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Date:	Thu, 4 Apr 2013 09:55:17 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Mike Galbraith <efault@....de>, Paul Turner <pjt@...gle.com>,
	Alex Shi <alex.shi@...el.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH 2/5] sched: factor out code to should_we_balance()

Hello, Peter.

On Tue, Apr 02, 2013 at 12:29:42PM +0200, Peter Zijlstra wrote:
> On Tue, 2013-04-02 at 12:00 +0200, Peter Zijlstra wrote:
> > On Tue, 2013-04-02 at 18:50 +0900, Joonsoo Kim wrote:
> > > 
> > > It seems that there is some misunderstanding about this patch.
> > > In this patch, we don't iterate all groups. Instead, we iterate on
> > > cpus of local sched_group only. So there is no penalty you mentioned.
> > 
> > OK, I'll go stare at it again..
> 
> Ah, I see, you're doing should_we_balance() _before_
> find_busiest_group() and instead you're doing another for_each_cpu() in
> there.
> 
> I'd write the thing like:
> 
> static bool should_we_balance(struct lb_env *env)
> {
> 	struct sched_group *sg = env->sd->groups;
> 	struct cpumask *sg_cpus, *sg_mask;
> 	int cpu, balance_cpu = -1;
> 
> 	if (env->idle == CPU_NEWLY_IDLE)
> 		return true;
> 
> 	sg_cpus = sched_group_cpus(sg);
> 	sg_mask = sched_group_mask(sg);
> 
> 	for_each_cpu_and(cpu, sg_cpus, env->cpus) {
> 		if (!cpumask_test_cpu(cpu, sg_mask))
> 			continue;
> 
> 		if (!idle_cpu(cpu))
> 			continue;
> 
> 		balance_cpu = cpu;
> 		break;
> 	}
> 
> 	if (balance_cpu == -1)
> 		balance_cpu = group_balance_cpu(sg);
> 
> 	return balance_cpu == env->dst_cpu;
> }

Okay. It looks nice.

> 
> I also considered doing the group_balance_cpu() first to avoid having
> to do the idle_cpu() scan, but that's a slight behavioural change
> afaict.

In my quick thought, we can avoid it through below way.

balance_cpu = group_balance_cpu(sg);
if (idle_cpu(balance_cpu))
	return balance_cpu == env->dst_cpu;
else
	do idle_cpus() scan loop

Is it your idea? If not, please let me know your idea.

Thanks.

> 
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