linux-kernel - Re: [PATCH v6 5/7] sched/fair: fix idle balance when remaining tasks are all non-CFS tasks

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Date:	Tue, 20 Jan 2015 07:18:37 +0800
From:	Wanpeng Li <wanpeng.li@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Wanpeng Li <wanpeng.li@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@....com>,
	Kirill Tkhai <ktkhai@...allels.com>,
	linux-kernel@...r.kernel.org, riel@...hat.com, jason.low2@...com,
	fweisbec@...il.com, Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH v6 5/7] sched/fair: fix idle balance when remaining tasks
 are all non-CFS tasks

On Mon, Jan 19, 2015 at 01:45:28PM +0100, Peter Zijlstra wrote:
>On Wed, Nov 26, 2014 at 08:44:05AM +0800, Wanpeng Li wrote:
>> The overload indicator is used for knowing when we can totally avoid load
>> balancing to a cpu that is about to go idle. We can avoid load balancing
>> when no cpu has cfs task and both rt and deadline have push/pull mechanism
>> to do their own balancing.
>> 
>> However, rq->nr_running on behalf of the total number of each class tasks
>> on the cpu, do idle balance when remaining tasks are all non-CFS tasks does
>> not make any sense.
>> 
>> This patch fix it by idle balance when there are still other CFS tasks in
>> the rq's root domain.
>> 
>
>Please always try and Cc the people who touched that code last; for the
>idle_balance bits commit 4486edd12b5a ("sched/fair: Implement fast
>idling of CPUs when the system is partially loaded") gives a fair clue
>as to who that would be.
>
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index 31f1e4d..f7dd978 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -1269,7 +1269,8 @@ static inline void add_nr_running(struct rq *rq, unsigned count)
>>  
>>  	rq->nr_running = prev_nr + count;
>>  
>> -	if (prev_nr < 2 && rq->nr_running >= 2) {
>> +	if (prev_nr < 2 && rq->nr_running >= 2 &&
>> +		rq->cfs.h_nr_running > 0) {
>>  #ifdef CONFIG_SMP
>>  		if (!rq->rd->overload)
>>  			rq->rd->overload = true;
>
>Here 3882ec643997 ("nohz: Use IPI implicit full barrier against
>rq->nr_running r/w") might be a clue.
>
>Also, this is wrong, it breaks NOHZ_FULL.

Got it. If the modification in update_sg_lb_stats of my patch is still
corrrect, if so I can send out a new version.

Regards,
Wanpeng Li 
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