linux-kernel - Re: [PATCH V9 09/14] perf: make events stream always parsable

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Message-ID: <20130812085227.GA27162@twins.programming.kicks-ass.net>
Date:	Mon, 12 Aug 2013 10:52:27 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Adrian Hunter <adrian.hunter@...el.com>
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 V9 09/14] perf: make events stream always parsable

On Mon, Aug 05, 2013 at 07:26:29PM +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.
> 
> 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>

Acked-by: Peter Zijlstra <peterz@...radead.org>


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