linux-kernel - Re: [RFC][PATCH 1/5] sched/deadline: Less agressive dl

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Message-ID: <aD8cwJGQz6iCjhwz@jlelli-thinkpadt14gen4.remote.csb>
Date: Tue, 3 Jun 2025 18:03:12 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...hat.com, vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, clm@...a.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 1/5] sched/deadline: Less agressive dl_server
 handling

Hi,

On 20/05/25 11:45, Peter Zijlstra wrote:
> Chris reported that commit 5f6bd380c7bd ("sched/rt: Remove default
> bandwidth control") caused a significant dip in his favourite
> benchmark of the day. Simply disabling dl_server cured things.
> 
> His workload hammers the 0->1, 1->0 transitions, and the
> dl_server_{start,stop}() overhead kills it -- fairly obviously a bad
> idea in hind sight and all that.
> 
> Change things around to only disable the dl_server when there has not
> been a fair task around for a whole period. Since the default period
> is 1 second, this ensures the benchmark never trips this, overhead
> gone.
> 
> Fixes: 557a6bfc662c ("sched/fair: Add trivial fair server")
> Reported-by: Chris Mason <clm@...a.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  include/linux/sched.h   |    1 +
>  kernel/sched/deadline.c |   31 +++++++++++++++++++++++++++----
>  2 files changed, 28 insertions(+), 4 deletions(-)
> 
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -702,6 +702,7 @@ struct sched_dl_entity {
>  	unsigned int			dl_defer	  : 1;
>  	unsigned int			dl_defer_armed	  : 1;
>  	unsigned int			dl_defer_running  : 1;
> +	unsigned int			dl_server_idle    : 1;
>  
>  	/*
>  	 * Bandwidth enforcement timer. Each -deadline task has its
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1215,6 +1215,8 @@ static void __push_dl_task(struct rq *rq
>  /* a defer timer will not be reset if the runtime consumed was < dl_server_min_res */
>  static const u64 dl_server_min_res = 1 * NSEC_PER_MSEC;
>  
> +static bool dl_server_stopped(struct sched_dl_entity *dl_se);
> +
>  static enum hrtimer_restart dl_server_timer(struct hrtimer *timer, struct sched_dl_entity *dl_se)
>  {
>  	struct rq *rq = rq_of_dl_se(dl_se);
> @@ -1234,6 +1236,7 @@ static enum hrtimer_restart dl_server_ti
>  
>  		if (!dl_se->server_has_tasks(dl_se)) {
>  			replenish_dl_entity(dl_se);
> +			dl_server_stopped(dl_se);
>  			return HRTIMER_NORESTART;
>  		}
>  
> @@ -1639,8 +1642,10 @@ void dl_server_update_idle_time(struct r
>  void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
>  {
>  	/* 0 runtime = fair server disabled */
> -	if (dl_se->dl_runtime)
> +	if (dl_se->dl_runtime) {
> +		dl_se->dl_server_idle = 0;
>  		update_curr_dl_se(dl_se->rq, dl_se, delta_exec);
> +	}
>  }
>  
>  void dl_server_start(struct sched_dl_entity *dl_se)
> @@ -1663,7 +1668,7 @@ void dl_server_start(struct sched_dl_ent
>  		setup_new_dl_entity(dl_se);
>  	}
>  
> -	if (!dl_se->dl_runtime)
> +	if (!dl_se->dl_runtime || dl_se->dl_server_active)
>  		return;
>  
>  	dl_se->dl_server_active = 1;
> @@ -1672,7 +1677,7 @@ void dl_server_start(struct sched_dl_ent
>  		resched_curr(dl_se->rq);
>  }
>  
> -void dl_server_stop(struct sched_dl_entity *dl_se)
> +static void __dl_server_stop(struct sched_dl_entity *dl_se)
>  {
>  	if (!dl_se->dl_runtime)
>  		return;
> @@ -1684,6 +1689,24 @@ void dl_server_stop(struct sched_dl_enti
>  	dl_se->dl_server_active = 0;
>  }
>  
> +static bool dl_server_stopped(struct sched_dl_entity *dl_se)
> +{
> +	if (!dl_se->dl_server_active)
> +		return false;
> +
> +	if (dl_se->dl_server_idle) {
> +		__dl_server_stop(dl_se);
> +		return true;
> +	}
> +
> +	dl_se->dl_server_idle = 1;
> +	return false;
> +}
> +
> +void dl_server_stop(struct sched_dl_entity *dl_se)
> +{
> +}

What if we explicitly set the server to idle (instead of ignoring the
stop) where this gets called in dequeue_entities()? Also, don't we need
to actually stop the server if we are changing its parameters from
sched_fair_server_write()?

Thanks,
Juri