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

Russell King - ARM Linux
2010-12-10T19:36:55Z

On Fri, Dec 10, 2010 at 07:10:54PM +0100, Peter Zijlstra wrote:
> On Fri, 2010-12-10 at 17:56 +0000, Russell King - ARM Linux wrote:
> > On Fri, Dec 10, 2010 at 02:47:46PM +0100, Peter Zijlstra wrote:
> > > inline void update_rq_clock(struct rq *rq)
> > > {
> > > - int cpu = cpu_of(rq);
> > > - u64 irq_time;
> > > + s64 delta;
> > >
> > > if (rq->skip_clock_update)
> > > return;
> > >
> > > - rq->clock = sched_clock_cpu(cpu);
> > > - irq_time = irq_time_cpu(cpu);
> > > - if (rq->clock - irq_time > rq->clock_task)
> > > - rq->clock_task = rq->clock - irq_time;
> > > + delta = sched_clock_cpu(cpu_of(rq)) - rq->clock;
> > > + rq->clock += delta;
> >
> > Hmm. Can you tell me how this is different to:
> >
> > new_clock = sched_clock_cpu(cpu_of(rq));
> > delta = new_clock - rq->clock;
> > rq->clock = new_clock;
> >
> > which I think may be simpler in terms of 64-bit math for 32-bit compilers
> > to deal with?
>
> Its not, I could write it like that, the only reason I didn't is because
> it uses an extra variable. If gcc on 32bit targets really generates
> hideous code for it I'll happily change it.

Well, I can't tell you what kind of code this produces on ARM, as it
doesn't appear to apply to any kernel I've tried. So, I assume it's
against some scheduler development tree rather than Linus' tree?