linux-kernel - Re: [PATCH 2/4] perf_event: add PERF_COUNT_HW_REF_CPU

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Message-ID: <20111211154742.GA1213@elte.hu>
Date:	Sun, 11 Dec 2011 16:47:42 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Stephane Eranian <eranian@...gle.com>,
	linux-kernel@...r.kernel.org, acme@...radead.org,
	robert.richter@....com, ming.m.lin@...el.com, andi@...stfloor.org,
	asharma@...com
Subject: Re: [PATCH 2/4] perf_event: add PERF_COUNT_HW_REF_CPU_CYCLES generic
 PMU event


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Sun, 2011-12-11 at 09:01 +0100, Ingo Molnar wrote:
> > > +     PERF_COUNT_HW_REF_CPU_CYCLES            = 9,
> > 
> > Btw., that was what 'bus cycles' tried to do a long time ago: 
> > the constant, non-variable baseline heartbeat of the system. 
> 
> This isn't about that. Its about exposing the third fixed 
> purpose counter. Intel, in their infinite wisdom, created a 
> fixed purpose counter for which there is no equivalent in the 
> general purpose events.
> 
> Our fixed purpose counter support is predicated on the 
> assumption that there is, and simply maps any event code to 
> also include the fixed purpose counter if appropriate.
> 
> There not being an event to map from has thus far avoided 
> exposing this third fixed purpose event.
> 
> The problem with remapping BUS_CYCLES is that BUS_CYCLES (now) 
> is something you can program on the {2,4,8} general purpose 
> counters, whereas this new thing can only ever be ran from the 
> 1 fixed purpose counter.

Okay - if we want/need 3 variants then i have no objections.

Thanks,

	Ingo
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