Re: [PATCH 07/18] sched: Add sched_ext hooks for proxy execution

[Peter Zijlstra]

On Mon, Aug 31, 2026 at 03:42:17PM +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,
>  - once proxy resolution has settled for the current pick.
> 
> Introduce scx_allow_proxy_exec(), scx_proxy_donor_start() and
> scx_proxy_resolved(), 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@xxxxxxxxxx>
> ---
>  kernel/sched/core.c    | 12 +++++++-----
>  kernel/sched/ext/ext.c | 13 +++++++++++++
>  kernel/sched/ext/ext.h |  6 ++++++
>  3 files changed, 26 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index bb79fcfa21cd2..d578dd635f2eb 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7217,13 +7217,12 @@ static void __sched notrace __schedule(int sched_mode)
>  		}
>  	} else if (!preempt && prev_state) {
>  		/*
> -		 * We pass task_is_blocked() as the should_block arg
> -		 * in order to keep mutex-blocked tasks on the runqueue
> -		 * for slection with proxy-exec (without proxy-exec
> -		 * task_is_blocked() will always be false).
> +		 * Keep mutex-blocked tasks on the runqueue for proxy execution
> +		 * only when their scheduling class allows it. Without proxy
> +		 * execution, task_is_blocked() always returns false.
>  		 */
>  		try_to_block_task(rq, prev, &prev_state,
> -				  !task_is_blocked(prev));
> +				  !task_is_blocked(prev) || !scx_allow_proxy_exec(prev));
>  		switch_count = &prev->nvcsw;
>  	}
>  
> @@ -7244,6 +7243,7 @@ static void __sched notrace __schedule(int sched_mode)
>  			}
>  			if (next == rq->idle) {
>  				zap_balance_callbacks(rq);
> +				scx_proxy_resolved(rq);
>  				goto keep_resched;
>  			}
>  		}
> @@ -7264,6 +7264,8 @@ 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_resolved(rq);
>  	} else {
>  		rq_set_donor(rq, next);
>  	}

Can you expand on the need for scx_proxy_resolved() ? I understand the
other two, but this one I'm struggling with a bit.