Re: [PATCH v2 4/7] sched/deadline: reschedule if stop task slip in after pull operations

[Wanpeng Li]

On 5/29/15 10:16 PM, Peter Zijlstra wrote:
> On Wed, May 13, 2015 at 02:01:04PM +0800, Wanpeng Li wrote:
> > pull_dl_task can drop (and re-acquire) rq->lock, this means a stop task
> > can slip in, in which case we need to reschedule. This patch add the
> > reschedule when the scenario occurs.
> >
> > Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxxxxxx>
> > ---
> >   kernel/sched/deadline.c | 16 +++++++++++++++-
> >   1 file changed, 15 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index e49b1e6..7d4c4fc 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -1750,7 +1750,13 @@ static void switched_from_dl(struct rq *rq, struct task_struct *p)
> >   	if (!task_on_rq_queued(p) || rq->dl.dl_nr_running)
> >   		return;
> >   
> > -	if (pull_dl_task(rq))
> > +	/*
> > +	 * pull_dl_task() can drop (and re-acquire) rq->lock; this
> > +	 * means a stop task can slip in, in which case we need to
> > +	 * reschedule.
> > +	 */
> > +	if (pull_dl_task(rq) ||
> > +		(rq->stop && task_on_rq_queued(rq->stop)))
> >   		resched_curr(rq);
> But, the waking of the stop task will already have done the preemption
> check and won (obviously). So the wakeup should already have done the
> resched_curr().

Indeed, thanks for your pointing out. :)

> So why?
>
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