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SubjectRe: [PATCH 29/52] sched: Implement NUMA scanning backoff
On 12/02/2012 01:43 PM, Ingo Molnar wrote:
> Back off slowly from scanning, up to sysctl_sched_numa_scan_period_max
> (1.6 seconds). Scan faster again if we were forced to switch to
> another node.

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8f0e6ba..59fea2e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -865,8 +865,10 @@ static void task_numa_placement(struct task_struct *p)
> }
> }
>
> - if (max_node != p->numa_max_node)
> + if (max_node != p->numa_max_node) {
> sched_setnuma(p, max_node, task_numa_shared(p));
> + goto out_backoff;
> + }
>
> p->numa_migrate_seq++;
> if (sched_feat(NUMA_SETTLE) &&

Is that correct?

It looks like the code only jumps to the out_backoff label
after resetting p->numa_scan_period to sysctl_sched_numa_scan_period_min
in sched_setnuma?

Should it not be the other way around, slowly increasing the process's
numa_scan_period when we do NOT do a sched_setnuma call for the process
at all?

> @@ -882,7 +884,11 @@ static void task_numa_placement(struct task_struct *p)
> if (shared != task_numa_shared(p)) {
> sched_setnuma(p, p->numa_max_node, shared);
> p->numa_migrate_seq = 0;
> + goto out_backoff;
> }
> + return;

We can never reach the backoff code, except by an explicit goto,
which is only there after a call to sched_setnuma.

That is the opposite from what the changelog suggests...

> +out_backoff:
> + p->numa_scan_period = min(p->numa_scan_period * 2, sysctl_sched_numa_scan_period_max);
> }
>
> /*
>



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