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Date:   Tue, 17 Nov 2020 12:06:20 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Valentin Schneider <valentin.schneider@arm.com>
Cc:     Oleksandr Natalenko <oleksandr@natalenko.name>,
        linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org,
        tglx@linutronix.de, rostedt@goodmis.org
Subject: Re: WARNING at kernel/sched/core.c:2013
 migration_cpu_stop+0x2e3/0x330
Message-ID: <20201117110620.GG3121378@hirez.programming.kicks-ass.net>
References: <a1fd0d9c6c8cd90a74879b61467ae48d@natalenko.name>
 <jhj3619y63v.mognet@arm.com>
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On Mon, Nov 16, 2020 at 10:00:14AM +0000, Valentin Schneider wrote:
> 
> On 15/11/20 22:32, Oleksandr Natalenko wrote:
> > Hi.
> >
> > I'm running v5.10-rc3-rt7 for some time, and I came across this splat in 
> > dmesg:
> >
> > ```
> > [118769.951010] ------------[ cut here ]------------
> > [118769.951013] WARNING: CPU: 19 PID: 146 at kernel/sched/core.c:2013 
> 
> Err, I didn't pick up on this back then, but isn't that check bogus? If the
> task is enqueued elsewhere, it's valid for it not to be affined
> 'here'. Also that is_migration_disabled() check within is_cpu_allowed()
> makes me think this isn't the best thing to call on a remote task.
> 
> ---
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 1218f3ce1713..47d5b677585f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2010,7 +2010,7 @@ static int migration_cpu_stop(void *data)
>  		 * valid again. Nothing to do.
>  		 */
>  		if (!pending) {
> -			WARN_ON_ONCE(!is_cpu_allowed(p, cpu_of(rq)));
> +			WARN_ON_ONCE(!cpumask_test_cpu(task_cpu(p), p->cpus_ptr));

Ho humm.. bit of a mess that. I'm trying to figure out if we need that
is_per_cpu_kthread() test here or not.

I suppose not, what we want here is to ensure the CPU is in cpus_mask
and not care about the whole hotplug mess.

Would it makes sense to replace both instances in migration_cpu_stop()
with:

	WARN_ON_ONCE(!cpumask_test_cpu(task_cpu(p), p->cpus_mask));

?