* Thomas Gleixner <tglx@xxxxxxxxxxxxx> [2009-03-17 11:22:24]:
> On Mon, 16 Mar 2009, Arun R Bharadwaj wrote:
> > @@ -627,6 +628,16 @@ __mod_timer(struct timer_list *timer, un
> >
> > new_base = __get_cpu_var(tvec_bases);
> >
> > + current_cpu = smp_processor_id();
> > + preferred_cpu = get_nohz_load_balancer();
> > + if (get_sysctl_timer_migration() && idle_cpu(current_cpu) &&
> > + !pinned && preferred_cpu != -1) {
> > + new_base = per_cpu(tvec_bases, preferred_cpu);
> > + timer_set_base(timer, new_base);
> > + timer->expires = expires;
> > + internal_add_timer(new_base, timer);
> > + goto out_unlock;
> > + }
>
> Err. This change breaks the timer->base logic. Why can't it just
> select the base and use the existing code ?
>
Sure, I'll take care of this.
> > @@ -198,8 +200,16 @@ switch_hrtimer_base(struct hrtimer *time
> > {
> > struct hrtimer_clock_base *new_base;
> > struct hrtimer_cpu_base *new_cpu_base;
> > + int current_cpu, preferred_cpu;
> > +
> > + current_cpu = smp_processor_id();
> > + preferred_cpu = get_nohz_load_balancer();
> > + if (get_sysctl_timer_migration() && !pinned && preferred_cpu != -1
> > + && idle_cpu(current_cpu))
> > + new_cpu_base = &per_cpu(hrtimer_bases, preferred_cpu);
> > + else
> > + new_cpu_base = &__get_cpu_var(hrtimer_bases);
> >
> > - new_cpu_base = &__get_cpu_var(hrtimer_bases);
> > new_base = &new_cpu_base->clock_base[base->index];
>
> Hmm. This can lead to high latencies when you enqueue the timer on
> the other CPU simply because we can not reprogram the timer hardware
> on the other CPU in the CONFIG_HIGH_RES=y case.
>
> Let's assume we are on CPU0 and try to enqueue the timer on CPU1,
> where the next timer expiry is 5ms away. The timer which we enqueue
> is due in 500us. So you introduce 4.5ms latency.
>
We are moving timers to the ilb which wakes up every jiffy.
So we can move the timer to the ilb only if it's expiry
time is greater than 1 jiffy. Else we can fire on the same CPU.
This would prevent any latency from being added, right?
--arun
> Thanks,
>
> tglx
>
>
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