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| -rw-r--r-- | Documentation/trace/ftrace.txt | 83 |
1 files changed, 83 insertions, 0 deletions
diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt index 6f51fed45f2d..53d6a3c51d87 100644 --- a/Documentation/trace/ftrace.txt +++ b/Documentation/trace/ftrace.txt @@ -1842,6 +1842,89 @@ 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 current 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 new +current (=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 (swap), then read + the snapshot from this file in the same format as + "trace" (described above in the section "The File + System"). Both reads snapshot and tracing are executable + in parallel. When the spare buffer is allocated, echoing + 0 frees it, and echoing else (positive) values clear the + snapshot contents. + More details are shown in the table below. + + status\input | 0 | 1 | else | + --------------+------------+------------+------------+ + not allocated |(do nothing)| alloc+swap | EINVAL | + --------------+------------+------------+------------+ + allocated | free | swap | clear | + --------------+------------+------------+------------+ + +Here is an example of using the snapshot feature. + + # 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 + # 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 |