linux-kernel - Re: [RFC] [PATCH 1/1] hrtimers: Cache next hrtimer

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Date:	Mon, 31 Aug 2009 09:08:26 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Ashwin Chaugule <ashwinc@...eaurora.org>
cc:	linux-kernel@...r.kernel.org, mingo@...hat.com
Subject: Re: [RFC] [PATCH 1/1] hrtimers: Cache next hrtimer

On Mon, 31 Aug 2009, Ashwin Chaugule wrote:
> Thomas Gleixner wrote:
> 
> >   That's not hard to fix by allowing the reprogramming to skip when the
> > new expiry time is the same as the old one.
> > 
> > I think that allowing the reprogram to skip is catching more cases
> > than the cached pointer. If the cached pointer is the one which gets
> > removed we might still reprogram with the same expiry value.
> >   
> Um. Wouldn't the cached pointer point to the first (oldest) hrtimer in the
> series of timers with the same expires value ? Then it would be the last
> hrtimer to be removed. I'm walking through the rbtree now to confirm this.
> 
> > Can you please try the delta patch on top of the last one I sent ?
> >   
> This looks very good ! Our results are almost similar now. However, I think
> that with this new
> patch we're still arming the timer needlessly at least once. This is the
> case when we're trying to remove the first (oldest) hrtimer with
> timer->expires = cpu->expires_next, but we forgo the reprogramming, when we
> really shouldn't. At this point, with the current scheme of things, we will
> needlessly wakeup and simply correct the expires_next value by
> traversing up the rbtree, to the parent node.

That only happens, when that timer is the last one in both trees. Then
the resulting reprogramming value is KTIME_MAX and we skip the
reprogramming. That's easy to fix by removing the KTIME_MAX check.
 
> If we knew in advance that this to-be-removed timer, was the oldest hrtimer
> of the series, then we could force reprogram, such that we wake up only when
> the parent node timer is really going to expire. This may make a noticeable
> difference in power for some devices. 
>
> Another question is, what happens when base->first of REALTIME and MONOTONIC
> both have the same expires ?

The skip_equal check covers both. We find out which timer of the two
bases is expiring next and compare the result against
cpu_base->expires_next. If both are identical we skip.

Thanks,

	tglx

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