linux-kernel - Re: [PATCH 1/6] hrtimer: Provide clock_was_set

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Date:	Wed, 11 Jul 2012 17:56:25 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Prarit Bhargava <prarit@...hat.com>,
	John Stultz <johnstul@...ibm.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, stable@...r.kernel.org
Subject: Re: [PATCH 1/6] hrtimer: Provide clock_was_set_delayed()

On Wed, 2012-07-11 at 17:18 +0200, Thomas Gleixner wrote:

> Right. I think with the atomic update of the offset in the timer
> interrupt we are on the safe side. The main problem of timers expiring
> early forever is covered by this.
> 
> Thinking more about it.
> 
> If time goes backwards, then the IPI is pointless. The already armed
> clockevent device will fire too early, hrtimer_interrupt will update
> and just rearm it. That's one "spurious" event.
> 
> So we only need it in the case of time going forward. 
> 
> Though with the leap second the maximum observable delay is 1 second
> on a completely idle core. Surely nothing to worry about for an event
> which happens rarely. So we could safely avoid the whole delayed
> business and just do the timerfd notification, though I wonder if even
> that is necessary in the leap second case.
> 
> On NOHZ=n systems the IPI is pointless as well. The maximum lateness
> will be 10ms for HZ=100. Nothing we should worry about.
> 
> That leaves NOHZ enabled systems and there we might be clever and
> avoid the IPIs to those cores which are not idle and let the tick
> interrupt deal with it. And we can make the calls async and just let
> them raise the hrtimer softirq on those cores, which will run the
> hrtimer interrupt code and take care of everything.
> 
> Thoughts?


static void nohz_hrtimer_softirq(void *unused)
{
	raise_softirq(HRTIMER_SOFTIRQ);
}

static void kick_nohz_cpus(void)
{
	smp_call_function_many(nohz.idle_cpus_mask, nohz_hrtimer_softirq, NULL, 0);
}

Same problem as before though, can't be sending IPIs while in hardirq
context.. 

And you cannot do the same trick with a CFD as with the CSD, some CPUs
might need it again while others are still pending.

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