This patch adds snapshot description in ftrace documentation. This description includes what the snapshot is and how to use it.
Signed-off-by: Hiraku Toyooka <hiraku.toyooka...@hitachi.com> Cc: Steven Rostedt <rost...@goodmis.org> Cc: Frederic Weisbecker <fweis...@gmail.com> Cc: Ingo Molnar <mi...@redhat.com> Cc: Rob Landley <r...@landley.net> Cc: linux-...@vger.kernel.org Cc: linux-kernel@vger.kernel.org --- Documentation/trace/ftrace.txt | 97 ++++++++++++++++++++++++++++++++++++++++ 1 files changed, 97 insertions(+), 0 deletions(-) diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt index 6f51fed..68ac294 100644 --- a/Documentation/trace/ftrace.txt +++ b/Documentation/trace/ftrace.txt @@ -1842,6 +1842,103 @@ an error. # cat buffer_size_kb 85 +Snapshot +-------- +CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature +available to all non latency tracers. (Latency tracers which +record max latency, such as "irqsoff" or "wakeup", can't use +this feature, since those are already using the snapshot +mechanism internally.) + +Snapshot preserves a trace buffer at a particular point in time +without stopping tracing. Ftrace swaps the current buffer with a +spare buffer, and tracing continues in the (previous) spare +buffer. + +The following debugfs files in "tracing" are related to this +feature: + + snapshot: + + This is used to take a snapshot and to read the output + of the snapshot. Echo 1 into this file to allocate a + spare buffer and to take a snapshot, then read the + snapshot from the file in the same format as "trace" + (described above in the section "The File System"). Both + reads snapshot and tracing are executable in parallel. + Echoing 0 erases the snapshot contents. + + snapshot_allocate: + + This is used to pre-allocate or free a spare buffer. + Echo 1 into this file to pre-allocate a spare buffer if + you don't want to fail in the next snapshot due to + memory allocation failure, or if you don't want to lose + older trace data while allocating buffer. Echo 0 to free + the spare buffer when the snapshot becomes unnecessary. + If you take the next snapshot again, you can reuse the + buffer, then just erase the snapshot contents by echoing + 1 into the "snapshot" file, instead of freeing the + buffer. + + Reads from this file display whether the spare buffer is + allocated. When current_tracer is changed, the allocated + spare buffer is freed. If the next tracer is one of the + latency tracers, this value turns into 1 and can't be + changed, or else the value starts with 0. + + +Here is an example of using the snapshot feature. + + # echo 1 > snapshot_allocate (if you want to pre-allocate the spare buffer) + # echo 1 > events/sched/enable + # echo 1 > snapshot + # cat snapshot +# tracer: nop +# +# entries-in-buffer/entries-written: 71/71 #P:8 +# +# _-----=> irqs-off +# / _----=> need-resched +# | / _---=> hardirq/softirq +# || / _--=> preempt-depth +# ||| / delay +# TASK-PID CPU# |||| TIMESTAMP FUNCTION +# | | | |||| | | + <idle>-0 [005] d... 2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120 + sleep-2242 [005] d... 2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120 prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120 +[...] + <idle>-0 [002] d... 2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120 + # cat trace +# tracer: nop +# +# entries-in-buffer/entries-written: 77/77 #P:8 +# +# _-----=> irqs-off +# / _----=> need-resched +# | / _---=> hardirq/softirq +# || / _--=> preempt-depth +# ||| / delay +# TASK-PID CPU# |||| TIMESTAMP FUNCTION +# | | | |||| | | + <idle>-0 [007] d... 2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120 + snapshot-test-2-2229 [002] d... 2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120 +[...] + + +If you try to use this snapshot feature when current tracer is +one of the latency tracers, you will get the following results. + + # echo wakeup > current_tracer + # cat snapshot_allocate +1 + # echo 1 > snapshot_allocate +bash: echo: write error: Device or resource busy + # echo 1 > snapshot +bash: echo: write error: Device or resource busy + # cat snapshot +cat: snapshot: Device or resource busy + ----------- More details can be found in the source code, in the -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
