linux-kernel - Re: [PATCH 02/11] sched/debug: Stop and start server based on if it was active

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Message-ID: <aQxKo68TJge5dRZI@jlelli-thinkpadt14gen4.remote.csb>
Date: Thu, 6 Nov 2025 08:13:39 +0100
From: Juri Lelli <juri.lelli@...hat.com>
To: Andrea Righi <arighi@...dia.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>, Tejun Heo <tj@...nel.org>,
	David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>, Shuah Khan <shuah@...nel.org>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Christian Loehle <christian.loehle@....com>,
	Emil Tsalapatis <emil@...alapatis.com>,
	Luigi De Matteis <ldematteis123@...il.com>,
	sched-ext@...ts.linux.dev, bpf@...r.kernel.org,
	linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/11] sched/debug: Stop and start server based on if it
 was active

Hi,

On 29/10/25 20:08, Andrea Righi wrote:
> From: Joel Fernandes <joelagnelf@...dia.com>
> 
> Currently the DL server interface for applying parameters checks
> CFS-internals to identify if the server is active. This is error-prone
> and makes it difficult when adding new servers in the future.
> 
> Fix it, by using dl_server_active() which is also used by the DL server
> code to determine if the DL server was started.
> 
> Acked-by: Tejun Heo <tj@...nel.org>
> Reviewed-by: Juri Lelli <juri.lelli@...hat.com>
> Reviewed-by: Andrea Righi <arighi@...dia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
> ---
>  kernel/sched/debug.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 6cf9be6eea49a..e71f6618c1a6a 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -354,6 +354,8 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
>  		return err;
>  
>  	scoped_guard (rq_lock_irqsave, rq) {
> +		bool is_active;
> +
>  		runtime  = rq->fair_server.dl_runtime;
>  		period = rq->fair_server.dl_period;
>  
> @@ -376,8 +378,11 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
>  			return  -EINVAL;
>  		}
>  
> -		update_rq_clock(rq);
> -		dl_server_stop(&rq->fair_server);
> +		is_active = dl_server_active(&rq->fair_server);
> +		if (is_active) {
> +			update_rq_clock(rq);
> +			dl_server_stop(&rq->fair_server);
> +		}
>  
>  		retval = dl_server_apply_params(&rq->fair_server, runtime, period, 0);
>  
> @@ -385,7 +390,7 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
>  			printk_deferred("Fair server disabled in CPU %d, system may crash due to starvation.\n",
>  					cpu_of(rq));
>  
> -		if (rq->cfs.h_nr_queued)
> +		if (is_active)
>  			dl_server_start(&rq->fair_server);

Something that I noticed while reviewing this series is that we still
start back a server even if the user put its runtime to zero (disabling
it) and I don't think we want to do that. It's not of course related to
this change or this series per-se, but something we probably want to fix
independently.

Thanks,
Juri