linux-kernel - Re: [RFC/PATCH] perf x86: Add off-core event constraints for Sandy/IvyBridge micro architecture

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Date:	Tue, 29 Jan 2013 16:49:19 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFC/PATCH] perf x86: Add off-core event constraints for
 Sandy/IvyBridge micro architecture

On Mon, Jan 28, 2013 at 06:49:55PM +0100, Stephane Eranian wrote:
> On Sun, Jan 27, 2013 at 6:33 PM, Jiri Olsa <jolsa@...hat.com> wrote:
> > hi,
> > I was looking at the offcore stuff and it looks like we might
> > be missing some constraints for offcore response events on
> > Sandy/IvyBridge.
> >
> > The table 18.8.5 (Off-core Response Performance Monitoring)
> > in Intel SDM states PMC0 for 0xb7 and PMC3 for 0xbb, but
> > there's no other explanation or related description.
> >
> > I can't say/ack if the counters looks bad or right with or
> > without the patch so far.. so just curious ;-)
> >
> Those are artificial constraints which should not be there.
> Remember that offcore_rsp uses an extra MSR which has
> to be shared by all the counters on the PMU. So a way to
> handle the sharing of that extra MSR is to impose an
> artificial constraint on the event itself. If it can only run
> on one counter, then you get the management of the
> extra MSR for free, i.e., only one event gets it.
> 
> In perf_events, we use a more sophisticated dynamic scheme
> which does not use this artificial constraint. We can measure
> the event multiple times and share the extra MSR if possible
> (same value). Why multiple times you might ask? For instance,
> with different priv levels.
> 
> 
> Hope this helps.

nice, thanks a lot for explanation

jirka
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