linux-kernel - Re: [PATCH] tracing: Resets the trace buffer after a snapshot

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Message-ID: <20200106124141.1fc400a1@gandalf.local.home>
Date:   Mon, 6 Jan 2020 12:41:41 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Frank A. Cancio Bello" <frank@...eralsoftwareinc.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        saiprakash.ranjan@...eaurora.org, nachukannan@...il.com
Subject: Re: [PATCH] tracing: Resets the trace buffer after a snapshot

On Sun, 5 Jan 2020 05:31:13 -0500
"Frank A. Cancio Bello" <frank@...eralsoftwareinc.com> wrote:

> 
> Thank you both for your answers. I'm wondering what would be the reason
> for not resetting the trace buffer after it gets swapped with the snapshot
> buffer. Given that resetting it's not expensive, I would say that is not
> performance, so I'm intrigued ;)

Is it not expensive? It calls synchronize_rcu()! which is very
expensive.

When I have used the snapshot buffer, It was usually to capture things
that happen at various times, but still look for the next trace. By
alternating, I do get to see where the last snapshot happened. It
basically doubles the size of the buffer.

> 
> If it's OK, I will send two patches then, one documenting explicitly
> that the trace buffer it will not be reset after be swapped and the
> implications of this, and the second one changing the documentation of
> the field trace_array->max_buffer that I now realized that say:
> 
> "
> /*
> ...
> * The buffers for the max_buffer are set up the same as the trace_buffer
> * When a snapshot is taken, the buffer of the max_buffer is swapped
> * with the buffer of the trace_buffer and the buffers are reset for
> * the trace_buffer so the tracing can continue.

It is reset partially by the latency tracers, and this is where it gets
confusing. Instead of a full reset, as the latency tracer only cares
about a specific start and end, it records where the start and end is,
and only modifies that. Look at the time_start of the trace_buffer.

Hmm, it may be possible to have an option just update that, which
should give the same effect.

-- Steve