Re: [PATCH 08/18] sched: Introduce WF_ON_RQ wake flag

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Aug 31, 2026 at 03:42:18PM +0200, Andrea Righi wrote:
> ttwu_runnable() handles wakeups for tasks which are already on the
> runqueue, but scheduling classes cannot distinguish that path from a
> full wakeup activation in wakeup_preempt().
> 
> Pass WF_ON_RQ to wakeup_preempt() from ttwu_runnable() so scheduling
> classes can handle already-runnable wakeups separately.
> 
> This is a preparatory change to support proxy execution with sched_ext.
> No functional change.
> 
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
>  kernel/sched/core.c  | 2 +-
>  kernel/sched/sched.h | 1 +
>  2 files changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index d578dd635f2eb..f142ab455b797 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3908,7 +3908,7 @@ static int ttwu_runnable(struct task_struct *p, int wake_flags)
>  		 * When on_rq && !on_cpu the task is preempted, see if
>  		 * it should preempt the task that is current now.
>  		 */
> -		wakeup_preempt(rq, p, wake_flags);
> +		wakeup_preempt(rq, p, wake_flags | WF_ON_RQ);
>  	}
>  	ttwu_do_wakeup(p);
>  	return 1;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 56ac104f1f825..f30d122909e7b 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2538,6 +2538,7 @@ static inline int task_on_rq_migrating(struct task_struct *p)
>  #define WF_MIGRATED		0x20 /* Internal use, task got migrated */
>  #define WF_CURRENT_CPU		0x40 /* Prefer to move the wakee to the current CPU. */
>  #define WF_RQ_SELECTED		0x80 /* ->select_task_rq() was called */
> +#define WF_ON_RQ		0x100 /* Wakeup of an already runnable task */

I are confused again... help?

wakeup_preempt() can only ever be called for tasks that are on the
runqueue. That is more or less the point of wakeup-preemption.

Even the already-runnable distinction would mean things like
move_queued_task() should also have this. The task is new to this
runqueue, but was definitely already runnable before.

What actual distinction are you needing?