linux-kernel - Re: [RFC][PATCH 2/5] perf: Turn the group counter values into delta values

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Message-ID: <20100830132138.GA24828@console-pimps.org>
Date:	Mon, 30 Aug 2010 14:21:38 +0100
From:	Matt Fleming <matt@...sole-pimps.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Zhang Rui <rui.zhang@...el.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Robert Richter <robert.richter@....com>,
	Lin Ming <ming.m.lin@...el.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Don Zickus <dzickus@...hat.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Len Brown <lenb@...nel.org>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: [RFC][PATCH 2/5] perf: Turn the group counter values into
 delta values

On Mon, Aug 30, 2010 at 02:54:01PM +0200, Peter Zijlstra wrote:
> On Mon, 2010-08-30 at 13:13 +0100, Matt Fleming wrote:
> > Change the semantics of the PERF_FORMAT_GROUP records and make them
> > delta values, i.e. the difference in value between consecutive reads of
> > the hardware counters. These delta values will be used in a subsequent
> > patch to calculate weighted values (by multiplying the deltas with the
> > time difference between reads of the counters). Calculating these deltas
> > is easiest to do in the kernel (as opposed to the userland tools)
> > because we can reset the counter whenever we read from it.
> > 
> > Note that even though this patch changes the semantics of
> > PERF_FORMAT_GROUP records there were no users of them anyway (the tools
> > didn't even understand them).
> 
> libpfmon might be using it,.. I know some people occasionally reported
> bugs for this so there are users.
> 
> Also, there's no real synchonization between read,read and read,sample
> so its racy to do this, simply generate deltas in post-processing.

Ah, fair enough. Thanks, I'll do this with post-processing.
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