linux-kernel - Re: [PATCH 3/3] sched, fair: Stop searching for tasks in newidle balance if there are runnable tasks

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Date:	Thu, 24 Apr 2014 15:08:02 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jason Low <jason.low2@...com>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
	"alex.shi@...aro.org" <alex.shi@...aro.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"efault@....de" <efault@....de>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"aswin@...com" <aswin@...com>,
	"chegu_vinod@...com" <chegu_vinod@...com>
Subject: Re: [PATCH 3/3] sched, fair: Stop searching for tasks in newidle
 balance if there are runnable tasks

On Thu, Apr 24, 2014 at 12:32:31PM +0100, Peter Zijlstra wrote:
> On Thu, Apr 24, 2014 at 11:30:59AM +0100, Morten Rasmussen wrote:
> > On Thu, Apr 24, 2014 at 02:30:35AM +0100, Jason Low wrote:
> > > @@ -6704,7 +6703,12 @@ static int idle_balance(struct rq *this_rq)
> > >  		interval = msecs_to_jiffies(sd->balance_interval);
> > >  		if (time_after(next_balance, sd->last_balance + interval))
> > >  			next_balance = sd->last_balance + interval;
> > > -		if (pulled_task)
> > > +
> > > +		/*
> > > +		 * Stop searching for tasks to pull if there are
> > > +		 * now runnable tasks on this rq.
> > > +		 */
> > > +		if (pulled_task || this_rq->nr_running > 0)
> > 
> > Should this be cfs tasks instead?
> > 
> > +		if (pulled_task || this_rq->cfs.h_nr_running > 0)
> > 
> > 3.15-rc2 commit 35805ff8f4fc535ac85330170d3c56829c87c677 seems to
> > indicate that using rq->nr_running may lead to trouble.
> > 
> > The other two patches look good to me.
> 
> No, this really wants to be nr_running, we want to bail the idle
> balancer when there's anything runnable present.
> 
> Note how out: is very careful to return -1 (which results in RETRY_TASK)
> when rq->nr_running != rq->cfs.h_nr_running.
> 
> That same out: test also makes problem that commit fixes impossible
> again.

I should have done my homework properly. I may be missing something, but
don't we risk bailing out of idle balance if there is a throttled rt
task and go straight to idle?
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