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Message-ID: <499ba227.077e420a.56d6.ffffc38b@mx.google.com>
Date: Wed, 18 Feb 2009 06:35:34 +0100
From: Frederic Weisbecker <fweisbec@...il.com>
To: Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>,
linux-kernel@...r.kernel.org
Subject: [RFC][PATCH 2/2] tracing/function-graph-tracer: provide documentation for the function graph tracer
This patch provides the documentation to use the function graph tracer on the ftrace.txt
file.
I know how evil is my english, so don't hesitate to throw some tomatoes and I will fix
the wicked sentences :-)
Note: I guess it could also find its way on 2.6.29, except that it describes the absolute timestamp
field which is not present in the 2.6.29 version. May be I should split it up in two parts?
There are also some pending things such as explanations for set_ftrace_pid, hardirq signals and
context switches.
I would also like to put a link to http://people.redhat.com/mingo/tip.git/tracing-quickstart.txt
from which I picked some ideas and which provides a more quick start and a more practical use of this tracer,
but I don't know if you would accept an external url.
Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
---
Documentation/ftrace.txt | 226 ++++++++++++++++++++++++++++++++++++++++++++++
1 files changed, 226 insertions(+), 0 deletions(-)
diff --git a/Documentation/ftrace.txt b/Documentation/ftrace.txt
index 758fb42..055bcd2 100644
--- a/Documentation/ftrace.txt
+++ b/Documentation/ftrace.txt
@@ -129,6 +129,10 @@ of ftrace. Here is a list of some of the key files:
set_ftrace_pid: Have the function tracer only trace a single thread.
+ set_graph_function: Select the function where the trace have to 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
@@ -143,6 +147,12 @@ Here is the list of current tracers that may be configured.
function - function tracer that uses mcount to trace all 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 like a primitive C code source.
+
sched_switch - traces the context switches between tasks.
irqsoff - traces the areas that disable interrupts and saves
@@ -1226,6 +1236,163 @@ 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.
+
+Probing on both extremities 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
+
+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
+
+# 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 one 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 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 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")
+
+will produce:
+
+ 1) | __might_sleep() {
+ 1) | /* I'm a comment! */
+ 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.
+
dynamic ftrace
--------------
@@ -1427,6 +1594,65 @@ 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, the following concerns only the latter.
+
+If you want to trace only one function and all of its childs, you just have
+to echo its name on set_graph_function:
+
+echo __do_fault > set_graph_function
+
+will produce the following:
+
+ 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 select several functions:
+
+echo sys_open > set_graph_function
+echo sys_close >> set_graph_function
+
+Now if you want to go back to trace all functions
+
+echo > set_graph_function
+
+
trace_pipe
----------
--
1.6.1
--
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