Re: [PATCH 05/16] sched: Add sched_ext hooks for proxy execution

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Sep 22, 2026 at 06:51:44PM +0200, Andrea Righi wrote:
> Proxy execution splits the scheduling context (the donor) from the
> execution context (the lock owner). sched_ext needs to observe that
> split at three points in __schedule():
> 
>  - whether a blocked EXT task can be retained on the runqueue as a
>    donor,
>  - when a donor's scheduling context starts driving a lock owner,
>  - after proxy resolution, when deferred reenqueue work that was blocked
>    by an active proxy relationship can be retried.
> 
> Introduce scx_allow_proxy_exec(), scx_proxy_donor_start() and
> scx_proxy_reenqueue_retry(), and add their call sites in __schedule().
> The implementations are empty here and are filled in by the sched_ext
> changes that follow, so that all the sched core changes needed by proxy
> execution stay together in the preparatory patches.
> 
> SCHED_PROXY_EXEC still depends on !SCHED_CLASS_EXT, so the new hooks are
> inert: they are compiled out with CONFIG_SCHED_CLASS_EXT=n and
> unreachable otherwise.
> 
> This is a preparatory change to support proxy execution with sched_ext.
> No functional change.
> 
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---

> @@ -7243,6 +7242,7 @@ static void __sched notrace __schedule(int sched_mode)
>  			}
>  			if (next == rq->idle) {
>  				zap_balance_callbacks(rq);
> +				scx_proxy_reenqueue_retry(rq, next);
>  				goto keep_resched;
>  			}
>  		}
> @@ -7263,6 +7263,8 @@ static void __sched notrace __schedule(int sched_mode)
>  			donor->sched_class->put_prev_task(rq, donor, donor);
>  			donor->sched_class->set_next_task(rq, donor, true);
>  		}
> +		scx_proxy_donor_start(rq);
> +		scx_proxy_reenqueue_retry(rq, next);
>  	} else {
>  		rq_set_donor(rq, next);
>  	}

> diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
> index f60894dbf0623..aaa6ee66917e8 100644
> --- a/kernel/sched/ext/ext.c
> +++ b/kernel/sched/ext/ext.c

> @@ -1110,6 +1115,10 @@ static void schedule_deferred_locked(struct rq *rq)
>  	schedule_deferred(rq);
>  }
>  
> +void scx_proxy_reenqueue_retry(struct rq *rq)
> +{
> +}

Doesn't match its prototype, also, I'm a little confused about the next
argument in the next == rq->idle case. 

Hmm, you seem to be using the argument like:

  bool proxy = next != rq->donor;

And I suppose that works. But that seems to be about the tick, not
putting current back on a dsq.

> diff --git a/kernel/sched/ext/ext.h b/kernel/sched/ext/ext.h
> index 0012f708a5504..cca3f7c97b788 100644
> --- a/kernel/sched/ext/ext.h
> +++ b/kernel/sched/ext/ext.h
> @@ -20,6 +20,9 @@ void scx_rq_deactivate(struct rq *rq);
>  int scx_check_setscheduler(struct task_struct *p, int policy);
>  bool task_should_scx(int policy);
>  bool scx_allow_ttwu_queue(const struct task_struct *p);
> +bool scx_allow_proxy_exec(const struct task_struct *p);
> +void scx_proxy_donor_start(struct rq *rq);
> +void scx_proxy_reenqueue_retry(struct rq *rq, struct task_struct *next);
>  void init_sched_ext_class(void);
>  void __scx_update_idle(struct rq *rq, bool idle, bool do_notify);
>