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

[K Prateek Nayak <kprateek.nayak@amd.com>]

Hello John,

On 3/13/2025 3:41 AM, John Stultz wrote:

[..snip..]

> 
> 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);
> +}
> +
>   /*
>    * __schedule() is the main scheduler function.
>    *
> @@ -6856,6 +6873,10 @@ static void __sched notrace __schedule(int sched_mode)
>   		 * changes to task_struct made by pick_next_task().
>   		 */
>   		RCU_INIT_POINTER(rq->curr, next);
> +
> +		if (!task_current_donor(rq, next))
> +			proxy_tag_curr(rq, next);

I don't see any dependency on rq->curr for task_current_donor() check.
Could this check be moved outside of the if-else block to avoid
duplicating in both places since rq_set_donor() was called just after
pick_next_task() or am I missing something?

> +
>   		/*
>   		 * The membarrier system call requires each architecture
>   		 * to have a full memory barrier after updating
> @@ -6890,6 +6911,10 @@ static void __sched notrace __schedule(int sched_mode)
>   		/* Also unlocks the rq: */
>   		rq = context_switch(rq, prev, next, &rf);
>   	} else {
> +		/* In case next was already curr but just got blocked_donor */
> +		if (!task_current_donor(rq, next))
> +			proxy_tag_curr(rq, next);
> +
>   		rq_unpin_lock(rq, &rf);
>   		__balance_callbacks(rq);
>   		raw_spin_rq_unlock_irq(rq);

-- 
Thanks and Regards,
Prateek