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

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Date:	Wed, 08 Apr 2015 19:39:15 -0700
From:	Jason Low <jason.low2@...com>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"riel@...hat.com" <riel@...hat.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"pjt@...gle.com" <pjt@...gle.com>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	"efault@....de" <efault@....de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"iamjoonsoo.kim@....com" <iamjoonsoo.kim@....com>,
	"svaidy@...ux.vnet.ibm.com" <svaidy@...ux.vnet.ibm.com>,
	"tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
	jason.low2@...com
Subject: Re: sched: Improve load balancing in the presence of idle CPUs

On Wed, 2015-04-08 at 16:42 +0530, Srikar Dronamraju wrote:
> * Jason Low <jason.low2@...com> [2015-04-07 17:07:46]:
> > @@ -7687,7 +7700,7 @@ static inline bool nohz_kick_needed(struct rq *rq)
> >  	int nr_busy, cpu = rq->cpu;
> >  	bool kick = false;
> > 
> > -	if (unlikely(rq->idle_balance))
> > +	if (unlikely(idle_cpu(cpu)))
> >  		return false;
> 
> 
> The only other place that we use idle_balance is
> run_rebalance_domains(). Would it make sense to just use idle_cpu() in
> run_rebalance_domains() and remove rq->idle_balance?

Hi Srikar,

So the idle_balance is used for storing the idle state of the CPU before
calling the softirq, for load balancing decisions. In that case, we may
need to keep this extra variable in order to store that information.

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