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

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Date:	Mon, 23 Apr 2012 17:34:54 +0200
From:	Juri Lelli <juri.lelli@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	tglx@...utronix.de, mingo@...hat.com, rostedt@...dmis.org,
	cfriesen@...tel.com, oleg@...hat.com, fweisbec@...il.com,
	darren@...art.com, johan.eker@...csson.com, p.faure@...tech.ch,
	linux-kernel@...r.kernel.org, claudio@...dence.eu.com,
	michael@...rulasolutions.com, fchecconi@...il.com,
	tommaso.cucinotta@...up.it, nicola.manica@...i.unitn.it,
	luca.abeni@...tn.it, dhaval.giani@...il.com, hgu1972@...il.com,
	paulmck@...ux.vnet.ibm.com, raistlin@...ux.it,
	insop.song@...csson.com, liming.wang@...driver.com
Subject: Re: [PATCH 05/16] sched: SCHED_DEADLINE policy implementation.

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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