linux-kernel - Re: [PATCH] x86, AMD: Set sched_clock

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Date:	Wed, 15 Feb 2012 16:30:37 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Ingo Molnar <mingo@...e.hu>, Borislav Petkov <bp@...64.org>,
	Venki Pallipadi <venki@...gle.com>,
	Stephane Eranian <eranian@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Robert Richter <robert.richter@....com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Andreas Herrmann <andreas.herrmann3@....com>
Subject: Re: [PATCH] x86, AMD: Set sched_clock_stable

On Tue, 2012-02-07 at 13:08 +0100, Borislav Petkov wrote:
> Stephane Eranian reported that doing a scheduler latency measurements
> with perf on AMD doesn't work out as expected due to the fact that the
> sched_clock() granularity is too coarse, i.e. done in jiffies due to the
> sched_clock_stable not set, which, if set, would mean that we get to use
> the TSC as sample source which would give us much higher precision.

FWIW that's not entirely accurate, the problem with !sched_clock_stable
is that the time between CPUs can drift a little. So while timestamps on
the same CPU are high-res, when comparing them across CPUs you can
observe funnies.


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