Re: [PATCH sched_ext/for-7.1] sched_ext: Documentation: Add ops.dequeue() to task lifecycle

[Kuba Piecuch]

Hi Andrea,

On Mon Apr 6, 2026 at 11:47 AM UTC, Andrea Righi wrote:
> Document ops.dequeue() in the sched_ext task lifecycle now that its
> semantics are well-defined.
>
> Also update the pseudo-code to use task_is_runnable() consistently and
> clarify the case where ops.dispatch() does not refill the time slice.
>
> Fixes: ebf1ccff79c4 ("sched_ext: Fix ops.dequeue() semantics")
> Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
> ---
>  Documentation/scheduler/sched-ext.rst | 24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)
>
> diff --git a/Documentation/scheduler/sched-ext.rst b/Documentation/scheduler/sched-ext.rst
> index 404b4e4c33f7e..9f03650abfeba 100644
> --- a/Documentation/scheduler/sched-ext.rst
> +++ b/Documentation/scheduler/sched-ext.rst
> @@ -422,23 +422,29 @@ by a sched_ext scheduler:
>  
>          ops.runnable();         /* Task becomes ready to run */
>  
> -        while (task is runnable) {
> +        while (task_is_runnable(task)) {
>              if (task is not in a DSQ && task->scx.slice == 0) {
>                  ops.enqueue();  /* Task can be added to a DSQ */
>  
> -                /* Any usable CPU becomes available */
> +                /* Task property change (i.e., affinity, nice, etc.)? */
> +                if (sched_change(task)) {
> +                    ops.dequeue(); /* Exiting BPF scheduler custody */

Doesn't the task also go through quiescent -> runnable here? The full path
being dequeue -> quiescent -> (actual property change) -> runnable -> enqueue.

I guess we should be accurate here since quiescent and runnable are present
elsewhere in the pseudocode.

> +                    continue;
> +                }
> +            }
>  
> -                ops.dispatch(); /* Task is moved to a local DSQ */
> +            /* Any usable CPU becomes available */
> +
> +            ops.dispatch();     /* Task is moved to a local DSQ */

s/local/terminal/?

> +            ops.dequeue();      /* Exiting BPF scheduler custody */
>  
> -                ops.dequeue(); /* Exiting BPF scheduler */
> -            }
>              ops.running();      /* Task starts running on its assigned CPU */
>  
> -            while task_is_runnable(p) {
> -                while (task->scx.slice > 0 && task_is_runnable(p))
> -                    ops.tick();     /* Called every 1/HZ seconds */
> +            while (task_is_runnable(task) && task->scx.slice > 0) {
> +                ops.tick();     /* Called every 1/HZ seconds */
>  
> -                ops.dispatch();     /* task->scx.slice can be refilled */
> +                if (task->scx.slice == 0)
> +                    ops.dispatch(); /* task->scx.slice can be refilled */
>              }
>  
>              ops.stopping();     /* Task stops running (time slice expires or wait) */

Thanks,
Kuba