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

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Message-ID: <551B8FF3.70608@linux.vnet.ibm.com>
Date:	Wed, 01 Apr 2015 11:58:03 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Jason Low <jason.low2@...com>,
	Morten Rasmussen <morten.rasmussen@....com>
CC:	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>
Subject: Re: [PATCH V2] sched: Improve load balancing in the presence of idle
 CPUs

On 03/31/2015 11:00 PM, Jason Low wrote:
> 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.
> 

Alright I see. But it is one additional wake up. And the wake up will be
within the cluster. We will not wake up any CPU in the neighboring
cluster unless there are tasks to be pulled. So, we can wake up a core
out of a deep idle state and never a cluster in the problem described.
In terms of energy efficiency, this is not so bad a scenario, is it?

Regards
Preeti U Murthy

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