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-rw-r--r--Documentation/ftrace.txt1134
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diff --git a/Documentation/ftrace.txt b/Documentation/ftrace.txt
index 803b1318b13d..fd9a3e693813 100644
--- a/Documentation/ftrace.txt
+++ b/Documentation/ftrace.txt
@@ -15,31 +15,31 @@ 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.
+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).
+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.
+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)
+( 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)
@@ -50,90 +50,124 @@ 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.
-
- trace_max_latency: Some of the tracers record the max latency.
- For example, the time interrupts are disabled.
- This time is saved in this file. The max trace
- will also be stored, and displayed by either
- "trace" or "latency_trace". A new max trace will
- only be recorded if the latency is greater than
- the value in this file. (in microseconds)
-
- 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.
-
- 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.)
+ 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
@@ -141,36 +175,66 @@ The Tracers
Here is the list of current tracers that may be configured.
- function - function tracer that uses mcount to trace all functions.
+ "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 between tasks.
+ "sched_switch"
- 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.
+ Traces the context switches and wakeups between tasks.
- preemptoff - Similar to irqsoff but traces and records the amount of
- time for which preemption is disabled.
+ "irqsoff"
- preemptirqsoff - Similar to irqsoff and preemptoff, but traces and
- records the largest time for which irqs and/or preemption
- is disabled.
+ 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.
- wakeup - Traces and records the max latency that it takes for
- the highest priority task to get scheduled after
- it has been woken up.
+ "preemptoff"
- nop - This is not a tracer. To remove all tracers from tracing
- simply echo "nop" into current_tracer.
+ 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).
+Here are typical examples of using the tracers when controlling
+them only with the debugfs interface (without using any
+user-land utilities).
Output format:
--------------
@@ -187,16 +251,16 @@ Here is an example of the output format of the file "trace"
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.
+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.
+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
@@ -205,8 +269,8 @@ context switches.
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:
+Wake ups are represented by a "+" and the context switches are
+shown as "==>". The format is:
Context switches:
@@ -220,19 +284,20 @@ Wake ups are represented by a "+" and the context switches are shown as
<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).
+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.
+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
#
@@ -259,20 +324,20 @@ irqsoff latency trace v1.1.5 on 2.6.26-rc8
<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).
-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 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:
+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.
@@ -308,12 +373,12 @@ The above is mostly meaningful for kernel developers.
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.
+ 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.
@@ -321,14 +386,15 @@ The above is mostly meaningful for kernel developers.
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:
+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
+ noblock nostacktrace nosched-tree nouserstacktrace nosym-userobj
-To disable one of the options, echo in the option prepended with "no".
+To disable one of the options, echo in the option prepended with
+"no".
echo noprint-parent > /debug/tracing/trace_options
@@ -338,8 +404,8 @@ To enable an option, leave off the "no".
Here are the available options:
- print-parent - On function traces, display the calling function
- as well as the function being traced.
+ 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
@@ -348,15 +414,16 @@ Here are the available options:
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 - 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 - this will also display the function address as well
+ as the function name.
sym-addr:
bash-4000 [01] 1477.606694: simple_strtoul <c0339346>
@@ -366,35 +433,41 @@ Here are the available options:
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.
+ 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.
+ 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.
+ 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.
+ 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
+ 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:
+ 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 - TBD (any users??)
+ 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
@@ -431,18 +504,19 @@ of how to use it.
[...]
-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.
+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.
+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
@@ -463,10 +537,10 @@ The task states are:
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.
+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
@@ -474,12 +548,12 @@ file system interface.
echo 1 > /proc/sys/kernel/ftrace_enabled
-To disable ftrace_enabled simply replace the '1' with '0' in
-the above commands.
+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.
+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
@@ -487,17 +561,18 @@ 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.
+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.
+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:
+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
@@ -532,10 +607,11 @@ irqsoff latency trace v1.1.5 on 2.6.26
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.
+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:
@@ -586,24 +662,24 @@ irqsoff latency trace v1.1.5 on 2.6.26-rc8
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.
+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.
+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.
+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
@@ -637,11 +713,12 @@ preemptoff latency trace v1.1.5 on 2.6.26-rc8
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.
+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
#
@@ -700,28 +777,30 @@ preemptoff latency trace v1.1.5 on 2.6.26-rc8
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.
+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.
+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.
+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:
@@ -741,11 +820,13 @@ 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.
+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.
+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
@@ -781,9 +862,10 @@ preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
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.
+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:
@@ -871,40 +953,42 @@ preemptirqsoff latency trace v1.1.5 on 2.6.26-rc8
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'.
+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.
+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.
+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
@@ -934,17 +1018,16 @@ wakeup latency trace v1.1.5 on 2.6.26-rc8
<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.
-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.
+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.
@@ -1001,24 +1084,25 @@ 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
+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.
+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
@@ -1048,14 +1132,15 @@ set; otherwise this tracer is a nop.
[...]
-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:
+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;
[...]
@@ -1070,10 +1155,10 @@ int main(int argc, char *argv[]) {
}
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.
+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
@@ -1152,49 +1237,297 @@ int main (int argc, char **argv)
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.)
+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).
+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.
+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.
+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.
+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:
+Two files are used, one for enabling and one for disabling the
+tracing of specified functions. They are:
set_ftrace_filter
@@ -1202,8 +1535,8 @@ and
set_ftrace_notrace
-A list of available functions that you can add to these files is listed
-in:
+A list of available functions that you can add to these files is
+listed in:
available_filter_functions
@@ -1240,8 +1573,8 @@ hrtimer_interrupt
sys_nanosleep
-Perhaps this is not enough. The filters also allow simple wild cards.
-Only the following are currently available
+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>
@@ -1251,9 +1584,9 @@ 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.
+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
@@ -1299,7 +1632,8 @@ 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:
+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
@@ -1331,7 +1665,8 @@ hrtimer_get_res
hrtimer_init_sleeper
-The set_ftrace_notrace prevents those functions from being traced.
+The set_ftrace_notrace prevents those functions from being
+traced.
# echo '*preempt*' '*lock*' > /debug/tracing/set_ftrace_notrace
@@ -1353,13 +1688,75 @@ Produces:
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.
+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 &
@@ -1387,38 +1784,45 @@ is live.
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.
+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.
+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.
+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.
+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.