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

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Message-ID: <20130712095600.GT25631@dyad.programming.kicks-ass.net>
Date:	Fri, 12 Jul 2013 11:56:00 +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 V5 09/12] perf: make events stream always parsable

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.

Why not always insert right after the header?

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