linux-kernel - Re: sched: Improve load balancing in the presence of idle CPUs

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Message-ID: <1427828056.2492.24.camel@j-VirtualBox>
Date:	Tue, 31 Mar 2015 11:54:16 -0700
From:	Jason Low <jason.low2@...com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	peterz@...radead.org, mingo@...nel.org,
	Daniel Lezcano <daniel.lezcano@...aro.org>, riel@...hat.com,
	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: sched: Improve load balancing in the presence of idle CPUs

On Tue, 2015-03-31 at 14:07 +0530, Preeti U Murthy wrote:
> Hi Jason,
> 
> On 03/31/2015 12:25 AM, Jason Low wrote:
> > Hi Preeti,
> > 
> > I noticed that another commit 4a725627f21d converted the check in
> > nohz_kick_needed() from idle_cpu() to rq->idle_balance, causing a
> > potentially outdated value to be used if this cpu is able to pull tasks
> > using rebalance_domains(), and nohz_kick_needed() directly returning
> > false.
> 
> I see that rebalance_domains() will be run at the end of the scheduler
> tick interrupt handling. trigger_load_balance() only sets the softirq,
> it does not call rebalance_domains() immediately. So the call graph
> would be:

Oh right, since that only sets the softirq, this wouldn't be the issue,
though we would need these changes if we were to incorporate any sort of
nohz_kick_needed() logic into the nohz_idle_balance() code path correct?

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