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

Mel Gorman
2013-07-11T13:03:28Z

On Thu, Jul 11, 2013 at 02:30:38PM +0200, Peter Zijlstra wrote:
> 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?

We do this

load_balance
-> active_load_balance_cpu_stop
-> move_one_task
-> can_migrate_task
-> migrate_improves_locality

If the conditions are right then it'll move the task to the preferred node
for a number of PTE scans. Of course there is no guarantee that the necessary
conditions will occur but I was wary of taking more drastic steps in the
scheduler such as retrying on every fault until the migration succeeds.

--
Mel Gorman
SUSE Labs