linux-kernel - Re: [RT] BUG in sched/cpupri.c

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Message-ID: <Ydgo2lENzywieaZL@donbot>
Date:   Fri, 7 Jan 2022 11:49:46 +0000
From:   John Keeping <john@...anate.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Valentin Schneider <valentin.schneider@....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 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?

> @ -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);
> 
> [...]