Re: [PATCH 1/2] sched/fair: Do not let idle entities preempt others

[Madadi Vineeth Reddy]

Hi Abel,

On 21/02/25 16:42, Abel Wu wrote:
> A task with SCHED_IDLE policy doesn't preempt others by definition, and
> the semantics are intended to be preserved when extending to cgroups
> introduced in commit 304000390f88 ("sched: Cgroup SCHED_IDLE support").
> 
> But current implementation allows idle entities to preempt each other
> on wakeup, which seems not behave as expected especially after
> commit faa42d29419d ("sched/fair: Make SCHED_IDLE entity be preempted in strict hierarchy")
> so fix this by explicitly skip wakeup preemption for idle entities.
> 
> Fixes: 304000390f88 ("sched: Cgroup SCHED_IDLE support")
> Signed-off-by: Abel Wu <wuyun.abel@xxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1c0ef435a7aa..4340178f29b7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8778,12 +8778,15 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
>  	pse_is_idle = se_is_idle(pse);
>  
>  	/*
> -	 * Preempt an idle entity in favor of a non-idle entity (and don't preempt
> -	 * in the inverse case).
> +	 * Preempt an idle entity in favor of a non-idle entity.
>  	 */
>  	if (cse_is_idle && !pse_is_idle)
>  		goto preempt;
> -	if (cse_is_idle != pse_is_idle)
> +
> +	/*
> +	 * IDLE entities do not preempt others.
> +	 */
> +	if (unlikely(pse_is_idle))
>  		return;

It makes sense that idle tasks don't preempt each other. The patch
preserves the original behavior: preemption occurs when the currently
running entity (cse) is SCHED_IDLE while the waking entity (pse) is not,
and it correctly returns when cse is not SCHED_IDLE while pse is
SCHED_IDLE.

Reviewed-by: Madadi Vineeth Reddy <vineethr@xxxxxxxxxxxxx>

Thanks,
Madadi Vineeth Reddy

>  
>  	/*