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

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Date:	Thu, 02 Apr 2015 10:22:07 -0700
From:	Jason Low <jason.low2@...com>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"riel@...hat.com" <riel@...hat.com>,
	"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: sched: Improve load balancing in the presence of idle CPUs

On Thu, 2015-04-02 at 10:17 +0100, Morten Rasmussen wrote:
> On Thu, Apr 02, 2015 at 06:59:07AM +0100, Jason Low wrote:

> > Also, below is an example patch.
> > 
> > (Without the conversion to idle_cpu(), the check for rq->idle_balance
> > would not be accurate anymore)

> I think this should reduce the latency Preeti is seeing and avoid
> unnecessary wake-ups, however, it may not be quite as aggressive in
> spreading tasks quickly. It will stop the chain-of-kicks as soon as the
> balancer cpu has pulled only one task. The source cpu may still be
> having two tasks and other cpus may still have more than two tasks
> running.

Yeah, good point. I'll wait and see if Preeti finds this to improve
scheduling behavior. If this only helps a little though, we can also try
to make it more aggressive in spreading tasks.

> Depending on how bad it is, we could consider kicking another cpu if the
> imbalance is still significant after the balancer cpu has pulled a task.


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