linux-kernel - Re: [PATCH 18/18] sched: Swap tasks when reschuling if a CPU on a target node is imbalanced

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Date:	Thu, 1 Aug 2013 16:48:58 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 18/18] sched: Swap tasks when reschuling if a CPU on a
 target node is imbalanced

On Thu, Aug 01, 2013 at 10:29:58AM +0530, Srikar Dronamraju wrote:
> > @@ -904,6 +908,8 @@ static int task_numa_find_cpu(struct task_struct *p, int nid)
> >  	src_eff_load *= src_load + effective_load(tg, src_cpu, -weight, -weight);
> > 
> >  	for_each_cpu(cpu, cpumask_of_node(nid)) {
> > +		struct task_struct *swap_candidate = NULL;
> > +
> >  		dst_load = target_load(cpu, idx);
> > 
> >  		/* If the CPU is idle, use it */
> > @@ -922,12 +928,41 @@ static int task_numa_find_cpu(struct task_struct *p, int nid)
> >  		 * migrate to its preferred node due to load imbalances.
> >  		 */
> >  		balanced = (dst_eff_load <= src_eff_load);
> > -		if (!balanced)
> > -			continue;
> > +		if (!balanced) {
> > +			struct rq *rq = cpu_rq(cpu);
> > +			unsigned long src_faults, dst_faults;
> > +
> > +			/* Do not move tasks off their preferred node */
> > +			if (rq->curr->numa_preferred_nid == nid)
> > +				continue;
> > +
> > +			/* Do not attempt an illegal migration */
> > +			if (!cpumask_test_cpu(cpu, tsk_cpus_allowed(rq->curr)))
> > +				continue;
> > +
> > +			/*
> > +			 * Do not impair locality for the swap candidate.
> > +			 * Destination for the swap candidate is the source cpu
> > +			 */
> > +			if (rq->curr->numa_faults) {
> > +				src_faults = rq->curr->numa_faults[task_faults_idx(nid, 1)];
> > +				dst_faults = rq->curr->numa_faults[task_faults_idx(src_cpu_node, 1)];
> > +				if (src_faults > dst_faults)
> > +					continue;
> > +			}
> > +
> > +			/*
> > +			 * The destination is overloaded but running a task
> > +			 * that is not running on its preferred node. Consider
> > +			 * swapping the CPU tasks are running on.
> > +			 */
> > +			swap_candidate = rq->curr;
> > +		}
> > 
> >  		if (dst_load < min_load) {
> >  			min_load = dst_load;
> >  			dst_cpu = cpu;
> > +			*swap_p = swap_candidate;
> 
> Are we some times passing a wrong candidate?
> Lets say the first cpu balanced is false and we set the swap_candidate,
> but find the second cpu(/or later cpus) to be idle or has lesser effective load, then we
> could be sending the task that is running on the first cpu as the swap
> candidate.

Then at the second or later CPU swap_candidate == NULL so swap_p is
cleared too.

-- 
Mel Gorman
SUSE Labs
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