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

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Date:	Thu, 26 Mar 2015 22:07:21 -0700
From:	Jason Low <jason.low2@...com>
To:	Wanpeng Li <wanpeng.li@...ux.intel.com>
Cc:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>, peterz@...radead.org,
	mingo@...nel.org, riel@...hat.com, daniel.lezcano@...aro.org,
	vincent.guittot@...aro.org, srikar@...ux.vnet.ibm.com,
	pjt@...gle.com, benh@...nel.crashing.org, efault@....de,
	linux-kernel@...r.kernel.org, iamjoonsoo.kim@....com,
	svaidy@...ux.vnet.ibm.com, tim.c.chen@...ux.intel.com,
	morten.rasmussen@....com, jason.low2@...com
Subject: Re: [PATCH V2] sched: Improve load balancing in the presence of
 idle CPUs

On Fri, 2015-03-27 at 10:12 +0800, Wanpeng Li wrote:
> Hi Preeti,
> On Thu, Mar 26, 2015 at 06:32:44PM +0530, Preeti U Murthy wrote:
> >
> >1. An ILB CPU was chosen from the first numa domain to trigger nohz idle
> >load balancing [Given the experiment, upto 6 CPUs per core could be
> >potentially idle in this domain.]
> >
> >2. However the ILB CPU would call load_balance() on itself before
> >initiating nohz idle load balancing.
> >
> >3. Given cores are SMT8, the ILB CPU had enough opportunities to pull
> >tasks from its sibling cores to even out load.
> >
> >4. Now that the ILB CPU was no longer idle, it would abort nohz idle
> >load balancing
> 
> I don't see abort nohz idle load balancing when ILB CPU was no longer idle 
> in nohz_idle_balance(), could you explain more in details?

Hi Wanpeng,

In nohz_idle_balance(), there is a check for need_resched() so if the
cpu has something to run, it should exit nohz_idle_balance(), which may
cause it to not do the idle balancing on the other CPUs.



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