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

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Date:	Wed, 22 Oct 2014 14:39:04 +0900
From:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>
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()

(2014/10/21 18:21), Peter Zijlstra wrote:
> 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.
>

When (private + shared) was not 0, I did not want to change behavior of
update_task_scan_period(). But I understood your comment. I'll update it.

Thanks,
Yasuaki Ishimatsu

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