linux-kernel - Re: [PATCH V11 09/15] perf: make events stream always parsable

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Date:	Wed, 14 Aug 2013 16:00:29 +0300
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
CC:	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>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...gle.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH V11 09/15] perf: make events stream always parsable

On 14/08/13 15:48, 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.
> 
> In this respect parsing samples requires that the sample_type and ID
> values are recorded.  For example, perf tools records struct perf_event_attr
> and the IDs within the perf.data file.  Those must be read first
> before it is possible to parse samples found later in the perf.data file.
> 
> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>

Forgot to add Peter's ack which is here:

	http://marc.info/?l=linux-kernel&m=137629757013526&w=2
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