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

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