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

Peter Zijlstra
2013-07-11T11:24:50Z

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?

> + 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.

> + /* Update the tasks preferred node if necessary */
> + if (max_faults && max_nid != p->numa_preferred_nid)
> + p->numa_preferred_nid = max_nid;
> }
>
> /*
> --
> 1.8.1.4
>