linux-kernel - Re: [PATCH] sched/fair: Care divide error in update_task_scan

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Date:	Wed, 8 Oct 2014 10:31:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org, riel@...hat.com,
	tkhai@...dex.ru, mgorman@...e.de
Subject: Re: [PATCH] sched/fair: Care divide error in
 update_task_scan_period()

On Wed, Oct 08, 2014 at 03:43:11PM +0900, Yasuaki Ishimatsu wrote:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index bfa3c86..fb7dc3f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1496,18 +1496,26 @@ static void update_task_scan_period(struct task_struct *p,
>  			slot = 1;
>  		diff = slot * period_slot;
>  	} else {
> -		diff = -(NUMA_PERIOD_THRESHOLD - ratio) * period_slot;
> +		if (unlikely((private + shared) == 0))
> +			/*
> +			 * This is a rare case. The trigger is node offline.
> +			 */
> +			diff = 0;
> +		else {
> +			diff = -(NUMA_PERIOD_THRESHOLD - ratio) * period_slot;
> 
> -		/*
> -		 * Scale scan rate increases based on sharing. There is an
> -		 * inverse relationship between the degree of sharing and
> -		 * the adjustment made to the scanning period. Broadly
> -		 * speaking the intent is that there is little point
> -		 * scanning faster if shared accesses dominate as it may
> -		 * simply bounce migrations uselessly
> -		 */
> -		ratio = DIV_ROUND_UP(private * NUMA_PERIOD_SLOTS, (private + shared));
> -		diff = (diff * ratio) / NUMA_PERIOD_SLOTS;
> +			/*
> +			 * Scale scan rate increases based on sharing. There is
> +			 * an inverse relationship between the degree of sharing
> +			 * and the adjustment made to the scanning period.
> +			 * Broadly speaking the intent is that there is little
> +			 * point scanning faster if shared accesses dominate as
> +			 * it may simply bounce migrations uselessly
> +			 */
> +			ratio = DIV_ROUND_UP(private * NUMA_PERIOD_SLOTS,
> +							(private + shared));
> +			diff = (diff * ratio) / NUMA_PERIOD_SLOTS;
> +		}
>  	}
> 
>  	p->numa_scan_period = clamp(p->numa_scan_period + diff,

Yeah, so I don't like the patch nor do I really like the function as it
stands -- which I suppose is part of why I don't like the patch.

The problem I have with the function is that its very inconsistent in
behaviour. In the early return path it sets numa_scan_period and
numa_next_scan, in the later return path it sets numa_scan_period and
numa_faults_locality.

I feel both return paths should affect the same set of variables, esp.
the non clearing of numa_faults_locality in the early path seems weird.

The thing I suppose I don't like about the patch is its added
indentation and the fact that the simple +1 thing wasn't considered.
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