Re: [PATCH 1/2] sched/eevdf: Cap protection when current has the shortest slice

[Christian Loehle <christian.loehle@arm.com>]

On 9/30/26 09:47, Christian Loehle wrote:
> Changing a task's slice does not discard its remaining request. With
> PLACE_REL_DEADLINE, the preserved deadline can therefore extend beyond
> the new slice after sched_setattr() reduces it.
> 
> set_protect_slice() starts from that deadline and only applies the slice
> cap when another entity has a shorter slice. If current itself has the
> shortest slice, a later pick can protect it for the remainder of the old
> request. update_curr() then need not request another selection when a
> competitor becomes eligible.
> 
> For a preserved deadline d beyond the new virtual slice vslice:
> 
>            v                 v + vslice                  d
>            |---------------------|----------------------|
>     old:   |<---------------- protection -------------->|
>     new:   |<---- protection --->|
> 
> Commit aae2a33ea662 ("sched/eevdf: Ensure that vprot will never go above a
> min slice") caps protection even when the ineligibility boundary is later,
> but leaves this slice == se->slice case uncovered.
> 
> Apply the minimum-slice cap unconditionally, including when it is
> current's own slice. Keep the earlier ineligibility boundary for the
> PREEMPT_SHORT case when a shorter slice is competing. This preserves the
> outstanding request while limiting protection at each fresh pick.
> 
> Fixes: 82e9d0456e06 ("sched/fair: Avoid re-setting virtual deadline on 'migrations'")
> Signed-off-by: Christian Loehle <christian.loehle@arm.com>
> ---
>  kernel/sched/fair.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 85bf02570473..868c3911337a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1128,13 +1128,12 @@ static inline void set_protect_slice(struct cfs_rq *cfs_rq, struct sched_entity
>  		slice = cfs_rq_min_slice(cfs_rq);
>  
>  	slice = min(slice, se->slice);
> +	/* A preserved deadline can extend beyond the current slice. */
> +	vprot = min_vruntime(vprot, se->vruntime + calc_delta_fair(slice, se));
>  
>  	/* If there are shorter slices than se's one */
> -	if (slice != se->slice) {
> -		vprot = min_vruntime(vprot, se->vruntime + calc_delta_fair(slice, se));
> -		if (sched_feat(PREEMPT_SHORT))
> -			vprot = min_vruntime(vprot, ineligible_vruntime(cfs_rq));
> -	}
> +	if (sched_feat(PREEMPT_SHORT) && slice != se->slice)
> +		vprot = min_vruntime(vprot, ineligible_vruntime(cfs_rq));
>  
>  	se->vprot = vprot;
>  }

Elif (+CC) made me aware that apart from the slice protection boundary and vruntime, which is:
set_protect_slice(): deadline, se->vruntime
update_protect_slice(): vprot, min(se->vruntime, avg_vruntime())

this now mirrors update_protect_slice() so should probably be merged into one, I'll resend.