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

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Date:	Mon, 30 Mar 2015 13:03:03 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	"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>,
	"jason.low2@...com" <jason.low2@...com>
Subject: Re: [PATCH V2] sched: Improve load balancing in the presence of idle
 CPUs

On Mon, Mar 30, 2015 at 12:06:32PM +0100, Peter Zijlstra wrote:
> On Fri, Mar 27, 2015 at 05:56:51PM +0000, Morten Rasmussen wrote:
> 
> > I agree that it is hard to predict how many additional cpus you need,
> > but I don't think you necessarily need that information as long as you
> > start by filling up the cpu that was kicked to do the
> > nohz_idle_balance() first.
> 
> > Reducing unnecessary wakeups is quite important for energy consumption
> > and something a lot of effort is put into. You really don't want to wake
> > up another cluster/package unnecessarily just because there was only one
> > nohz-idle cpu left in the previous one which could have handled the
> > additional load. It gets even worse if the other cluster is less
> > energy-efficient (big.LITTLE).
> 
> So the only way to get tasks to cross your cluster is by balancing that
> domain. At this point we'll compute sg stats for either group
> (=cluster).
> 
> The only thing we need to ensure is that it doesn't view the small
> cluster as overloaded (as long as it really isn't of course), as long as
> its not viewed as overloaded it will not pull tasks from it into the big
> cluster, no matter how many ILBs we run before the ILB duty cpu's
> rebalance_domains() call.
> 
> I'm really not seeing the problem here.

I see. The group_classify() should take care of it in all cases of
balancing across clusters. You would be iterating over all cpus in the
other cluster running rebalance_domains() if the balancer cpu happens to
be the last one in the little cluster though. However, within the
cluster (in case you have 2 or more nohz-idle cpus) you still take a
double hit. No?
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