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

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Date:	Mon, 6 Apr 2009 12:56:17 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Arun R Bharadwaj <arun@...ux.vnet.ibm.com>
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
Subject: Re: [v4 RFC PATCH 4/4] timers: logic to move non pinned timers

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.

>  /*
>   * 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:

  			    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.

> +		}
>  	}
>  	return new_base;
>  }

Thanks,

	tglx
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