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Date: Thu, 23 Dec 2021 11:58:10 +0000
From: John Keeping <john@...anate.com>
To: Valentin Schneider <valentin.schneider@....com>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>,
linux-rt-users@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
linux-kernel@...r.kernel.org
Subject: Re: [RT] BUG in sched/cpupri.c
On Wed, Dec 22, 2021 at 07:48:33PM +0000, Valentin Schneider wrote:
> On 22/12/21 18:46, Dietmar Eggemann wrote:
> > On 21.12.21 17:45, John Keeping wrote:
> >> On Tue, 21 Dec 2021 16:11:34 +0000
> >> Valentin Schneider <valentin.schneider@....com> wrote:
> >>
> >>> On 20/12/21 18:35, Dietmar Eggemann wrote:
> >
> > [...]
> >
> >>> 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);
> >
> > switched_from_rt() -> rt_queue_pull_task(, pull_rt_task)
> > pull_rt_task()->tell_cpu_to_push()->irq_work_queue_on(&rq->rd->rto_push_work,)
> > rto_push_irq_work_func() -> push_rt_task(rq, true)
> >
> > seems to be the only way with pull=true.
> >
> > In my tests, rq->rt.rt_nr_running seems to be 0 when it happens.
> >
> > [ 22.288537] CPU3 switched_to_rt: p=[ksoftirqd/3 35]
> > [ 22.288554] rt_mutex_setprio: CPU3 p=[ksoftirqd/3 35] pi_task=[rcu_preempt 11] queued=1 running=0 prio=98 oldprio=120
> > [ 22.288636] CPU3 switched_from_rt: p=[ksoftirqd/3 35] rq->rt.rt_nr_running=0
> > ^^^^^^^^^^^^^^^^^^^^^^
> > [ 22.288649] rt_mutex_setprio: CPU3 p=[ksoftirqd/3 35] queued=1 running=1 prio=120 oldprio=98
> > [ 22.288681] CPU3 push_rt_task: next_task=[rcu_preempt 11] migr_dis=1 rq->curr=[ksoftirqd/3 35] pull=1
> > ^^^^^^^^^^ ^^^^^^
>
> mark_wakeup_next_waiter() first deboosts the previous owner and then
> wakeups the next top waiter. Seems like you somehow have the wakeup happen
> before the push_rt_task IRQ work is run. Also, tell_cpu_to_push() should
> only pick a CPU that is in rq->rd->rto_mask, which requires having at least
> 2 RT tasks there...
>
> Now, that wakeup from the rtmutex unlock would give us a resched_curr() via
> check_preempt_curr() if required, which is good, though I think we are
> still missing some for sched_setscheduler() (there are no wakeups
> there). So if we just have to live with an IRQ work popping in before we
> get to preempt_schedule_irq() (or somesuch), then perhaps the below would
> be sufficient.
With this patch I ran 100 reboots without hitting the BUG, so it looks
like this is the solution!
If you post this as a patch, feel free to add:
Tested-by: John Keeping <john@...anate.com>
> ---
>
> 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..8f3e3a1367b6 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1890,6 +1890,16 @@ static int push_rt_task(struct rq *rq, bool pull)
> if (!next_task)
> return 0;
>
> + /*
> + * It's possible that the next_task slipped in of higher priority than
> + * current, or current has *just* changed priority. If that's the case
> + * just reschedule current.
> + */
> + if (unlikely(next_task->prio < rq->curr->prio)) {
> + resched_curr(rq);
> + return 0;
> + }
> +
> retry:
> if (is_migration_disabled(next_task)) {
> struct task_struct *push_task = NULL;
> @@ -1922,16 +1932,6 @@ static int push_rt_task(struct rq *rq, bool pull)
> if (WARN_ON(next_task == rq->curr))
> return 0;
>
> - /*
> - * It's possible that the next_task slipped in of
> - * higher priority than current. If that's the case
> - * just reschedule current.
> - */
> - if (unlikely(next_task->prio < rq->curr->prio)) {
> - resched_curr(rq);
> - return 0;
> - }
> -
> /* We might release rq lock */
> get_task_struct(next_task);
>
> @@ -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)
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