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ftrace: update txt document

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Impact: Documentation update only

A lot of changes have gone into ftrace. This patch updates
the ftrace.txt document.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Steven Rostedt 2008-11-03 15:15:08 -05:00 committed by Ingo Molnar
parent 457d2ee225
commit 9b803c0fc3
1 changed files with 71 additions and 96 deletions
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167
Documentation/ftrace.txt
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@ -50,26 +50,26 @@ 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
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
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
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
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
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 +82,11 @@ 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.
iter_ctrl : This file lets the user control the amount of data
iter_ctrl: This file lets the user control the amount of data
that is displayed in one of the above output
files.
trace_max_latency : Some of the tracers record the max latency.
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
@ -94,29 +94,26 @@ 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)
trace_entries : This sets or displays the number of trace
entries each CPU buffer can hold. The tracer buffers
are the same size for each CPU. The displayed number
is the size of the CPU buffer and not total size. The
trace_entries: This sets or displays the number of bytes 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).
Since each entry is smaller than a page, if the last
allocated page has room for more entries than were
requested, the rest of the page is used to allocate
entries.
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 "none".
is set to "nop".
NOTE: It is planned on changing the allocated buffers
from being the number of possible CPUS to
the number of online CPUS.
tracing_cpumask : This is a mask that lets the user only trace
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
@ -130,14 +127,11 @@ of ftrace. Here is a list of some of the key files:
be traced. If a function exists in both set_ftrace_filter
and set_ftrace_notrace, the function will _not_ be traced.
available_filter_functions : When a function is encountered the first
time by the dynamic tracer, it is recorded and
later the call is converted into a nop. This file
lists the functions that have been recorded
by the dynamic tracer and these functions can
be used to set the ftrace filter by the above
"set_ftrace_filter" file. (See the section "dynamic ftrace"
below 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
@ -145,7 +139,7 @@ The Tracers
Here is the list of current tracers that may be configured.
ftrace - function tracer that uses mcount to trace all functions.
function - function tracer that uses mcount to trace all functions.
sched_switch - traces the context switches between tasks.
@ -166,8 +160,8 @@ Here is the list of current tracers that may be configured.
the highest priority task to get scheduled after
it has been woken up.
none - This is not a tracer. To remove all tracers from tracing
simply echo "none" into current_tracer.
nop - This is not a tracer. To remove all tracers from tracing
simply echo "nop" into current_tracer.
Examples of using the tracer
@ -182,7 +176,7 @@ Output format:
Here is an example of the output format of the file "trace"
--------
# tracer: ftrace
# tracer: function
#
# TASK-PID CPU# TIMESTAMP FUNCTION
# | | | | |
@ -192,7 +186,7 @@ Here is an example of the output format of the file "trace"
--------
A header is printed with the tracer name that is represented by the trace.
In this case the tracer is "ftrace". Then a header showing the format. Task
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
@ -1003,22 +997,20 @@ 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.
ftrace
------
function
--------
ftrace is not only the name of the tracing infrastructure, but it
is also a name of one of the tracers. The tracer is the function
tracer. Enabling the function tracer can be done from the
debug file system. Make sure the ftrace_enabled is set otherwise
this tracer is a nop.
This tracer is the function tracer. Enabling the function tracer
can be done from the debug file system. Make sure the ftrace_enabled is
set; otherwise this tracer is a nop.
# sysctl kernel.ftrace_enabled=1
# echo ftrace > /debug/tracing/current_tracer
# echo function > /debug/tracing/current_tracer
# echo 1 > /debug/tracing/tracing_enabled
# usleep 1
# echo 0 > /debug/tracing/tracing_enabled
# cat /debug/tracing/trace
# tracer: ftrace
# tracer: function
#
# TASK-PID CPU# TIMESTAMP FUNCTION
# | | | | |
@ -1040,10 +1032,10 @@ this tracer is a nop.
[...]
Note: ftrace 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
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
@ -1077,18 +1069,31 @@ 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.)
