Messages in this thread | | | Subject | Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM | From | Peter Zijlstra <> | Date | Wed, 08 Dec 2010 15:04:36 +0100 |
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On Wed, 2010-12-08 at 12:55 +0000, Russell King - ARM Linux wrote: > On Wed, Dec 08, 2010 at 01:40:15PM +0100, Peter Zijlstra wrote: > > 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 > > Hmm, you're right. In which case it's purely down to sched_clock() > only being able to cover 4s - which seems to be far too small a gap. > > I'm not sure that the unstable sched clock stuff makes much sense to > be enabled - we don't have an unstable clock, we just don't have the > required number of bits for the scheduler to work correctly.
We can perhaps make part of the HAVE_UNSTABLE_SCHED_CLOCK depend on SMP and only deal with the short wraps (and maybe monotonicity) on UP.
> Nevertheless, this provides a good way to find this kind of wrap bug. > Even with cnt_32_to_63, we still don't get a 64-bit sched_clock(), so > this bug will still be there. Even with a 64-bit clock, the bug will > still be there. It's basically crap code.
You're referring to the clock_task bit from Venki? Yes that needs fixing.
> Maybe it's better that on ARM, we just don't implement sched_clock() > at all?
If you have a high res clock source that's cheap to read it would be better if we can simply fix the infrastructure such that we can make use of it.
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