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SubjectRe: [PATCH] tracing: Resets the trace buffer after a snapshot
On Fri, Jan 03, 2020 at 11:40:01AM -0500, Steven Rostedt wrote:
> On Tue, 31 Dec 2019 03:58:22 -0500
> "Frank A. Cancio Bello" <frank@generalsoftwareinc.com> wrote:
>
> > Currently, when a snapshot is taken the trace_buffer and the
> > max_buffer are swapped. After this swap, the "new" trace_buffer is
> > not reset. This produces an odd behavior: after a snapshot is taken
> > the previous snapshot entries become available to the next reader of
> > the trace_buffer as far as the reading occurs before the buffer is
> > refilled with new entries by a writer.
>
> I consider this a feature not a bug ;-)
>
> Anyway, this behavior should be determined by an option. Care to create
> one? (reset_on_snapshot?) I would keep the default behavior the same,
> but document this a bit better.

I relate to what Steve said as well. It is not strictly a bug per-se. An
option to do this would be nice but I am doubting a user will really turn on
such option (or even know an option exists) ;-). I would say leave it in the
current state unless some usecase is disrupted by the current behavior..

thanks!

- Joel


>
> Thanks!
>
> -- Steve
>
> >
> > This patch resets the trace buffer after a snapshot is taken.
> >
> > Signed-off-by: Frank A. Cancio Bello <frank@generalsoftwareinc.com>
> > ---
> >
> > The following commands illustrate this odd behavior:
> >
> > # cd /sys/kernel/debug/tracing
> > # echo nop > current_tracer
> > # echo 1 > tracing_on
> > # echo m1 > trace_marker
> > # echo 1 > snapshot
> > # echo m2 > trace_marker
> > # echo 1 > snapshot
> > # cat trace
> > # tracer: nop
> > #
> > # entries-in-buffer/entries-written: 1/1 #P:2
> > #
> > # _-----=> irqs-off
> > # / _----=> need-resched
> > # | / _---=> hardirq/softirq
> > # || / _--=> preempt-depth
> > # ||| / delay
> > # TASK-PID CPU# |||| TIMESTAMP FUNCTION
> > # | | | |||| | |
> > bash-550 [000] .... 50.479755: tracing_mark_write: m1
> >
> >
> > kernel/trace/trace.c | 7 +++++--
> > 1 file changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > index ddb7e7f5fe8d..58373b5ae0cf 100644
> > --- a/kernel/trace/trace.c
> > +++ b/kernel/trace/trace.c
> > @@ -6867,10 +6867,13 @@ tracing_snapshot_write(struct file *filp, const char __user *ubuf, size_t cnt,
> > break;
> > local_irq_disable();
> > /* Now, we're going to swap */
> > - if (iter->cpu_file == RING_BUFFER_ALL_CPUS)
> > + if (iter->cpu_file == RING_BUFFER_ALL_CPUS) {
> > update_max_tr(tr, current, smp_processor_id(), NULL);
> > - else
> > + tracing_reset_online_cpus(&tr->trace_buffer);
> > + } else {
> > update_max_tr_single(tr, current, iter->cpu_file);
> > + tracing_reset_cpu(&tr->trace_buffer, iter->cpu_file);
> > + }
> > local_irq_enable();
> > break;
> > default:
>

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