Re: [PATCH v23 8/9] sched: Add blocked_donor link to task for smarter mutex handoffs

[K Prateek Nayak]

Hello Peter, John,

On 10/30/2025 5:48 AM, John Stultz wrote:
> @@ -958,7 +964,34 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
>  
>  	raw_spin_lock_irqsave(&lock->wait_lock, flags);
>  	debug_mutex_unlock(lock);
> -	if (!list_empty(&lock->wait_list)) {
> +
> +	if (sched_proxy_exec()) {
> +		raw_spin_lock(&current->blocked_lock);
> +		/*
> +		 * If we have a task boosting current, and that task was boosting
> +		 * current through this lock, hand the lock to that task, as that
> +		 * is the highest waiter, as selected by the scheduling function.
> +		 */
> +		donor = current->blocked_donor;
> +		if (donor) {

Any concerns on new waiters always appearing as donors and in-turn
starving the long time waiters on the list?

> +			struct mutex *next_lock;
> +
> +			raw_spin_lock_nested(&donor->blocked_lock, SINGLE_DEPTH_NESTING);
> +			next_lock = __get_task_blocked_on(donor);
> +			if (next_lock == lock) {
> +				next = donor;
> +				__set_task_blocked_on_waking(donor, next_lock);
> +				wake_q_add(&wake_q, donor);
> +				current->blocked_donor = NULL;
> +			}
> +			raw_spin_unlock(&donor->blocked_lock);
> +		}
> +	}
-- 
Thanks and Regards,
Prateek