Messages in this thread | | | Date | Fri, 3 Jan 2020 17:37:11 -0500 | From | Joel Fernandes <> | Subject | Re: [PATCH] tracing: Resets the trace buffer after a snapshot |
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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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