Re: [RFC PATCH v15 6/7] sched: Fix proxy/current (push,pull)ability

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Mar 12, 2025 at 03:11:36PM -0700, John Stultz wrote:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b4f7b14f62a24..3596244f613f8 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6722,6 +6722,23 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
>  }
>  #endif /* SCHED_PROXY_EXEC */
>  
> +static inline void proxy_tag_curr(struct rq *rq, struct task_struct *owner)
> +{
> +	if (!sched_proxy_exec())
> +		return;
> +	/*
> +	 * pick_next_task() calls set_next_task() on the chosen task
> +	 * at some point, which ensures it is not push/pullable.
> +	 * However, the chosen/donor task *and* the mutex owner form an
> +	 * atomic pair wrt push/pull.
> +	 *
> +	 * Make sure owner we run is not pushable. Unfortunately we can
> +	 * only deal with that by means of a dequeue/enqueue cycle. :-/
> +	 */
> +	dequeue_task(rq, owner, DEQUEUE_NOCLOCK | DEQUEUE_SAVE);
> +	enqueue_task(rq, owner, ENQUEUE_NOCLOCK | ENQUEUE_RESTORE);
> +}

So this is probably fine at this point; but we should eventually look at
fixing this.

We can probably look at (ab)using sched_class::set_cpus_allowed() for
this.