Re: [PATCH 2/6] sched: Simplify migration_cpu_stop()

Valentin Schneider
2021-02-24T15:49:27Z

On 24/02/21 13:24, Peter Zijlstra wrote:
> @@ -1950,31 +1931,20 @@ static int migration_cpu_stop(void *data
> goto out;
>
> if (pending) {
> - p->migration_pending = NULL;
> + if (p->migration_pending == pending)
> + p->migration_pending = NULL;
> complete = true;
> }
>
> - /* migrate_enable() -- we must not race against SCA */
> - if (dest_cpu < 0) {
> - /*
> - * When this was migrate_enable() but we no longer
> - * have a @pending, a concurrent SCA 'fixed' things
> - * and we should be valid again. Nothing to do.
> - */
> - if (!pending) {
> - WARN_ON_ONCE(!cpumask_test_cpu(task_cpu(p), &p->cpus_mask));
> - goto out;
> - }
> -

This is fixed by 5+6, but at this patch I think you can have double
completions - I thought this was an issue, but briefly looking at
completion stuff it might not. In any case, consider:

task_cpu(p) == Y

SCA(p, X);
SCA(p, Y);

SCA(p, Y) will uninstall SCA(p, X)'s pending and complete.

migration/Y kicked by SCA(p, X) will grab arg->pending, which is still
SCA(p, X)'s pending and also complete.

> + if (dest_cpu < 0)
> dest_cpu = cpumask_any_distribute(&p->cpus_mask);
> - }
>
> if (task_on_rq_queued(p))
> rq = __migrate_task(rq, &rf, p, dest_cpu);
> else
> p->wake_cpu = dest_cpu;
>
> - } else if (dest_cpu < 0 || pending) {
> + } else if (pending) {
> /*
> * This happens when we get migrated between migrate_enable()'s
> * preempt_enable() and scheduling the stopper task. At that