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


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