Messages in this thread |  | | Date | Wed, 31 Jul 2019 22:20:46 +0200 | From | luca abeni <> | Subject | Re: [PATCH 4/5] sched/deadline: Cleanup on_dl_rq() handling |
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On Wed, 31 Jul 2019 18:32:47 +0100 Dietmar Eggemann <dietmar.eggemann@arm.com> wrote: [...] > >>>> static void dequeue_dl_entity(struct sched_dl_entity *dl_se) > >>>> { > >>>> + if (!on_dl_rq(dl_se)) > >>>> + return; > >>> > >>> Why allow double dequeue instead of WARN? > >> > >> As I was saying to Valentin, it can currently happen that a task > >> could have already been dequeued by update_curr_dl()->throttle > >> called by dequeue_task_dl() before calling __dequeue_task_dl(). Do > >> you think we should check for this condition before calling into > >> dequeue_dl_entity()? > > > > Yes, that's what ->dl_throttled is for, right? And !->dl_throttled > > && !on_dl_rq() is a BUG. > > OK, I will add the following snippet to the patch. > Although it's easy to provoke a situation in which DL tasks are > throttled, I haven't seen a throttling happening when the task is > being dequeued.
This is a not-so-common situation, that can happen with periodic tasks (a-la rt-app) blocking on clock_nanosleep() (or similar) after executing for an amount of time comparable with the SCHED_DEADLINE runtime.
It might happen that the task consumed a little bit more than the remaining runtime (but has not been throttled yet, because the accounting happens at every tick)... So, when dequeue_task_dl() invokes update_task_dl() the runtime becomes negative and the task is throttled.
This happens infrequently, but if you try rt-app tasksets with multiple tasks and execution times near to the runtime you will see it happening, sooner or later.
[...] > @@ -1592,6 +1591,10 @@ static void __dequeue_task_dl(struct rq *rq, > struct task_struct *p) static void dequeue_task_dl(struct rq *rq, > struct task_struct *p, int flags) { > update_curr_dl(rq); > + > + if (p->dl.dl_throttled) > + return;
Sorry, I missed part of the previous discussion, so maybe I am missing something... But I suspect this "return" might be wrong (you risk to miss a call to task_non_contending(), coming later in this function).
Maybe you cound use if (!p->dl_throttled) __dequeue_task_dl(rq, p)
Or did I misunderstand something?
Thanks, Luca
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