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Message-ID: <87r1a4775a.mognet@arm.com>
Date: Wed, 22 Dec 2021 19:48:33 +0000
From: Valentin Schneider <valentin.schneider@....com>
To: Dietmar Eggemann <dietmar.eggemann@....com>,
John Keeping <john@...anate.com>
Cc: 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 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.
> What about slightly changing the layout in switched_from_rt() (only lightly tested):
>
>
> @@ -2322,7 +2338,15 @@ 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;
> +
> + if (task_current(rq, p) && (p->sched_class < &rt_sched_class)) {
> + resched_curr(rq);
> + return;
> + }
> +
> + if (rq->rt.rt_nr_running)
> return;
>
> rt_queue_pull_task(rq);
If !rq->rt.rt_nr_running then there's no point in issuing a reschedule (at
least from RT's perspective; p->sched_class->switched_to() takes care of
the rest)
---
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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