Re: [PATCH 05/16] sched: Select a preferred node with the most numa hinting faults

Mel Gorman
2013-07-11T12:53:08Z

On Thu, Jul 11, 2013 at 01:23:59PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 11, 2013 at 10:46:49AM +0100, Mel Gorman wrote:
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1593,6 +1593,7 @@ static void __sched_fork(struct task_struct *p)
> > p->numa_scan_seq = p->mm ? p->mm->numa_scan_seq : 0;
> > p->numa_migrate_seq = p->mm ? p->mm->numa_scan_seq - 1 : 0;
> > p->numa_scan_period = sysctl_numa_balancing_scan_delay;
> > + p->numa_preferred_nid = -1;
>
> (1)
>
> > p->numa_work.next = &p->numa_work;
> > p->numa_faults = NULL;
> > #endif /* CONFIG_NUMA_BALANCING */
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 904fd6f..c0bee41 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -793,7 +793,8 @@ unsigned int sysctl_numa_balancing_scan_delay = 1000;
> >
> > static void task_numa_placement(struct task_struct *p)
> > {
> > - int seq;
> > + int seq, nid, max_nid = 0;
>
> Should you not start with max_nid = -1?
>

For consistency, yes.

> > + unsigned long max_faults = 0;
> >
> > if (!p->mm) /* for example, ksmd faulting in a user's mm */
> > return;
> > @@ -802,7 +803,19 @@ static void task_numa_placement(struct task_struct *p)
> > return;
> > p->numa_scan_seq = seq;
> >
> > - /* FIXME: Scheduling placement policy hints go here */
> > + /* Find the node with the highest number of faults */
> > + for (nid = 0; nid < nr_node_ids; nid++) {
> > + unsigned long faults = p->numa_faults[nid];
> > + p->numa_faults[nid] >>= 1;
> > + if (faults > max_faults) {
> > + max_faults = faults;
> > + max_nid = nid;
> > + }
> > + }
> > +
>
> It is rather unlikely; but suppose the entire ->numa_faults[] array is 0, you'd
> somehow end up selecting max_nid := 0. Which seems inconsistent with \1.
>

This happens more often than you might imagine due to THP false sharing
getting count as shared. If the entire numa_faults[] array is 0 then
max_faults == 0 and it will not update preferred_nid due to the
"max_faults" check below.

> > + /* Update the tasks preferred node if necessary */
> > + if (max_faults && max_nid != p->numa_preferred_nid)
> > + p->numa_preferred_nid = max_nid;
> > }
> >

--
Mel Gorman
SUSE Labs