Re: [PATCH 05/16] sched: SCHED_DEADLINE policy implementation.

[Juri Lelli]

On 04/23/2012 04:25 PM, Peter Zijlstra wrote:
> On Fri, 2012-04-06 at 09:14 +0200, Juri Lelli wrote:
> > +/*
> > + * This is the bandwidth enforcement timer callback. If here, we know
> > + * a task is not on its dl_rq, since the fact that the timer was running
> > + * means the task is throttled and needs a runtime replenishment.
> > + *
> > + * However, what we actually do depends on the fact the task is active,
> > + * (it is on its rq) or has been removed from there by a call to
> > + * dequeue_task_dl(). In the former case we must issue the runtime
> > + * replenishment and add the task back to the dl_rq; in the latter, we just
> > + * do nothing but clearing dl_throttled, so that runtime and deadline
> > + * updating (and the queueing back to dl_rq) will be done by the
> > + * next call to enqueue_task_dl().
>
> OK, so that comment isn't entirely clear to me, how can that timer still
> be active when the task isn't? You start the timer when you throttle it,
> at that point it cannot in fact dequeue itself anymore.
>
> The only possibility I see is the one mentioned with the dl_task() check
> below, that someone else called sched_setscheduler() on it.

Ok, I was also stuck at this point when I first reviewed v3.
Then I convinced myself that, even if probably always true,
the p->on_rq check would prevent weird situations like for
example: by the time I block on a mutex, go to sleep or whatever,
I am throttled, then the dl_timer fires and I'm still !on_rq.
But I didn't see this happening ever actually...

> > + */
> > +static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
> > +{
> > +       unsigned long flags;
> > +       struct sched_dl_entity *dl_se = container_of(timer,
> > +                                                    struct sched_dl_entity,
> > +                                                    dl_timer);
> > +       struct task_struct *p = dl_task_of(dl_se);
> > +       struct rq *rq = task_rq_lock(p,&flags);
> > +
> > +       /*
> > +        * We need to take care of a possible races here. In fact, the
> > +        * task might have changed its scheduling policy to something
> > +        * different from SCHED_DEADLINE (through sched_setscheduler()).
> > +        */
> > +       if (!dl_task(p))
> > +               goto unlock;
> > +
> > +       dl_se->dl_throttled = 0;
> > +       if (p->on_rq) {
> > +               enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
> > +               if (task_has_dl_policy(rq->curr))
> > +                       check_preempt_curr_dl(rq, p, 0);
> > +               else
> > +                       resched_task(rq->curr);
> > +       }
>
> So I can't see how that cannot be true.
>
> > +unlock:
> > +       task_rq_unlock(rq, p,&flags);
> > +
> > +       return HRTIMER_NORESTART;
> > +}
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