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

Russell King - ARM Linux
2010-12-10T17:57:39Z

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?

In terms of the wrap-around, I don't see this as any different from the
above, as:

rq->clock += sched_clock_cpu(cpu_of(rq)) - rq_clock;
rq->clock = rq->clock + sched_clock_cpu(cpu_of(rq)) - rq_clock;
rq->clock = sched_clock_cpu(cpu_of(rq));