On Tue, 21 Dec 2021 16:11:34 +0000
Valentin Schneider <valentin.schneider@xxxxxxx> wrote:
> On 20/12/21 18:35, Dietmar Eggemann wrote:
> > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> > index ef8228d19382..798887f1eeff 100644
> > --- a/kernel/sched/rt.c
> > +++ b/kernel/sched/rt.c
> > @@ -1895,9 +1895,17 @@ static int push_rt_task(struct rq *rq, bool pull)
> > struct task_struct *push_task = NULL;
> > int cpu;
> >
> > + if (WARN_ON_ONCE(!rt_task(rq->curr))) {
> > + printk("next_task=[%s %d] rq->curr=[%s %d]\n",
> > + next_task->comm, next_task->pid, rq->curr->comm, rq->curr->pid);
> > + }
> > +
> > if (!pull || rq->push_busy)
> > return 0;
> >
> > + if (!rt_task(rq->curr))
> > + return 0;
> > +
>
> If current is a DL/stopper task, why not; if that's CFS (which IIUC is your
> case), that's buggered: we shouldn't be trying to pull RT tasks when we
> have queued RT tasks and a less-than-RT current, we should be rescheduling
> right now.
>
> I'm thinking this can happen via rt_mutex_setprio() when we demote an RT-boosted
> CFS task (or straight up sched_setscheduler()):
> check_class_changed()->switched_from_rt() doesn't trigger a resched_curr(),
> so I suspect we get to the push/pull callback before getting a
> resched (I actually don't see where we'd get a resched in that case other
> than at the next tick).
>
> IOW, feels like we want the below. Unfortunately I can't reproduce the
> issue locally (yet), so that's untested.
This patch doesn't make any difference for me - I hit the BUG on the
first boot with this applied.
> ---
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index fd7c4f972aaf..7d61ceec1a3b 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -2467,10 +2467,13 @@ static void switched_from_dl(struct rq *rq, struct task_struct *p)
> * this is the right place to try to pull some other one
> * from an overloaded CPU, if any.
> */
> - if (!task_on_rq_queued(p) || rq->dl.dl_nr_running)
> + if (!task_on_rq_queued(p))
> return;
>
> - deadline_queue_pull_task(rq);
> + if (!rq->dl.dl_nr_running)
> + deadline_queue_pull_task(rq);
> + else if (task_current(rq, p) && (p->sched_class < &dl_sched_class))
> + resched_curr(rq);
> }
>
> /*
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index ef8228d19382..1ea2567612fb 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -2322,10 +2322,13 @@ static void switched_from_rt(struct rq *rq, struct task_struct *p)
> * we may need to handle the pulling of RT tasks
> * now.
> */
> - if (!task_on_rq_queued(p) || rq->rt.rt_nr_running)
> + if (!task_on_rq_queued(p))
> return;
>
> - rt_queue_pull_task(rq);
> + if (!rq->rt.rt_nr_running)
> + rt_queue_pull_task(rq);
> + else if (task_current(rq, p) && (p->sched_class < &rt_sched_class))
> + resched_curr(rq);
> }
>
> void __init init_sched_rt_class(void)
