linux-kernel - Re: [PATCH 4/7] sched_ext: Add a DL server for sched

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Message-ID: <aULQ7kPm-RqHWGDL@jlelli-thinkpadt14gen4.remote.csb>
Date: Wed, 17 Dec 2025 16:49:02 +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>, sched-ext@...ts.linux.dev,
	bpf@...r.kernel.org, linux-kselftest@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] sched_ext: Add a DL server for sched_ext tasks

Hi!

On 17/12/25 10:35, Andrea Righi wrote:
> sched_ext currently suffers starvation due to RT. The same workload when
> converted to EXT can get zero runtime if RT is 100% running, causing EXT
> processes to stall. Fix it by adding a DL server for EXT.

...

> v4: - initialize EXT server bandwidth reservation at init time and
>       always keep it active (Andrea Righi)
>     - check for rq->nr_running == 1 to determine when to account idle
>       time (Juri Lelli)
> v3: - clarify that fair is not the only dl_server (Juri Lelli)
>     - remove explicit stop to reduce timer reprogramming overhead
>       (Juri Lelli)
>     - do not restart pick_task() when it's invoked by the dl_server
>       (Tejun Heo)
>     - depend on CONFIG_SCHED_CLASS_EXT (Andrea Righi)
> v2: - drop ->balance() now that pick_task() has an rf argument
>       (Andrea Righi)
> 
> Tested-by: Christian Loehle <christian.loehle@....com>
> Co-developed-by: Joel Fernandes <joelagnelf@...dia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
> Signed-off-by: Andrea Righi <arighi@...dia.com>
> ---

...

> @@ -3090,6 +3123,15 @@ static void switching_to_scx(struct rq *rq, struct task_struct *p)
>  static void switched_from_scx(struct rq *rq, struct task_struct *p)
>  {
>  	scx_disable_task(p);
> +
> +	/*
> +	 * After class switch, if the DL server is still active, restart it so
> +	 * that DL timers will be queued, in case SCX switched to higher class.
> +	 */
> +	if (dl_server_active(&rq->ext_server)) {
> +		dl_server_stop(&rq->ext_server);
> +		dl_server_start(&rq->ext_server);
> +	}
>  }

We might have discussed this already, in that case I forgot, sorry. But,
why we do need to start the server again if switched from scx? Couldn't
make sense of the comment that is already present.

Thanks,
Juri