Subject: Re: [.32-rc3] scheduler: iwlagn consistently high in "waiting for
 CPU"
From: Mike Galbraith <efault@gmx.de>
To: Frans Pop <elendil@planet.nl>
Cc: Arjan van de Ven <arjan@infradead.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Ingo Molnar <mingo@elte.hu>, Peter Zijlstra <peterz@infradead.org>,
       linux-wireless@vger.kernel.org
In-Reply-To: <200910071910.53907.elendil@planet.nl>
References: <200910051500.55875.elendil@planet.nl>
	 <20091005072428.16ce40e4@infradead.org>
	 <200910061749.02805.elendil@planet.nl>
	 <200910071910.53907.elendil@planet.nl>
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Date: Wed, 07 Oct 2009 20:10:40 +0200
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On Wed, 2009-10-07 at 19:10 +0200, Frans Pop wrote:
> On Tuesday 06 October 2009, Frans Pop wrote:
> > I've checked for 2.6.31.1 now and iwlagn is listed high there too when
> > the system is idle, but with normal values of 60-100 ms. And phy0 has
> > normal values of below 10 ms.
> > I've now rebooted with today's mainline git; phy0 now frequently shows
> > with values of around 100 ms too (i.e. higher than last time).
> >
> > Both still go way down as soon as the system is given work to do.
> >
> > With a 5 second sleep I was unable to get any significant latencies (I
> > started perf on a latencytop refresh and did a manual refresh as it
> > finished to see what happened during the perf run). The perf run does
> > seem to affect the latencies.
> > I've uploaded a chart for a 10s sleep during which I got latencies of
> > 101ms for iwlagn and 77ms for phy0:
> > http://people.debian.org/~fjp/tmp/kernel/.
> 
> Mike privately sent me a script to try to capture the latencies with perf,
> but the perf output does not show any high latencies at all. It looks as if
> we may have found a bug in latencytop here instead.

Maybe.  I have a little perturbation measurement proggy which I just
fired up to verify both perf and latencytop's numbers here.  It's a dirt
simply cycle counter tool, which calibrates itself, sums perturbations
over a period of time and emit stats.  Here, all three are in violent
agreement wrt how long "pert" is waiting for cpu.

	-Mike

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