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

Peter Zijlstra
2013-07-11T12:31:37Z

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?