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

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Date:   Wed, 8 Nov 2023 13:44:01 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Bristot de Oliveira <bristot@...nel.org>
Cc:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Ingo Molnar <mingo@...hat.com>,
        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 Tue, Nov 07, 2023 at 07:50:28PM +0100, Daniel Bristot de Oliveira 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;

Should we rather not cap the runtime, something like so?

Because the above also causes period drift, which we do not want.

---
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 58b542bf2893..1453a2cd0680 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -829,10 +829,12 @@ static inline void setup_new_dl_entity(struct sched_dl_entity *dl_se)
  */
 static void replenish_dl_entity(struct sched_dl_entity *dl_se)
 {
+	struct sched_dl_entity *pi_se = pi_of(dl_se);
 	struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
 	struct rq *rq = rq_of_dl_rq(dl_rq);
+	u64 dl_runtime = pi_se->dl_runtime;
 
-	WARN_ON_ONCE(pi_of(dl_se)->dl_runtime <= 0);
+	WARN_ON_ONCE(dl_runtime <= 0);
 
 	/*
 	 * This could be the case for a !-dl task that is boosted.
@@ -851,10 +853,13 @@ static void replenish_dl_entity(struct sched_dl_entity *dl_se)
 	 * arbitrary large.
 	 */
 	while (dl_se->runtime <= 0) {
-		dl_se->deadline += pi_of(dl_se)->dl_period;
-		dl_se->runtime += pi_of(dl_se)->dl_runtime;
+		dl_se->deadline += pi_se->dl_period;
+		dl_se->runtime += dl_runtime;
 	}
 
+	if (dl_se->zerolax && dl_se->runtime > dl_runtime)
+		dl_se->runtime = dl_runtime;
+
 	/*
 	 * At this point, the deadline really should be "in
 	 * the future" with respect to rq->clock. If it's