linux-kernel - Re: [PATCH 29/52] sched: Implement NUMA scanning backoff

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Date:	Mon, 03 Dec 2012 14:55:35 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Christoph Lameter <cl@...ux.com>, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [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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