[BUG] 2.6.37-rc3 massive interactivity regression on ARM

[Peter Zijlstra]

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.