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 18:31:49 +0800
From:	"Wangnan (F)" <wangnan0@...wei.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 2015/12/3 18:08, Peter Zijlstra wrote:
> 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.

Thank you for your attitude. We can start consider it seriously.

Now I'm working on perf side code to make a workable prototype.

>>   2. We must ensure all events attached this ring buffer has
>>      'PERF_SAMPLE_SIZE' selected.
> That can be easily enforced.

Yes.

>>   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.

So does it means we need to pad before outputing size?

>> 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.

That's good. We don't need to consider this :)

Before receiving your comment I'm thinking about modifying
DEFINE_OUTPUT_COPY() to write first sizeof(header) bytes at the
end of reserved area, so it would work automatically for every
possible events.

>>   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.

Thank you.

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