From 985ec20ad531f2641ab9d5193e37891fe959fc7d Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Wed, 18 Feb 2009 06:35:34 +0100 Subject: [PATCH] tracing/function-graph-tracer: provide documentation for the function graph tracer Update documentation for the function graph tracer. Signed-off-by: Frederic Weisbecker Cc: Arnaldo Carvalho de Melo Cc: Steven Rostedt Signed-off-by: Ingo Molnar --- Documentation/ftrace.txt | 226 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 226 insertions(+) diff --git a/Documentation/ftrace.txt b/Documentation/ftrace.txt index 758fb42a1b6..055bcd2992d 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 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 ---------- -- 2.41.1