* Thomas Gleixner <tglx@xxxxxxxxxxxxx> [2009-04-06 17:35:09]:
> Arun,
>
> On Mon, 6 Apr 2009, Arun R Bharadwaj wrote:
> > > > - new_cpu_base = &__get_cpu_var(hrtimer_bases);
> > > > +again:
> > > > + new_cpu_base = &per_cpu(hrtimer_bases, cpu);
> > > > new_base = &new_cpu_base->clock_base[base->index];
> > > >
> > > > if (base != new_base) {
> > > > @@ -220,6 +242,32 @@ switch_hrtimer_base(struct hrtimer *time
> > > > spin_unlock(&base->cpu_base->lock);
> > > > spin_lock(&new_base->cpu_base->lock);
> > > > timer->base = new_base;
> > > > +
> > > > + if (cpu == preferred_cpu) {
> > > > + /* Calculate clock monotonic expiry time */
> > > > + ktime_t expires = ktime_sub(hrtimer_get_expires(timer),
> > > > + new_base->offset);
> > > > +
> > > > + /*
> > > > + * Get the next event on target cpu from the
> > > > + * clock events layer.
> > > > + * This covers the highres=off nohz=on case as well.
> > > > + */
> > > > + ktime_t next = clockevents_get_next_event(cpu);
> > > > +
> > > > + ktime_t delta = ktime_sub(expires, next);
> > > > +
> > > > + /*
> > > > + * We do not migrate the timer when it is expiring
> > > > + * before the next event on the target cpu because
> > > > + * we cannot reprogram the target cpu hardware and
> > > > + * we would cause it to fire late.
> > > > + */
> > > > + if (delta.tv64 < 0) {
> > > > + cpu = smp_processor_id();
> > >
> > > You are missing a small but fatal detail here: You hold
> > > new_base->cpu_base->lock. So you need to do:
> > >
> >
> > I just moved the if block.. if (cpu==preferred_cpu) above the base
> > locking part to avoid the extra unlocking.
>
> That's not a good idea. You want to look at the next event with the
> base lock of the other cpu held. That prevents that expires the first
> pending timer right after you checked next_event and before you queue
> your timer, which then might become the first timer to expire but you
> can't reprogram the clock event device on the other cpu.
>
Okay... I understand. Thanks for the explanation :)
Will post all the changes together in a separate thread.
--arun
> Thanks,
>
> tglx
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