linux-kernel - Re: [PATCH v5 6/7] sched/deadline: Deferrable dl server

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Date:   Tue, 7 Nov 2023 22:20:55 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Daniel Bristot de Oliveira <bristot@...nel.org>
Cc:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vineeth Pillai <vineeth@...byteword.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Phil Auld <pauld@...hat.com>
Subject: Re: [PATCH v5 6/7] sched/deadline: Deferrable dl server

Hi Daniel,

On Tue, Nov 7, 2023 at 1:50 PM Daniel Bristot de Oliveira
<bristot@...nel.org> wrote:
>
> > The code is not doing what I intended because I thought it was doing overload
> > control on the replenishment, but it is not (my bad).
> >
>
> I am still testing but... it is missing something like this (famous last words).
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 1092ca8892e0..6e2d21c47a04 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -842,6 +842,8 @@ static inline void setup_new_dl_entity(struct sched_dl_entity *dl_se)
>   * runtime, or it just underestimated it during sched_setattr().
>   */
>  static int start_dl_timer(struct sched_dl_entity *dl_se);
> +static bool dl_entity_overflow(struct sched_dl_entity *dl_se, u64 t);
> +
>  static void replenish_dl_entity(struct sched_dl_entity *dl_se)
>  {
>         struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
> @@ -852,9 +854,18 @@ static void replenish_dl_entity(struct sched_dl_entity *dl_se)
>         /*
>          * This could be the case for a !-dl task that is boosted.
>          * Just go with full inherited parameters.
> +        *
> +        * Or, it could be the case of a zerolax reservation that
> +        * was not able to consume its runtime in background and
> +        * reached this point with current u > U.
> +        *
> +        * In both cases, set a new period.
>          */
> -       if (dl_se->dl_deadline == 0)
> -               replenish_dl_new_period(dl_se, rq);
> +       if (dl_se->dl_deadline == 0 ||
> +               (dl_se->dl_zerolax_armed && dl_entity_overflow(dl_se, rq_clock(rq)))) {
> +                       dl_se->deadline = rq_clock(rq) + pi_of(dl_se)->dl_deadline;
> +                       dl_se->runtime = pi_of(dl_se)->dl_runtime;
> +       }
>
>         if (dl_se->dl_yielded && dl_se->runtime > 0)
>                 dl_se->runtime = 0;

I was wondering does this mean GRUB needs to be enabled? Otherwise I
can see that "runtime / (deadline - t) > dl_runtime / dl_deadline"
will be true almost all the time due to the constraint of executing at
the 0-lax time.

Because at the 0-lax time, AFAICS this will be 100% > 30% (say if CFS
has a 30% reservation).

And I think even if GRUB is enabled, it is possible other DL task may
have reserved bandwidth.

Or is there a subtlety that makes that not possible?

thanks,

 - Joel