Date:   Fri, 7 Jan 2022 18:35:40 +0000
From:   John Keeping <john@metanate.com>
To:     Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc:     Valentin Schneider <valentin.schneider@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
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On Fri, Jan 07, 2022 at 03:25:21PM +0100, Dietmar Eggemann wrote:
> On 07/01/2022 12:49, John Keeping wrote:
> > On Fri, Jan 07, 2022 at 11:46:45AM +0100, Dietmar Eggemann wrote:
> >> On 22/12/2021 20:48, Valentin Schneider wrote:
> >>>  /*
> >>> 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:
> > 
> > Do you mean to replace just the one hunk from Valentin's patch with the
> > change below (keeping the rest), or are you saying that only the change
> > below is needed?
> 
> The latter.

Thanks!  I tested the patch below and can confirm that I no longer see
any BUGs with this applied.

Tested-By: John Keeping <john@metanate.com>

--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -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);
                if (cpu == -1 || cpu == rq->cpu)
                        return 0;