linux-kernel - Re: [RFC][PATCH 6/6] perf, tools: X86 RDPMC, RDTSC test

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Date:	Mon, 21 Nov 2011 16:37:22 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	mingo@...e.hu, William Cohen <wcohen@...hat.com>,
	linux-kernel@...r.kernel.org, Arun Sharma <asharma@...com>,
	Vince Weaver <vince@...ter.net>
Subject: Re: [RFC][PATCH 6/6] perf, tools: X86 RDPMC, RDTSC test

On Mon, 2011-11-21 at 16:29 +0100, Stephane Eranian wrote:
> Peter,
> 
> I don't see how this test and infrastructure handles the case where the event
> is multiplexed. I know there is time_enabled and time_running. But those are
> not sync'd to the moment of the rdpmc(). I think there needs to be some other
> timestamp in the mmap struct so the user can compute a delta to then add to
> time_enabled and time_running.

When the counter isn't actually on the PMU, ->index will be 0 and rdpmc
should not be attempted.

> Unless, we assume the two time metrics are there ONLY to compute a scaling
> ratio. In which case, I think, we don't need the delta because if we
> can do rdpmc()
> it means the event is currently scheduled and thus time_enabled and time_running
> are both ticking which means the scaling ratio does not change since the moment
> the event was scheduled in.

Right, you don't need delta to compute the scale, but its useful for
user-space time based measurements, Arun wanted to do something like
that.

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