linux-kernel - Re: [PATCH V5 09/12] perf: make events stream always parsable

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Date:	Fri, 12 Jul 2013 15:56:01 +0300
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel@...r.kernel.org, David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>, Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH V5 09/12] perf: make events stream always parsable

On 12/07/13 12:56, Peter Zijlstra wrote:
> On Thu, Jul 11, 2013 at 04:12:18PM +0300, Adrian Hunter wrote:
>> The event stream is not always parsable because the format of a sample
>> is dependent on the sample_type of the selected event.  When there
>> is more than one selected event and the sample_types are not the
>> same then parsing becomes problematic.  A sample can be matched to its
>> selected event using the ID that is allocated when the event is opened.
>> Unfortunately, to get the ID from the sample means first parsing it.
>>
>> This patch adds a new sample format bit PERF_SAMPLE_IDENTIFER that puts
>> the ID at a fixed position so that the ID can be retrieved without
>> parsing the sample.  For sample events, that is the first position
>> immediately after the header.  For non-sample events, that is the last
>> position.
> 
> There's events where this isn't a possible location; take PERF_RECORD_MMAP for
> instance; the tail is the complete filename.

PERF_RECORD_MMAP falls in the category I have called non-sample events.
Those events are appended with an ID sample.  perf tools parses the ID
sample backwards from header.size.  So the ID is at the last position
relative to header.size

> 
> Why not always insert right after the header?

It is for sample events i.e. PERF_RECORD_SAMPLE

> 
> Also; you forgot to update include/uapi/linux/perf_event.h:enum perf_event_type
> That's the format documentation.
> 

OK

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