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

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Message-ID: <1291840941.2909.40.camel@work-vm>
Date:	Wed, 08 Dec 2010 12:42:21 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	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
Subject: Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM

On Wed, 2010-12-08 at 15:44 +0100, Peter Zijlstra wrote:
> On Wed, 2010-12-08 at 14:28 +0000, Russell King - ARM Linux wrote:
> > So, what I'm saying is that if wrapping in 4 seconds is a problem,
> > then maybe we shouldn't be providing sched_clock() at all.
> 
> 4 seconds should be perfectly fine, we call it at least every scheduler
> tick (HZ) and NO_HZ will either have to limit the max sleep period or
> provide its own sleep duration (using a more expensive clock) so we can
> recover (much like GTOD already requires).
> 
> > Also, if wrapping below 64-bits is also a problem, again, maybe we
> > shouldn't be providing it there either.  Eg:
> 
> That's mostly due to hysterical raisins and we should just fix that, the
> simplest way is to use something like kernel/sched_clock.c to use
> sched_clock() deltas to make a running u64 value.
> 
> Like said, John Stultz was already looking at doing something like that
> because there's a number of architectures suffering this same problem
> and they're all already using part of the clocksource infrastructure to
> implement the sched_clock() interface simply because they only have a
> single hardware resource.

I'm not actively working on it right now, but trying to rework the
sched_clock code so its more like the generic timekeeping code is on my
list (I'm Looking to see if I can bump it up to the front in the near
future).

thanks
-john


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