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

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Date:	Mon, 6 Apr 2009 20:58:43 +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 12:56:17]:

> Arun,
> 
> On Mon, 6 Apr 2009, Arun R Bharadwaj wrote:
> >  
> > +ktime_t clockevents_get_next_event(int cpu)
> > +{
> > +	struct tick_device *td;
> > +	struct clock_event_device *dev;
> > +
> > +	td = &per_cpu(tick_cpu_device, cpu);
> > +	dev = td->evtdev;
> > +
> > +	return dev->next_event;
> > +}
> > +
> 
> Preferrably this function should be in the clock events code and a
> stub inline function which returns KTIME_MAX for non clock events
> archs is probably necessary as well.
>

Sure.

> >  /*
> >   * Switch the timer base to the current CPU when possible.
> >   */
> > @@ -198,8 +211,17 @@ switch_hrtimer_base(struct hrtimer *time
> >  {
> >  	struct hrtimer_clock_base *new_base;
> >  	struct hrtimer_cpu_base *new_cpu_base;
> > +	int cpu, preferred_cpu = -1;
> > +
> > +	cpu = smp_processor_id();
> > +	if (get_sysctl_timer_migration() && !pinned && idle_cpu(cpu)) {
> > +		preferred_cpu = get_nohz_load_balancer();
> > +		if (preferred_cpu >= 0)
> > +			cpu = preferred_cpu;
> > +	}
> >  
> > -	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.

>   			    spin_unlock(&new_base->cpu_base->lock);
> 			    spin_lock(&base->cpu_base->lock);
>   
> > +				goto again;
> > +			}
> 
>   Also you need to move
> 
> >  		timer->base = new_base;
> 
>   here to avoid a stale timer->base setting.
>

The above takes care of this as well.

--arun
> > +		}
> >  	}
> >  	return new_base;
> >  }
> 
> Thanks,
> 
> 	tglx
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