linux-kernel - Re: [.32-rc3] scheduler: iwlagn consistently high in "waiting for CPU"

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Date:	Wed, 07 Oct 2009 20:10:40 +0200
From:	Mike Galbraith <efault@....de>
To:	Frans Pop <elendil@...net.nl>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-wireless@...r.kernel.org
Subject: Re: [.32-rc3] scheduler: iwlagn consistently high in "waiting for
 CPU"

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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