Subject: Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM
From: john stultz <johnstul@us.ibm.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Russell King - ARM Linux <linux@arm.linux.org.uk>,
        Mikael Pettersson <mikpe@it.uu.se>,
        Venkatesh Pallipadi <venki@google.com>, Ingo Molnar <mingo@elte.hu>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org
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Date: Wed, 08 Dec 2010 12:42:21 -0800
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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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