linux-kernel - Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM

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Date:	Wed, 08 Dec 2010 13:40:15 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Mikael Pettersson <mikpe@...uu.se>,
	Venkatesh Pallipadi <venki@...gle.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	John Stultz <johnstul@...ibm.com>
Subject: Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM

On Sun, 2010-12-05 at 16:21 +0000, Russell King - ARM Linux wrote:

> I'm not sure that's the correct fix - it looks like sched_clock_cpu()
> should already be preventing scheduler clock time going backwards.
> 
> Hmm. IOP32x seems to have a 32-bit timer clocked at 200MHz.  That means
> it wraps once every 21s.  However, we have that converted to ns by an
> unknown multiplier and shift.  It seems that those are chosen to
> guarantee that we will cover only 4s without wrapping in the clocksource
> conversion.  Maybe that's not sufficient?
> 
> Could you try looking into sched_clock_cpu(), sched_clock_local() and
> sched_clock() to see whether anything odd stands out?

# git grep HAVE_UNSTABLE_SCHED_CLOCK arch/arm | wc -l
0

That code won't help if you don't enable it ;-)

John Stultz was also looking into making the kernel/sched_clock.c code
deal with short clocks.
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