Re: [PATCH] clockevents: avoid unnecessary reprograming of event timer

[Ingo Molnar <mingo@elte.hu>]

* Thomas Gleixner <tglx@linutronix.de> wrote:

> On Fri, 13 Feb 2009, Ingo Molnar wrote:
> > > +	if (ktime_equal(dev->next_event, expires))
> > > +		return 0;
> > > +
> > 
> > Hm, given that a good high-res source has nanoseconds resolution,
> > what's the chance of this optimization triggering in practice?
> > Near zero i think - unless we trigger useless reprogramming without
> > having added or removed any new timers - but then we should
> > concentrate on analyzing the reason for that redundant reprogramming.
> > 
> > Does it trigger often for you?
> 
> The only point where this should trigger is when we come out of C2/C3
> and switch the local APIC timer back on. There we reload the
> next_event which was discarded in the hardware before we entered C2/C3
> and switched to the broadcast timer. So this change would prevent the
> rearming of the local APIC after coming out of C2/C3 and probably give
> us some new C-State related headaches.

yes, that was my other thought. Albeit arguably the CE code should
set next_event to -1 or so when suspending, out of caution, right?

	Ingo