoint names are global to the
+kernel: they are considered as being the same whether they are in the
+core kernel image or in modules.
+
+If the tracepoint has to be used in kernel modules, an
+EXPORT_TRACEPOINT_SYMBOL_GPL() or EXPORT_TRACEPOINT_SYMBOL() can be
+used to export the defined tracepoints.
+
+* Probe / tracepoint example
+
+See the example provided in samples/tracepoints
+
+Compile them with your kernel. They are built during 'make' (not
+'make modules') when CONFIG_SAMPLE_TRACEPOINTS=m.
+
+Run, as root :
+modprobe tracepoint-sample (insmod order is not important)
+modprobe tracepoint-probe-sample
+cat /proc/tracepoint-sample (returns an expected error)
+rmmod tracepoint-sample tracepoint-probe-sample
+dmesg
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