From: Valentin Schneider <valentin.schneider@arm.com>
To: John Keeping <john@metanate.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>,
linux-rt-users@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Steven Rostedt <rostedt@goodmis.org>,
Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
linux-kernel@vger.kernel.org
Subject: Re: [RT] BUG in sched/cpupri.c
Date: Tue, 21 Dec 2021 17:22:34 +0000 [thread overview]
Message-ID: <87wnjx7u05.mognet@arm.com> (raw)
In-Reply-To: <20211221164528.3c84543f.john@metanate.com>
On 21/12/21 16:45, John Keeping wrote:
> On Tue, 21 Dec 2021 16:11:34 +0000
> Valentin Schneider <valentin.schneider@arm.com> 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.
>
Thanks for the swift testing!
Did you give Dietmar's patch a try? ITSM it lacks a resched_curr(), but if
we can somehow get to the push IRQ work before rescheduling (which I think
might happen if we try to resched_curr(this_rq)), then we need his
bailout.
next prev parent reply other threads:[~2021-12-21 17:22 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-18 14:25 [RT] BUG in sched/cpupri.c John Keeping
2021-12-20 17:35 ` Dietmar Eggemann
2021-12-21 16:11 ` Valentin Schneider
2021-12-21 16:45 ` John Keeping
2021-12-21 17:22 ` Valentin Schneider [this message]
2021-12-21 17:42 ` John Keeping
2021-12-22 17:46 ` Dietmar Eggemann
2021-12-22 18:45 ` John Keeping
2021-12-22 19:48 ` Valentin Schneider
2021-12-23 11:58 ` John Keeping
2021-12-23 14:05 ` Valentin Schneider
2022-01-07 10:46 ` Dietmar Eggemann
2022-01-07 11:49 ` John Keeping
2022-01-07 14:25 ` Dietmar Eggemann
2022-01-07 18:35 ` John Keeping
2022-01-14 18:25 ` Valentin Schneider
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