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 07:16:03 +0800
From:	Wanpeng Li <wanpeng.li@...ux.intel.com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
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
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 09:48:45AM -0800, Tim Chen wrote:
>On Mon, 2015-01-19 at 13:45 +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;
>> 
>
>After this change, you will not start cfs load balance if you have one
>cfs task and a few other non-cfs tasks on a run-queue.  In this case if
>there's a less loaded run queue available, the cfs scheduler would not
>move the cfs task there. So you will be forcing the deadline and rt
>scheduler to move their tasks away.  In this case, a cfs task will
>behave like a "higher" priority task than the rt and deadline tasks in
>the sense that it forces the other classes of tasks to be moved to
>accommodate cfs tasks.  I don't think this is the right behavior.
>
>Also, add_nr_running could add more than one cfs tasks.  So if there are
>more than one cfs tasks being added, you should load balance and that
>need to be checked in add_nr_running if you make the change there.

Got it, thanks.

Regards,
Wanpeng Li 

>
>Tim
>
>> 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.
>
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