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Commit 5752674e authored by Ingo Molnar's avatar Ingo Molnar
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Documentation/ftrace.txt: update 

- fix typos/grammos and clarify the text
- prettify the document some more

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 985ec20a
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Documentation/ftrace.txt
View file @ 5752674e
......@@ -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)
......@@ -50,26 +50,38 @@ 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
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.
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:
tracing_enabled: This sets or displays whether the current_tracer
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).
trace:
This file holds the output of the trace in a human
readable format (described below).
latency_trace:
latency_trace: This file shows the same trace but the information
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
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"
......@@ -82,11 +94,15 @@ of ftrace. Here is a list of some of the key files:
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
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.
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
......@@ -94,7 +110,9 @@ of ftrace. Here is a list of some of the key files:
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_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
......@@ -103,17 +121,21 @@ of ftrace. Here is a list of some of the key files:
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.)
( 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
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
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
......@@ -122,18 +144,26 @@ of ftrace. Here is a list of some of the key files:
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_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_ftrace_pid:
Have the function tracer only trace a single thread.
set_graph_function: Select the function where the trace have to start
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
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"
......@@ -145,44 +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"
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 like a primitive C code source.
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
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
"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
"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
"wakeup"
Traces and records the max latency that it takes for
the highest priority task to get scheduled after
it has been woken up.
nop - This is not a tracer. To remove all tracers from tracing
simply echo "nop" into current_tracer.
"hw-branch-tracer"
hw-branch-tracer - traces branches on all cpu's in a circular buffer.
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:
--------------
......@@ -199,16 +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
......@@ -217,8 +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:
......@@ -232,19 +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
#
......@@ -271,20 +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.
......@@ -333,14 +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
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
......@@ -350,8 +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
......@@ -360,15 +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>
......@@ -378,35 +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
......@@ -443,18 +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
......@@ -475,10 +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
......@@ -486,12 +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
......@@ -499,17 +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
......@@ -544,10 +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:
......@@ -598,24 +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
......@@ -649,11 +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
#
......@@ -712,28 +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:
......@@ -753,11 +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
......@@ -793,9 +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:
......@@ -883,40 +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
......@@ -946,17 +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.
......@@ -1013,24 +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
......@@ -1060,14 +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;
[...]
......@@ -1082,10 +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
......@@ -1186,10 +1259,11 @@ following format:
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.
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
......@@ -1198,8 +1272,8 @@ 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:
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]
......@@ -1239,25 +1313,34 @@ kernel module:
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 setting a dynamically allocated stack of return addresses on each
task_struct. Then 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.
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 extremities of a function leads to special features such as
Probing on both ends of a function leads to special features
such as:
_ measure of function's time execution
_ having a reliable call stack to draw function calls graph
- 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 see what happens inside your kernel
- 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
#
......@@ -1282,24 +1365,28 @@ _ you just want to see what happens inside your kernel
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.
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.
- 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.
- 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 one in case of reached duration thresholds.
- 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
......@@ -1328,8 +1415,8 @@ _ The overhead field precedes the duration one in case of reached duration thres
! 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.
- 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
......@@ -1351,8 +1438,9 @@ _ The task/pid field displays the thread cmdline and pid which executed the func
0) sh-4802 | + 49.370 us | }
_ The absolute time field is an absolute timestamp given by the clock since
it started. A snapshot of this time is given on each entry/exit of functions
- 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
......@@ -1377,9 +1465,10 @@ _ The absolute time field is an absolute timestamp given by the clock since
360.774530 | 1) 0.594 us | __phys_addr();
You can put some comments on specific functions by using ftrace_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 ftrace_printk() inside __might_sleep()
You can put some comments on specific functions by using
ftrace_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 ftrace_printk() inside __might_sleep()
ftrace_printk("I'm a comment!\n")
......@@ -1390,8 +1479,9 @@ will produce:
1) 1.449 us | }
You might find other useful features for this tracer on the "dynamic ftrace"
section such as tracing only specific functions or tasks.
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
--------------
......@@ -1399,43 +1489,45 @@ 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
......@@ -1443,8 +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
......@@ -1481,8 +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>
......@@ -1492,9 +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
......@@ -1540,7 +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
......@@ -1572,7 +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
......@@ -1595,18 +1689,20 @@ Produces:
We can see that there's no more lock or preempt tracing.
* Dynamic ftrace with the function graph tracer *
Dynamic ftrace with the function graph tracer
---------------------------------------------
Although what has been explained above concerns both the function tracer and
the function_graph_tracer, the following concerns only the latter.
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 childs, you just have
to echo its name on set_graph_function:
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
echo __do_fault > set_graph_function
will produce the following:
will produce the following "expanded" trace of the __do_fault()
function:
0) | __do_fault() {
0) | filemap_fault() {
......@@ -1643,23 +1739,24 @@ will produce the following:
0) 2.793 us | }
0) + 14.012 us | }
You can also select several functions:
You can also expand several functions at once:
echo sys_open > set_graph_function
echo sys_close >> set_graph_function
echo sys_open > set_graph_function
echo sys_close >> set_graph_function
Now if you want to go back to trace all functions
Now if you want to go back to trace all functions you can clear
this special filter via:
echo > set_graph_function
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 &
......@@ -1687,38 +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.
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