Messages in this thread | | | Date | Mon, 23 Apr 2012 17:34:54 +0200 | From | Juri Lelli <> | Subject | Re: [PATCH 05/16] sched: SCHED_DEADLINE policy implementation. |
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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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