linux-kernel - Re: [PATCH 08/16] sched: Reschedule task on preferred NUMA node once selected

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Date:	Thu, 11 Jul 2013 14:30:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	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 08/16] sched: Reschedule task on preferred NUMA node once
 selected

On Thu, Jul 11, 2013 at 10:46:52AM +0100, Mel Gorman wrote:
> @@ -829,10 +854,29 @@ static void task_numa_placement(struct task_struct *p)
>  		}
>  	}
>  
> -	/* Update the tasks preferred node if necessary */
> +	/*
> +	 * Record the preferred node as the node with the most faults,
> +	 * requeue the task to be running on the idlest CPU on the
> +	 * preferred node and reset the scanning rate to recheck
> +	 * the working set placement.
> +	 */
>  	if (max_faults && max_nid != p->numa_preferred_nid) {
> +		int preferred_cpu;
> +
> +		/*
> +		 * If the task is not on the preferred node then find the most
> +		 * idle CPU to migrate to.
> +		 */
> +		preferred_cpu = task_cpu(p);
> +		if (cpu_to_node(preferred_cpu) != max_nid) {
> +			preferred_cpu = find_idlest_cpu_node(preferred_cpu,
> +							     max_nid);
> +		}
> +
> +		/* Update the preferred nid and migrate task if possible */
>  		p->numa_preferred_nid = max_nid;
>  		p->numa_migrate_seq = 0;
> +		migrate_task_to(p, preferred_cpu);
>  	}
>  }

Now what happens if the migrations fails? We set numa_preferred_nid to max_nid
but then never re-try the migration. Should we not re-try the migration every
so often, regardless of whether max_nid changed?
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