linux-kernel - Re: [PATCH v2] sched/fair: Care divide error in update_task_scan

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Date:	Tue, 21 Oct 2014 11:21:12 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
Cc:	mingo@...hat.com, kernellwp@...il.com, riel@...hat.com,
	tkhai@...dex.ru, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/fair: Care divide error in
 update_task_scan_period()

On Thu, Oct 16, 2014 at 06:48:15PM +0900, Yasuaki Ishimatsu wrote:
> +++ b/kernel/sched/fair.c
> @@ -1466,6 +1466,7 @@ static void update_task_scan_period(struct task_struct *p,
> 
>  	unsigned long remote = p->numa_faults_locality[0];
>  	unsigned long local = p->numa_faults_locality[1];
> +	unsigned long total_faults = shared + private;
> 
>  	/*
>  	 * If there were no record hinting faults then either the task is
> @@ -1496,6 +1497,14 @@ static void update_task_scan_period(struct task_struct *p,
>  			slot = 1;
>  		diff = slot * period_slot;
>  	} else {
> +		/*
> +		 * This is a rare case. total_faults might become 0 after
> +		 * offlining node. In this case, total_faults is set to 1
> +		 * for avoiding divide error.
> +		 */
> +		if (unlikely(total_faults == 0))
> +			total_faults = 1;
> +
>  		diff = -(NUMA_PERIOD_THRESHOLD - ratio) * period_slot;
> 
>  		/*
> @@ -1506,7 +1515,7 @@ static void update_task_scan_period(struct task_struct *p,
>  		 * scanning faster if shared accesses dominate as it may
>  		 * simply bounce migrations uselessly
>  		 */
> -		ratio = DIV_ROUND_UP(private * NUMA_PERIOD_SLOTS, (private + shared));
> +		ratio = DIV_ROUND_UP(private * NUMA_PERIOD_SLOTS, (total_faults));
>  		diff = (diff * ratio) / NUMA_PERIOD_SLOTS;

So what was wrong with the 'normal' unconditional +1 approach? Also
you've got superfluous parenthese.
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