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

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Date:	Tue, 31 Mar 2015 10:30:08 -0700
From:	Jason Low <jason.low2@...com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	Morten Rasmussen <morten.rasmussen@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	"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
Subject: Re: [PATCH V2] sched: Improve load balancing in the presence of
 idle CPUs

On Tue, 2015-03-31 at 14:28 +0530, Preeti U Murthy wrote:

> Morten,

> I am a bit confused about the problem you are pointing to.

> I am unable to see the issue. What is it that I am missing ?

Hi Preeti,

Here is one of the potential issues that have been described from my
understanding.

In situations where there are just a few tasks to pull (for example,
there's 1 task to move).

Before, if CPU 1 calls run_rebalance_domains(), we'll pull the tasks to
this CPU 1 (which is already awake) and run the task on CPU 1.

Now, we'll pull the task to some idle CPU 2 and wake up CPU 2 in order
for the task to run. Meanwhile, CPU 1 may go idle, instead of running
the task on CPU 1 which was already awake.

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