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

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Message-ID: <20150401170418.GX18994@e105550-lin.cambridge.arm.com>
Date:	Wed, 1 Apr 2015 18:04:18 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	Jason Low <jason.low2@...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>
Subject: Re: sched: Improve load balancing in the presence of idle CPUs

On Wed, Apr 01, 2015 at 07:49:56AM +0100, Preeti U Murthy wrote:
> 
> On 04/01/2015 12:24 AM, Jason Low wrote:
> > 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 potentially outdated value to be used if this cpu is able to pull tasks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?
> 
> I am sorry I don't quite get this. Can you please elaborate?

I think the scenario is that we are in nohz_idle_balance() and decide to
bail out because we have pulled some tasks, but before leaving
nohz_idle_balance() we want to check if more balancing is necessary
using nohz_kick_needed() and potentially kick somebody to continue.

Note that the balance cpu is currently skipped in nohz_idle_balance(),
but if it wasn't the scenario would be possible.

In that case, we can't rely on rq->idle_balance as it would not be
up-to-date. Also, we may even want to use nohz_kick_needed(rq) where rq
!= this_rq, in which case we probably also want an updated status. It
seems that rq->idle_balance is only updated at each tick.

Or maybe I'm all wrong :)
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