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

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Date:   Wed, 8 Nov 2023 09:01:17 +0100
From:   Daniel Bristot de Oliveira <bristot@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.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 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.

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.

> 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