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

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Message-ID: <1427432472.2788.4.camel@j-VirtualBox>
Date:	Thu, 26 Mar 2015 22:01:12 -0700
From:	Jason Low <jason.low2@...com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	Wanpeng Li <wanpeng.li@...ux.intel.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 10:03 +0530, Preeti U Murthy wrote:
> Hi Wanpeng
> 
> On 03/27/2015 07:42 AM, 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?
> 
> When the ILB CPU pulls load in rebalance_domains(), its idle state
> is set to CPU_NOT_IDLE.
> 
> ""
> idle = idle_cpu(cpu) ? CPU_IDLE : CPU_NOT_IDLE;

Hi Preeti,

The "idle" variable is a local variable to the rebalance_domains()
function. In that case, that shouldn't have an affect on the idle value
that gets passed to nohz_idle_balance().

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