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

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Message-ID: <20231108182520.GD2992223@google.com>
Date:   Wed, 8 Nov 2023 18:25:20 +0000
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

On Wed, Nov 08, 2023 at 09:01:17AM +0100, Daniel Bristot de Oliveira wrote:
> On 11/8/23 04:20, Joel Fernandes wrote:
> > 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.
> 
> No grub needed. It will only happen if the fair server did not have any chance to run.
> 
> If it happens, it is not a problem, see that timeline I replied in the previous
> email.

Ah you're right, I mistakenly read your diff assuming you were calling
replenish_dl_new_period() on dl_entity_overflow(). Indeed the diff is needed
(I was actually wondering about why that was not done in my initial review as
well -- so its good we found it in discussion).

> We do not want a zerolax scheduler, because it breaks everything else. It is
> a deferred EDF, that looking from wall clock, composes an "zerolaxish" timeline.

Indeed. I was not intending that we do zerolax scheduler, I was merely
misreading the diff assuming you were throttling the DL-server once again at
the zerolax time.

thanks,

 - Joel