Re: [PATCH] sched/eevdf: Fix vprot across reweighting

[Kayra Cizmeci <kayracizmeci@gmail.com>]

> I don't get why you need to a se->rel_vprot and all these changes . If
> we want to ensure that protection will not be re enabled because of
> rescale, we just need the below

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 5c98d8dfce5d..e83836fe44ef 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4951,12 +4951,15 @@ static void reweight_eevdf(struct cfs_rq
> *cfs_rq, struct sched_entity *se,
>         update_load_set(&se->h_load, weight);
> 
>         if (on_rq) {
> -               if (rel_vprot)
> -                       se->vprot += avruntime;
>                 se->deadline += avruntime;
>                 se->rel_deadline = 0;
>                 se->vruntime = avruntime - se->vlag;
> 
> +               if (rel_vprot)
> +                       se->vprot += avruntime;
> +               else if (curr)
> +                       se->vprot = se->vruntime;
> +
>                 if (!curr)
>                         __enqueue_entity(cfs_rq, se);
>                 cfs_rq->h_nr_queued++;

> or the below which is even simpler because vprot, vlag and deadline
> are rescaled with the same ratio but it implies useless div64_long()
> when the protection is already canceled

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 5c98d8dfce5d..5d1b16780b52 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4936,7 +4936,7 @@ static void reweight_eevdf(struct cfs_rq
> *cfs_rq, struct sched_entity *se,
>                 se->vlag = entity_lag(cfs_rq, se, avruntime);
>                 se->deadline -= avruntime;
>                 se->rel_deadline = 1;
> -               if (curr && protect_slice(se)) {
> +               if (curr) {
>                         se->vprot -= avruntime;
>                         rel_vprot = true;
>                 }

Here:

https://lore.kernel.org/all/20261001164609.244156-1-kayracizmeci@gmail.com/
https://lore.kernel.org/all/e4700a93-ebd5-4c7e-88b5-d606437a13c0@arm.com/

Because on_rq can get false. And alive protection complicates things.
And this diff alone wouldn't work.

IDK what else we could do to fix this both cases cleanly tbh.
But there could be a cleaner way.

Thanks,

Kayra