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

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Message-ID: <51E3932C.5000905@intel.com>
Date:	Mon, 15 Jul 2013 09:14:04 +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 17:55, Peter Zijlstra wrote:
> On Fri, Jul 12, 2013 at 03:56:01PM +0300, Adrian Hunter wrote:
>>> 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
> 
> But why? Why make it different per PERF_RECORD type? 

There have always been two formats:

	1. PERF_RECORD_SAMPLE as defined by perf_output_sample()

	2. everything else as defined by __perf_event__output_id_sample()

The two formats are not the same, and there is no reason for them to be.

PERF_RECORD_SAMPLE is parsed forwards, so the ID is at the first position.

ID samples are parsed backwards, so the ID is at the last position (i.e. the
first position parsed).

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