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

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Date:	Sun, 11 Dec 2011 13:55:31 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
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

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.

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