linux-kernel - Re: [PATCH 3/3] perf record/report: Process events in order

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Message-Id: <1291640985-sup-4443@au1.ibm.com>
Date:	Tue, 07 Dec 2010 00:11:28 +1100
From:	Ian Munsie <imunsie@....ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 3/3] perf record/report: Process events in order

Excerpts from Thomas Gleixner's message of Tue Dec 07 00:04:20 +1100 2010:
> > I just moved them into this routine to keep all the dispatching in one
> > place, whether delayed or not. These particular events will still be
> > processed immediately when encountered in the file. Only >=
> > PERF_RECORD_MMAP && <= PERF_RECORD_SAMPLE will be delayed via the
> > perf_session__process_timed function.
> 
> Gah. This is nasty. I really prefer the explicit split of instant
> processed and possibly delayed events. That makes the code clear and
> easy to extend. I just want to add a new event type at the right place
> and not worry about magic comparisions in some other place.

Fair enough, I'll split them back out.

> > For instance, suppose we ran this on an old kernel without support for
> > timestamps on every event (so timestamps are only on sample events):
> > 
> > perf record -T
> > perf report
> > 
> > If perf report tried to process the events in order, all the events
> > without timestamps would be processed first -- including the
> > PERF_RECORD_EXIT event, which would cause every sample not to be
> > attributed. Falling back means we should get no worse than the old
> > behaviour, while an upgraded kernel will provide the timestamps and
> > should not fall back.
> 
> Ok, but you'll break existing code which does only care about sample
> ordering if you do that at the session level unconditionally.

Good point. I'll give this some thought overnight.

Cheers,
-Ian
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