Re: [RFC PATCH 04/16] sched/core: Activate blocked donor when no owner is found

[Andrea Righi <arighi@nvidia.com>]

On Wed, Aug 26, 2026 at 06:28:48AM +0000, K Prateek Nayak wrote:
> mutex_unlock_slowpath() follows:
> 
>     if (owner & MUTEX_FLAG_HANDOFF)
>         break /* ... and do __mutex_handoff() */
> 
>     if (atomic_long_try_cmpxchg_release(&lock->owner, &owner, __owner_flags(owner))) {
>         if (owner & MUTEX_FLAG_WAITERS)
>             break; /* ... and wake up the forst waiter. */

nit: s/forst/first/

> 
> MUTEX_FLAG_HANDOFF is only set by first-waiter after it has been woken
> up and in absence of MUTEX_FLAG_HANDOFF, the owner clears itself from
> the lock_word and wakes up the first waiter to try a
> __mutex_trylock_or_handoff().
> 
> MUTEX_FLAG_HANDOFF exists to prevent new optimistic spinners from
> trying to hijack the lock from waiter all the time and potentially
> starving them but it is not necessary for MUTEX_FLAG_HANDOFF to be
> always set in presence of a waiter.
> 
> If a blocked donor is deactivated when no owner is observed, it may not
> be woken up until it becomes the first waiter and is naturally woken up
> which breaks proxy in the interim.
> 
> Wake up the blocked donor and allow it to grab the lock when no owner is
> observed. If the task manages to grab the lock, the block chain will
> follow at the next proxy migration. If the task fails to grab the lock,
> same situation is restored and everyone migrated to the CPU of new
> owner.
> 
> Fixes: f13beb010e4a ("sched: Have try_to_wake_up() handle return-migration for PROXY_WAKING case")
> Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
> ---
> XXX: Is there a better way to handle this? If we can confirm a owner in
> find_proxy_task(), we don't need to do a spurious wakeup of every task
> observing !owner.
> 
> proxy_resched_idle() until owner appears in an option but it will spin
> until next owner appears.

IIUC, the owner can remain NULL until the waiter selected by mutex_unlock() gets
CPU time and acquires the mutex, so proxy_resched_idle() could spin for longer
than just the unlock critical section.

Maybe we could instead force a handoff from mutex_unlock_slowpath() when proxy
execution is enabled and the mutex has waiters? This would keep the owner
identifiable and avoid waking every task that happens to observe !owner.

> ---
>  kernel/sched/core.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 4cd69b08b415..9d9db7ccf01f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6809,6 +6809,19 @@ static inline void proxy_reacquire_rq_lock(struct rq *rq, struct rq_flags *rf)
>  	update_rq_clock(rq);
>  }
>  
> +static void
> +proxy_activate(struct rq *rq, struct rq_flags *rf, struct task_struct *p)
> +	__must_hold(__rq_lockp(rq))
> +{
> +	lockdep_assert_rq_held(rq);
> +	proxy_resched_idle(rq);
> +	proxy_release_rq_lock(rq, rf);
> +
> +	wake_up_process(p);
> +
> +	proxy_reacquire_rq_lock(rq, rf);
> +}
> +
>  /*
>   * If the blocked-on relationship crosses CPUs, migrate @p to the
>   * owner's CPU.
> @@ -6934,14 +6947,15 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
>  			/*
>  			 * If there is no owner, either clear blocked_on
>  			 * and return p (if it is current and safe to
> -			 * just run on this rq), or return-migrate the task.
> +			 * just run on this rq), or wake the task to try
> +			 * and grab the lock it is blocked on.
>  			 */
>  			__clear_task_blocked_on(p, NULL);
> -			if (task_current(rq, p)) {
> +			if (task_current(rq, p) || p->wake_cpu == task_cpu(p)) {
>  				p->is_blocked = 0;
>  				return p;
>  			}
> -			goto deactivate;
> +			goto activate;
>  		}
>  
>  		if (!READ_ONCE(owner->on_rq) || owner->se.sched_delayed) {
> @@ -7029,6 +7043,9 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
>  	}
>  	return owner;
>  
> +activate:
> +	proxy_activate(rq, rf, p);
> +	return NULL;
>  deactivate:
>  	proxy_deactivate(rq, p);
>  	return NULL;
> -- 
> 2.34.1
> 

Thanks,
-Andrea