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

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Message-ID: <CABPqkBSabGq=QQz4fvR3DuY2AyhKS6bYAsRugh4MgApLz0LqpA@mail.gmail.com>
Date:	Mon, 15 Jul 2013 13:53:31 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	LKML <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>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH V5 09/12] perf: make events stream always parsable

On Mon, Jul 15, 2013 at 8:14 AM, Adrian Hunter <adrian.hunter@...el.com> wrote:
> 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).
>
I am missing something here.
Why do we need an event ID for RECORD_MMAP records?
I understand those are requested by events, but do we care which one?
The information is global to the monitored process and not specific to an event.
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