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Date:   Wed, 13 Jul 2022 13:46:11 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Juri Lelli <juri.lelli@redhat.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        linux-rt-users <linux-rt-users@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>
Subject: Re: [PATCH] sched/deadline: Fix BUG_ON condition for deboosted tasks
Message-ID: <Ys6wgxiZbdjxyh8C@hirez.programming.kicks-ass.net>
References: <20220713075014.411739-1-juri.lelli@redhat.com>
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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@redhat.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?