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Message-ID: <f2d50e78-dc7b-6851-f12e-d702fbfea826@arm.com>
Date: Fri, 7 Jan 2022 11:46:45 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Valentin Schneider <valentin.schneider@....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 22/12/2021 20:48, 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:
[...]
>> 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...
True, this_cpu has rt_nr_running = 0 and *cpu* has rt_nr_running >= 2:
mark_wakeup_next_waiter()
(1) /* deboost */
rt_mutex_adjust_prio()
rt_mutex_setprio(current, ...)
rq = __task_rq_lock(current, );
check_class_changed(rq, p, prev_class, oldprio)
switched_from_rt()
if (!task_on_rq_queued(p) || rq->rt.rt_nr_running)
return;
rt_queue_pull_task(rq)
queue_balance_callback(rq, ..., pull_rt_task);
pull_rt_task()
tell_cpu_to_push()
*cpu* = rto_next_cpu(rq->rd)
irq_work_queue_on(&rq->rd->rto_push_work, *cpu*)
rto_push_irq_work_func()
push_rt_task(rq, true) <-- !!!
(2) /* waking the top waiter */
rt_mutex_wake_q_add(wqh, waiter);
> 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.
I think that's the case here but we are on the RT overloaded CPU (*cpu*).
>
>> 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)
Right.
[...]
> /*
> 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;
> + }
IMHO, that's the bit which prevents the BUG.
But this would also prevent the case in which rq->curr is an RT task
with lower prio than next_task.
Also `rq->curr = migration/X` goes still though which is somehow fine
since find_lowest_rq() bails out for if (task->nr_cpus_allowed == 1).
And DL tasks (like sugov:X go through and they can have
task->nr_cpus_allowed > 1 (arm64 slow-switching boards with shared
freuency domains with schedutil). cpupri_find_fitness()->convert_prio()
can handle task_pri, p->prio = -1 (CPUPRI_INVALID) although its somehow
by coincidence.
So maybe something like this:
@ -1898,6 +1898,11 @@ static int push_rt_task(struct rq *rq, bool pull)
if (!pull || rq->push_busy)
return 0;
+ if (rq->curr->sched_class != &rt_sched_class) {
+ resched_curr(rq);
+ return 0;
+ }
+
cpu = find_lowest_rq(rq->curr);
[...]
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