* Thomas Gleixner <tglx@xxxxxxxxxxxxx> [2009-04-06 12:56:17]:
> Arun,
>
> On Mon, 6 Apr 2009, Arun R Bharadwaj wrote:
> >
> > +ktime_t clockevents_get_next_event(int cpu)
> > +{
> > + struct tick_device *td;
> > + struct clock_event_device *dev;
> > +
> > + td = &per_cpu(tick_cpu_device, cpu);
> > + dev = td->evtdev;
> > +
> > + return dev->next_event;
> > +}
> > +
>
> Preferrably this function should be in the clock events code and a
> stub inline function which returns KTIME_MAX for non clock events
> archs is probably necessary as well.
>
Sure.
> > /*
> > * Switch the timer base to the current CPU when possible.
> > */
> > @@ -198,8 +211,17 @@ switch_hrtimer_base(struct hrtimer *time
> > {
> > struct hrtimer_clock_base *new_base;
> > struct hrtimer_cpu_base *new_cpu_base;
> > + int cpu, preferred_cpu = -1;
> > +
> > + cpu = smp_processor_id();
> > + if (get_sysctl_timer_migration() && !pinned && idle_cpu(cpu)) {
> > + preferred_cpu = get_nohz_load_balancer();
> > + if (preferred_cpu >= 0)
> > + cpu = preferred_cpu;
> > + }
> >
> > - 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.
> spin_unlock(&new_base->cpu_base->lock);
> spin_lock(&base->cpu_base->lock);
>
> > + goto again;
> > + }
>
> Also you need to move
>
> > timer->base = new_base;
>
> here to avoid a stale timer->base setting.
>
The above takes care of this as well.
--arun
> > + }
> > }
> > return new_base;
> > }
>
> Thanks,
>
> tglx
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