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

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Message-ID: <alpine.LFD.2.00.0902131029330.16972@localhost.localdomain>
Date:	Fri, 13 Feb 2009 10:39:24 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Michael Davidson <md@...gle.com>, mbligh@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] clockevents: avoid unnecessary reprograming of event
 timer

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.

Thanks,

	tglx
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