linux-kernel - Re: [PATCH] sched/deadline: Fix BUG

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Message-ID: <Ys7BYCvlgphAwQ+Y@localhost.localdomain>
Date:   Wed, 13 Jul 2022 14:58:08 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH] sched/deadline: Fix BUG_ON condition for deboosted tasks

On 13/07/22 13:46, Peter Zijlstra wrote:
> On Wed, Jul 13, 2022 at 09:50:14AM +0200, Juri Lelli wrote:
> > Tasks the are being deboosted from SCHED_DEADLINE might enter
> > enqueue_task_dl() one last time and hit an erroneous BUG_ON condition:
> > since they are not boosted anymore, the if (is_dl_boosted()) branch is
> > not taken, but the else if (!dl_prio) is and inside this one we
> > BUG_ON(!is_dl_boosted), which is of course false (BUG_ON triggered)
> > otherwise we had entered the if branch above. Long story short, the
> > current condition doesn't make sense and always leads to triggering of a
> > BUG.
> > 
> > Fix this by only checking enqueue flags, properly: ENQUEUE_REPLENISH has
> > to be present, but additional flags are not a problem.
> > 
> > Fixes: 2279f540ea7d ("sched/deadline: Fix priority inheritance with multiple scheduling classes")
> > Signed-off-by: Juri Lelli <juri.lelli@...hat.com>
> > ---
> >  kernel/sched/deadline.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index 5867e186c39a..0447d46f4718 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -1703,7 +1703,7 @@ static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags)
> >  		 * the throttle.
> >  		 */
> >  		p->dl.dl_throttled = 0;
> > -		BUG_ON(!is_dl_boosted(&p->dl) || flags != ENQUEUE_REPLENISH);
> > +		BUG_ON(!(flags & ENQUEUE_REPLENISH));
> 
> While there, can we perhaps make it less fatal? 

Yep. On it. Thanks!

Juri