Re: [PATCH sched_ext/for-7.3-fixes] sched_ext: Derive SCX_RQ_IN_WAKEUP from the core enqueue flags

[Andrea Righi <arighi@nvidia.com>]

Hi Tejun,

On Wed, Sep 16, 2026 at 12:00:24PM -1000, Tejun Heo wrote:
> schedule_deferred_locked() skips scheduling a deferred action while
> SCX_RQ_IN_WAKEUP is set and relies on the task_woken_scx() call that follows
> a wakeup enqueue to run it. enqueue_task_scx() sets the flag from the merged
> enqueue flags, which include the flags stashed for a remote activation.
> move_remote_task_to_local_dsq() thus sets SCX_RQ_IN_WAKEUP on the
> destination rq when the moved task was woken up, although no
> task_woken_scx() follows that activation.
> 
> An IMMED insert into a busy destination requests a local reenqueue during
> that enqueue. The request gets linked but not scheduled and stays pending
> until an unrelated wakeup or preemption on that CPU runs the deferred
> actions. The IMMED task sits behind the running task in the meantime. If
> nothing runs them before the scheduler is disabled, the request outlives the
> scheduler and points into its freed per-cpu area, which the next scheduler
> dereferences from run_deferred().
> 
> Test the core enqueue flags for the wakeup bit. Only the core's wakeup path
> is followed by task_woken_scx().
> 
> Fixes: 57ccf5ccdc56 ("sched_ext: Fix enqueue_task_scx() truncation of upper enqueue flags")
> Cc: stable@vger.kernel.org # v7.1+
> Reported-by: Andrea Righi <arighi@nvidia.com>
> Link: https://lore.kernel.org/all/20260916145807.3250167-1-arighi@nvidia.com/
> Signed-off-by: Tejun Heo <tj@kernel.org>

This looks better than my fix.

Reviewed-by: Andrea Righi <arighi@nvidia.com>

Thanks,
-Andrea

> ---
>  kernel/sched/ext/ext.c |    7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> --- a/kernel/sched/ext/ext.c
> +++ b/kernel/sched/ext/ext.c
> @@ -2141,7 +2141,12 @@ static void enqueue_task_scx(struct rq *
>  	int sticky_cpu = p->scx.sticky_cpu;
>  	u64 enq_flags = core_enq_flags | rq->scx.remote_activate_enq_flags;
>  
> -	if (enq_flags & ENQUEUE_WAKEUP)
> +	/*
> +	 * SCX_RQ_IN_WAKEUP promises a task_woken_scx() call once this enqueue
> +	 * returns. Only the core's wakeup path delivers one. The flags stashed
> +	 * for a remote activation may carry the wakeup bit without it.
> +	 */
> +	if (core_enq_flags & ENQUEUE_WAKEUP)
>  		rq->scx.flags |= SCX_RQ_IN_WAKEUP;
>  
>  	/*