linux-kernel - Re: [PATCH V2] sched: Improve load balancing in the presence of idle CPUs

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Message-ID: <20150330125450.GL24151@twins.programming.kicks-ass.net>
Date:	Mon, 30 Mar 2015 14:54:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"riel@...hat.com" <riel@...hat.com>,
	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"srikar@...ux.vnet.ibm.com" <srikar@...ux.vnet.ibm.com>,
	"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" <jason.low2@...com>
Subject: Re: [PATCH V2] sched: Improve load balancing in the presence of idle
 CPUs

On Mon, Mar 30, 2015 at 02:24:49PM +0200, Peter Zijlstra wrote:

> @@ -7647,6 +7648,8 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>  			break;
>  
>  		rq = cpu_rq(balance_cpu);
> +		if (rq == this_rq)
> +			done = true;
>  
>  		/*
>  		 * If time for next balance is due,

Equally note that if you change find_new_ilb() to not pick the very
first cpu in the mask you already get the bouncy bounce, because this
routine will in fact try and move things to the first cpu first.

So if you go change find_new_ilb() you should also amend the iteration
order here.
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