linux-kernel - Re: [PATCH 3/8] sched: don't consider other cpus in our group in case of NEWLY

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Date:	Wed, 20 Mar 2013 15:52:18 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/8] sched: don't consider other cpus in our group in
 case of NEWLY_IDLE

On Tue, Mar 19, 2013 at 03:20:57PM +0100, Peter Zijlstra wrote:
> On Thu, 2013-02-14 at 14:48 +0900, Joonsoo Kim wrote:
> > Commit 88b8dac0 makes load_balance() consider other cpus in its group,
> > regardless of idle type. When we do NEWLY_IDLE balancing, we should not
> > consider it, because a motivation of NEWLY_IDLE balancing is to turn
> > this cpu to non idle state if needed. This is not the case of other cpus.
> > So, change code not to consider other cpus for NEWLY_IDLE balancing.
> > 
> > With this patch, assign 'if (pulled_task) this_rq->idle_stamp = 0'
> > in idle_balance() is corrected, because NEWLY_IDLE balancing doesn't
> > consider other cpus. Assigning to 'this_rq->idle_stamp' is now valid.
> > 
> > Cc: Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> 
> Fair enough, good catch.
> 
> Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 0c6aaf6..97498f4 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5016,8 +5016,15 @@ static int load_balance(int this_cpu, struct rq *this_rq,
> >  		.cpus		= cpus,
> >  	};
> >  
> > +	/* For NEWLY_IDLE load_balancing, we don't need to consider
> > +	 * other cpus in our group */
> > +	if (idle == CPU_NEWLY_IDLE) {
> > +		env.dst_grpmask = NULL;
> > +		max_lb_iterations = 0;
> 
> Just a small nit; I don't think we'll ever get to evaluate
> max_lb_iterations when !dst_grpmask. So strictly speaking its
> superfluous to touch it.

Okay. In next spin, I will remove it and add a comment here.

Thanks.

> 
> > +	} else {
> > +		max_lb_iterations = cpumask_weight(env.dst_grpmask);
> > +	}
> >  	cpumask_copy(cpus, cpu_active_mask);
> > -	max_lb_iterations = cpumask_weight(env.dst_grpmask);
> >  
> >  	schedstat_inc(sd, lb_count[idle]);
> >  
> 
> 
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