linux-kernel - Re: sched: Improve load balancing in the presence of idle CPUs

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Date:	Wed, 01 Apr 2015 19:11:52 -0700
From:	Jason Low <jason.low2@...com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	peterz@...radead.org, mingo@...nel.org,
	Daniel Lezcano <daniel.lezcano@...aro.org>, riel@...hat.com,
	vincent.guittot@...aro.org, srikar@...ux.vnet.ibm.com,
	pjt@...gle.com, benh@...nel.crashing.org, efault@....de,
	linux-kernel@...r.kernel.org, iamjoonsoo.kim@....com,
	svaidy@...ux.vnet.ibm.com, tim.c.chen@...ux.intel.com,
	morten.rasmussen@....com, jason.low2@...com
Subject: Re: sched: Improve load balancing in the presence of idle CPUs

On Tue, 2015-03-31 at 14:07 +0530, Preeti U Murthy wrote:
> On 03/31/2015 12:25 AM, Jason Low wrote:

> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index fdae26e..ba8ec1a 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -7644,7 +7644,7 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> >  		 * balancing owner will pick it up.
> >  		 */
> >  		if (need_resched())
> > -			break;
> > +			goto end;
> 
> Why is this hunk needed?

In terms of the change in the need_resched() case, if the current CPU
doesn't complete iterating all of the CPUs, then this will make it not
update nohz.next_balance. This is so we can continue the balancing with
the next balancing owner without too much delay.

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