linux-kernel - Re: [PATCH 06/11] sched_ext: Add a DL server for sched

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Message-ID: <aQx_rJccmaJEOAUk@jlelli-thinkpadt14gen4.remote.csb>
Date: Thu, 6 Nov 2025 11:59:56 +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 06/11] sched_ext: Add a DL server for sched_ext tasks

Hi,

On 29/10/25 20:08, 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.
> 
> A kselftest is also provided later to verify:
> 
>  # ./runner -t rt_stall
>  ===== START =====
>  TEST: rt_stall
>  DESCRIPTION: Verify that RT tasks cannot stall SCHED_EXT tasks
>  OUTPUT:
>  # Runtime of EXT task (PID 23338) is 0.250000 seconds
>  # Runtime of RT task (PID 23339) is 4.750000 seconds
>  # EXT task got 5.00% of total runtime
>  ok 1 PASS: EXT task got more than 4.00% of runtime
>  =====  END  =====
> 
> 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)
> 
> Cc: Luigi De Matteis <ldematteis123@...il.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>
> ---

...

> @@ -1409,6 +1412,15 @@ static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int enq_flags
>  	if (enq_flags & SCX_ENQ_WAKEUP)
>  		touch_core_sched(rq, p);
>  
> +	if (rq->scx.nr_running == 1) {
> +		/* Account for idle runtime */
> +		if (!rq->nr_running)

Hummm, didn't we just add_nr_running(rq, 1) before gettng here?

> +			dl_server_update_idle_time(rq, rq->curr, &rq->ext_server);
> +
> +		/* Start dl_server if this is the first task being enqueued */
> +		dl_server_start(&rq->ext_server);
> +	}
> +

Thanks,
Juri