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

[Peter Zijlstra <peterz@infradead.org>]

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.