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

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Date:	Mon, 12 Dec 2011 18:32:15 +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 00:28 +0100, Stephane Eranian wrote:
> > This event counts the number of reference core cpu cycles.
> > Reference means that the event increments at a constant rate which
> > is not subject to core CPU frequency adjustments. The event may
> > not count when the processor is in halted (low power) state.
> > As such, it may not be equivalent to wall clock time. However,
> > when the processor is not halted state, the event keeps
> > a constant correlation with wall clock time.
> > 
> > Signed-off-by: Stephane Eranian <eranian@...gle.com>
> > ---
> >  include/linux/perf_event.h |    1 +
> >  1 files changed, 1 insertions(+), 0 deletions(-)
> > 
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index 564769c..0885561 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -54,6 +54,7 @@ enum perf_hw_id {
> >  	PERF_COUNT_HW_BUS_CYCLES		= 6,
> >  	PERF_COUNT_HW_STALLED_CYCLES_FRONTEND	= 7,
> >  	PERF_COUNT_HW_STALLED_CYCLES_BACKEND	= 8,
> > +	PERF_COUNT_HW_REF_CPU_CYCLES		= 9,
> >  
> >  	PERF_COUNT_HW_MAX,			/* non-ABI */
> >  };
> 
> Does it make sense to add this to the 'generic' events? Are 
> other archs going to use this?
> 
> That is, I already queued patch 1, I'm just wondering if the 
> generic bit makes sense, Even BUS_CYCLES seems to be a 
> questionable 'generic' event, but that's history and we can't 
> fix it.

If we named it in a generic way, with a generic, 
platform-independent meaning behind it, then it shouldn't be a 
problem. This is why i suggested naming it 'constant CPU cycles' 
- or 'constant freq cycles' or a variant of that.

Thanks,

	Ingo
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