Messages in this thread | | | Subject | Re: [RFC][PATCH 05/22] sched: SCHED_DEADLINE policy implementation | From | Raistlin <> | Date | Thu, 11 Nov 2010 02:02:05 +0100 |
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On Wed, 2010-11-10 at 20:43 +0100, Peter Zijlstra wrote: > On Fri, 2010-10-29 at 08:30 +0200, Raistlin wrote: > > + * We are being explicitly informed that a new instance is starting, > > + * and this means that: > > + * - the absolute deadline of the entity has to be placed at > > + * current time + relative deadline; > > + * - the runtime of the entity has to be set to the maximum value. > > When exactly are we a new instance? From a quick look dl_new gets set > after a sched_setscheduler() call, is that the only way? > One of the only two ways. Later in the queue, that flag is set by a new system call, i.e., sched_wait_interval, that can be used to inform the scheduler (for example at the end of a periodic/sporadic job) that an instance just ended. Moreover, it can be exploited by a task which want the scheduler to wake it up when it can be given its full runtime. It as been added as a consequence of the discussion happened in Dresden, at last year RTLWS, aside of my presentation...
Whether or not this could be useful, I don't know, and I accept comments as usual. My opinion is that it might be something worthwhile to have, especially from the point of view of hard real-time-ish scenarios, but we can remove it appears unnecessary.
> Could a task calling sched_setscheduler() on itself cheat the system? > I obviously might be wrong (especially at this time), but I would say no for the following reasons.
If you are an overrunning -deadline task calling sched_setscheduler() the deactivate_task->dequeue_task->dequeue_task_dl() below will trigger the bandwidth enforcement, i.e., will set dl_throttled=1 and start dl_timer: ... on_rq = p->se.on_rq; running = task_current(rq, p); if (on_rq) deactivate_task(rq, p, 0); if (running) p->sched_class->put_prev_task(rq, p); ...
Later, this enqueue: ... if (running) p->sched_class->set_curr_task(rq); if (on_rq) { activate_task(rq, p, 0); check_class_changed(rq, p, prev_class, oldprio, running); } ...
even if it will find dl_new=1, will not enqueue the task back in its dl_rq (since dl_throttled=1). The actual enqueueing happens at the firing of dl_timer, where an update instead than a replenishment will be performed, right because of the fact that dl_new=1. This means the runtime will be fully replenished and the deadline moved toward rq->clock+dl_se->dl_deadline.
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Thanks and Regards, Dario
-- <<This happens because I choose it to happen!>> (Raistlin Majere) ---------------------------------------------------------------------- Dario Faggioli, ReTiS Lab, Scuola Superiore Sant'Anna, Pisa (Italy)
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