On Sun, 15 Dec 2019 23:27:56 -0500
"Frank A. Cancio Bello" <frank@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> Clarify in a few places where the ring buffer and the "snapshot"
> buffer are cleared as a side effect of an operation.
>
> This will avoid users lost of tracing data because of these so far
> undocumented behavior.
>
> Signed-off-by: Frank A. Cancio Bello <frank@xxxxxxxxxxxxxxxxxxxxxx>
> ---
> Documentation/trace/ftrace.rst | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
> index d2b5657ed33e..5cc65314e16d 100644
> --- a/Documentation/trace/ftrace.rst
> +++ b/Documentation/trace/ftrace.rst
> @@ -95,7 +95,8 @@ of ftrace. Here is a list of some of the key files:
> current_tracer:
>
> This is used to set or display the current tracer
> - that is configured.
> + that is configured. Changing the current tracer clears
> + the ring buffer content as well as the "snapshot" buffer.
>
> available_tracers:
>
> @@ -126,7 +127,9 @@ of ftrace. Here is a list of some of the key files:
> This file holds the output of the trace in a human
> readable format (described below). Note, tracing is temporarily
> disabled when the file is open for reading. Once all readers
> - are closed, tracing is re-enabled.
> + are closed, tracing is re-enabled. Opening this file for
> + writing with the O_TRUNC flag clears the ring buffer content
> + as well as the "snapshot" buffer.
Writing O_TRUNC into trace does not touch the snapshot buffer. I just
tested to make sure (because that's not the behavior I remember making):
# cd /sys/kernel/tracing
# echo 1 > events/sched/enable
# echo 1 > snapshot
# head -15 snapshot
# tracer: nop
#
# entries-in-buffer/entries-written: 3563/3563 #P:8
#
# _-----=> irqs-off
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth
# ||| / delay
# TASK-PID CPU# |||| TIMESTAMP FUNCTION
# | | | |||| | |
bash-20130 [007] d.h. 243924.135391: sched_stat_runtime: comm=bash pid=20130 runtime=584815 [ns] vruntime=116509213 [ns]
bash-20130 [007] d... 243924.135853: sched_waking: comm=kworker/u16:2 pid=20112 prio=120 target_cpu=002
bash-20130 [007] d... 243924.135859: sched_wake_idle_without_ipi: cpu=2
bash-20130 [007] d... 243924.135861: sched_wakeup: comm=kworker/u16:2 pid=20112 prio=120 target_cpu=002
# echo 0 > tracing_on
# head -15 trace
# tracer: nop
#
# entries-in-buffer/entries-written: 1697/1697 #P:8
#
# _-----=> irqs-off
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth
# ||| / delay
# TASK-PID CPU# |||| TIMESTAMP FUNCTION
# | | | |||| | |
sshd-20125 [005] d... 244126.123095: sched_stat_runtime: comm=sshd pid=20125 runtime=101555 [ns] vruntime=153686839 [ns]
sshd-20125 [005] d... 244126.123100: sched_switch: prev_comm=sshd prev_pid=20125 prev_prio=120 prev_state=S ==> next_comm=swapper/5 next_pid=0 next_prio=120
bash-20130 [007] d... 244126.123173: sched_waking: comm=kworker/u16:3 pid=20190 prio=120 target_cpu=006
bash-20130 [007] d... 244126.123177: sched_wake_idle_without_ipi: cpu=6
# echo > trace
# head -15 trace
# tracer: nop
#
# entries-in-buffer/entries-written: 0/0 #P:8
#
# _-----=> irqs-off
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth
# ||| / delay
# TASK-PID CPU# |||| TIMESTAMP FUNCTION
# | | | |||| | |
# head -15 snapshot
# tracer: nop
#
# entries-in-buffer/entries-written: 3563/3563 #P:8
#
# _-----=> irqs-off
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth
# ||| / delay
# TASK-PID CPU# |||| TIMESTAMP FUNCTION
# | | | |||| | |
bash-20130 [007] d.h. 243924.135391: sched_stat_runtime: comm=bash pid=20130 runtime=584815 [ns] vruntime=116509213 [ns]
bash-20130 [007] d... 243924.135853: sched_waking: comm=kworker/u16:2 pid=20112 prio=120 target_cpu=002
bash-20130 [007] d... 243924.135859: sched_wake_idle_without_ipi: cpu=2
bash-20130 [007] d... 243924.135861: sched_wakeup: comm=kworker/u16:2 pid=20112 prio=120 target_cpu=002
>
> trace_pipe:
>
> @@ -490,6 +493,9 @@ of ftrace. Here is a list of some of the key files:
>
> # echo global > trace_clock
>
> + Setting a clock clears the ring buffer content as well as the
> + "snapshot" buffer.
> +
Yeah, this is true, because the sorting algorithm relies on the trace
clocks matching in the buffers. If this becomes an issue, I'm sure we
could change it.
-- Steve
> trace_marker:
>
> This is a very useful file for synchronizing user space
