Re: [PATCH 8/9] sched: Fix migrate_disable() vs set_cpus_allowed_ptr()

From: Valentin Schneider <valentin.schneider@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: tglx@linutronix.de, mingo@kernel.org,
	linux-kernel@vger.kernel.org, bigeasy@linutronix.de,
	qais.yousef@arm.com, swood@redhat.com, juri.lelli@redhat.com,
	vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
	rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
	bristot@redhat.com, vincent.donnefort@arm.com
Subject: Re: [PATCH 8/9] sched: Fix migrate_disable() vs set_cpus_allowed_ptr()
Date: Fri, 25 Sep 2020 11:09:31 +0100	[thread overview]
Message-ID: <jhjzh5ekw10.mognet@arm.com> (raw)
In-Reply-To: <20200925095615.GA2651@hirez.programming.kicks-ass.net>

On 25/09/20 10:56, Peter Zijlstra wrote:
> On Fri, Sep 25, 2020 at 11:05:28AM +0200, Peter Zijlstra wrote:
>> On Thu, Sep 24, 2020 at 08:59:33PM +0100, Valentin Schneider wrote:
>> > > @@ -2025,19 +2138,8 @@ static int __set_cpus_allowed_ptr(struct
>> > >       if (cpumask_test_cpu(task_cpu(p), new_mask))
>> > >               goto out;
>> >
>> > I think this needs a cancellation of any potential pending migration
>> > requests. Consider a task P0 running on CPU0:
>> >
>> >    P0                     P1                               P2
>> >
>> >    migrate_disable();
>> >    <preempt>
>> >                           set_cpus_allowed_ptr(P0, CPU1);
>> >                           // waits for completion
>> >                                                            set_cpus_allowed_ptr(P0, CPU0);
>> >                                                            // Already good, no waiting for completion
>> >    <resumes>
>> >    migrate_enable();
>> >    // task_cpu(p) allowed, no move_task()
>> >
>> > AIUI in this scenario P1 would stay forever waiting.
>>
>
>> The other approach is trying to handle that last condition in
>> move_task(), but I'm quite sure that's going to be aweful too :/
>
> Something like so perhaps?
>

That looks somewhat sane (not pretty, but we're past that :-)). I was
trying something similar in __set_cpus_allowed_ptr() itself, but this
condenses the completion / refcount logic in one place, so fewer
headaches.

> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2039,6 +2039,10 @@ static int move_task(struct rq *rq, stru
>       if (WARN_ON_ONCE(!pending))
>               return -EINVAL;
>
> +	/* Can the task run on the task's current CPU? If so, we're done */
> +	if (cpumask_test_cpu(task_cpu(p), &p->cpus_mask))
> +		goto easy;
> +
>       arg.done = &pending->done;
>
>       if (flags & SCA_MIGRATE_ENABLE) {
> @@ -2063,6 +2067,7 @@ static int move_task(struct rq *rq, stru
>                       if (task_on_rq_queued(p))
>                               rq = move_queued_task(rq, rf, p, dest_cpu);
>
> +easy:
>                       p->migration_pending = NULL;
>                       complete = true;
>               }
> @@ -2151,10 +2156,6 @@ static int __set_cpus_allowed_ptr(struct
>                       p->nr_cpus_allowed != 1);
>       }
>
> -	/* Can the task run on the task's current CPU? If so, we're done */
> -	if (cpumask_test_cpu(task_cpu(p), new_mask))
> -		goto out;
> -
>       return move_task(rq, &rf, p, dest_cpu, flags);
>
>  out: