linux-kernel - Re: [v4 RFC PATCH 4/4] timers: logic to move non pinned timers

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Date:	Mon, 6 Apr 2009 21:30:39 +0530
From:	Arun R Bharadwaj <arun@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org, linux-pm@...ts.linux-foundation.org,
	a.p.zijlstra@...llo.nl, ego@...ibm.com, mingo@...e.hu,
	andi@...stfloor.org, venkatesh.pallipadi@...el.com,
	vatsa@...ux.vnet.ibm.com, arjan@...radead.org,
	svaidy@...ux.vnet.ibm.com, Arun Bharadwaj <arun@...ux.vnet.ibm.com>
Subject: Re: [v4 RFC PATCH 4/4] timers: logic to move non pinned timers

* Thomas Gleixner <tglx@...utronix.de> [2009-04-06 17:35:09]:

> Arun,
> 
> On Mon, 6 Apr 2009, Arun R Bharadwaj wrote:
> > > > -	new_cpu_base = &__get_cpu_var(hrtimer_bases);
> > > > +again:
> > > > +	new_cpu_base = &per_cpu(hrtimer_bases, cpu);
> > > >  	new_base = &new_cpu_base->clock_base[base->index];
> > > >  
> > > >  	if (base != new_base) {
> > > > @@ -220,6 +242,32 @@ switch_hrtimer_base(struct hrtimer *time
> > > >  		spin_unlock(&base->cpu_base->lock);
> > > >  		spin_lock(&new_base->cpu_base->lock);
> > > >  		timer->base = new_base;
> > > > +
> > > > +		if (cpu == preferred_cpu) {
> > > > +			/* Calculate clock monotonic expiry time */
> > > > +			ktime_t expires = ktime_sub(hrtimer_get_expires(timer),
> > > > +							new_base->offset);
> > > > +
> > > > +			/*
> > > > +			 * Get the next event on target cpu from the
> > > > +			 * clock events layer.
> > > > +			 * This covers the highres=off nohz=on case as well.
> > > > +			 */
> > > > +			ktime_t next = clockevents_get_next_event(cpu);
> > > > +
> > > > +			ktime_t delta = ktime_sub(expires, next);
> > > > +
> > > > +			/*
> > > > +			 * We do not migrate the timer when it is expiring
> > > > +			 * before the next event on the target cpu because
> > > > +			 * we cannot reprogram the target cpu hardware and
> > > > +			 * we would cause it to fire late.
> > > > +			 */
> > > > +			if (delta.tv64 < 0) {
> > > > +				cpu = smp_processor_id();
> > > 
> > >   You are missing a small but fatal detail here: You hold
> > >   new_base->cpu_base->lock. So you need to do:
> > >
> > 
> > I just moved the if block.. if (cpu==preferred_cpu) above the base
> > locking part to avoid the extra unlocking.
> 
> That's not a good idea. You want to look at the next event with the
> base lock of the other cpu held. That prevents that expires the first
> pending timer right after you checked next_event and before you queue
> your timer, which then might become the first timer to expire but you
> can't reprogram the clock event device on the other cpu.
>

Okay... I understand. Thanks for the explanation :)
Will post all the changes together in a separate thread.

--arun
> Thanks,
> 
> 	tglx
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