When dynamic ftrace is initialized, it calls kstop_machine to make
the machine act like a uniprocessor so that it can freely modify code
without worrying about other processors executing that same code. At
initialization, the mcount calls are changed to call a "record_ip"
function. After this, the first time a kernel function is called,
it has the calling address saved in a hash table.
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).
Later on the ftraced kernel thread is awoken and will again call
kstop_machine if new functions have been recorded. The ftraced thread
will change all calls to mcount to "nop". Just calling mcount
and having mcount return has shown a 10% overhead. By converting
it to a nop, there is no measurable overhead to the system.
A new section called "__mcount_loc" is created that holds references
to all the mcount call sites in the .text section. This section is
compiled back into the original object. The final linker will add
all these references into a single table.
On boot up, before SMP is initialized, the dynamic ftrace code
scans this table and updates all the locations into nops. It also
records the locations, which are added to the available_filter_functions
list. Modules are processed as they are loaded and before they are
executed. When a module is unloaded, it also removes its functions from
the ftrace function list. This is automatic in the module unload
code, and the module author does not need to worry about it.
When tracing is enabled, kstop_machine is called to prevent races
with the CPUS executing code being modified (which can cause the
CPU to do undesireable things), and the nops are patched back
to calls. But this time, they do not call mcount (which is just
a function stub). They now call into the ftrace infrastructure.
One special side-effect to the recording of the functions being
traced is that we can now selectively choose which functions we
@ -1251,36 +1256,6 @@ Produces:
We can see that there's no more lock or preempt tracing.
ftraced
-------
As mentioned above, when dynamic ftrace is configured in, a kernel
thread wakes up once a second and checks to see if there are mcount
calls that need to be converted into nops. If there are not any, then
it simply goes back to sleep. But if there are some, it will call
kstop_machine to convert the calls to nops.
There may be a case in which you do not want this added latency.
Perhaps you are doing some audio recording and this activity might
cause skips in the playback. There is an interface to disable
and enable the "ftraced" kernel thread.
# echo 0 > /debug/tracing/ftraced_enabled
This will disable the calling of kstop_machine to update the
mcount calls to nops. Remember that there is a large overhead
to calling mcount. Without this kernel thread, that overhead will
exist.
If there are recorded calls to mcount, any write to the ftraced_enabled
file will cause the kstop_machine to run. This means that a
user can manually perform the updates when they want to by simply
echoing a '0' into the ftraced_enabled file.
The updates are also done at the beginning of enabling a tracer
that uses ftrace function recording.
trace_pipe
----------
@ -1289,14 +1264,14 @@ on the tracing is different. Every read from trace_pipe is consumed.
This means that subsequent reads will be different. The trace
is live.
# echo ftrace > /debug/tracing/current_tracer
# echo function > /debug/tracing/current_tracer
# cat /debug/tracing/trace_pipe > /tmp/trace.out &
[1] 4153
# echo 1 > /debug/tracing/tracing_enabled
# usleep 1
# echo 0 > /debug/tracing/tracing_enabled
# cat /debug/tracing/trace
# tracer: ftrace
# tracer: function
#
# TASK-PID CPU# TIMESTAMP FUNCTION
# | | | | |
@ -1317,7 +1292,7 @@ is live.
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 ftrace tracer _before_ cating the trace_pipe file.
to set the function tracer _before_ we "cat" the trace_pipe file.
trace entries
@ -1334,10 +1309,10 @@ number of entries.
65620
Note, to modify this, you must have tracing completely disabled. To do that,
echo "none" into the current_tracer. If the current_tracer is not set
to "none", an EINVAL error will be returned.
echo "nop" into the current_tracer. If the current_tracer is not set
to "nop", an EINVAL error will be returned.
# echo none > /debug/tracing/current_tracer
# echo nop > /debug/tracing/current_tracer
# echo 100000 > /debug/tracing/trace_entries
# cat /debug/tracing/trace_entries
100045