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Date: Mon, 30 Aug 2010 14:21:38 +0100
From: Matt Fleming <matt@console-pimps.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Zhang Rui <rui.zhang@intel.com>, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@elte.hu>, Frederic Weisbecker <fweisbec@gmail.com>,
        Robert Richter <robert.richter@amd.com>,
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        Don Zickus <dzickus@redhat.com>, Cyrill Gorcunov <gorcunov@gmail.com>,
        Len Brown <lenb@kernel.org>, Matthew Garrett <mjg59@srcf.ucam.org>
Subject: Re: [RFC][PATCH 2/5] perf: Turn the group counter values into
 delta values
Message-ID: <20100830132138.GA24828@console-pimps.org>
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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.