linux-kernel - Re: [RFC PATCH] perf/core: Put size of a sample at the end of it

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Date:	Thu, 3 Dec 2015 11:08:01 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Wang Nan <wangnan0@...wei.com>
Cc:	acme@...hat.com, paulus@...ba.org, kan.liang@...el.com,
	linux-kernel@...r.kernel.org, pi3orama@....com, lizefen@...wei.com,
	Adrian Hunter <adrian.hunter@...el.com>,
	David Ahern <dsahern@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Yunlong Song <yunlong.song@...wei.com>
Subject: Re: [RFC PATCH] perf/core: Put size of a sample at the end of it

On Wed, Dec 02, 2015 at 01:38:19PM +0000, Wang Nan wrote:
> This sloution requires user program (perf) do more things. At least
> following things and limitations should be considered:
> 
>  1. Before reading such ring buffer, perf must ensure all events which
>     may output to it is already stopped, so the 'head' pointer it get
>     is the end of the last record.

Right, this is tricky, this would not allow two snapshots to happen back
to back since that would then result in a bunch of missed events.

Aside from this issue its a rather nice idea.

>  2. We must ensure all events attached this ring buffer has
>     'PERF_SAMPLE_SIZE' selected.

That can be easily enforced.

>  3. There must no tracking events output to this ring buffer.

That is rather unfortunate, we'd best fix that up.

>  4. 2 bytes extra space is required for each record.

8, perf records must be 8 byte aligned and sized.

> Further improvement can be taken:
> 
>  1. If PERF_SAMPLE_SIZE is selected, we can avoid outputting the event
>     size in header. Which eliminate extra space cose;

That would mandate you always parse the stream backwards. Which seems
rather unfortunate. Also, no you cannot recoup the extra space, see the
alignment and size requirement.

>  2. We can find a way to append size information for tracking events
>     also.

The !sample records you mean? Yes those had better have them too.
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