linux-kernel - Re: [PATCH 06/14] sched_ext: Add a DL server for sched

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Message-ID: <aPYj-iOdvgUYQFpn@jlelli-thinkpadt14gen4.remote.csb>
Date: Mon, 20 Oct 2025 13:58:50 +0200
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>,
	Joel Fernandes <joelagnelf@...dia.com>, Tejun Heo <tj@...nel.org>,
	David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>, Shuah Khan <shuah@...nel.org>,
	sched-ext@...ts.linux.dev, bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Luigi De Matteis <ldematteis123@...il.com>
Subject: Re: [PATCH 06/14] sched_ext: Add a DL server for sched_ext tasks

Hi!

On 17/10/25 11:25, Andrea Righi wrote:
> From: Joel Fernandes <joelagnelf@...dia.com>
> 
> 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:
> TAP version 13
> 1..1
> ok 1 PASS: CFS task got more than 4.00% of runtime
> 
> [ arighi: drop ->balance() now that pick_task() has an rf argument ]
> 
> Cc: Luigi De Matteis <ldematteis123@...il.com>
> Co-developed-by: Andrea Righi <arighi@...dia.com>
> Signed-off-by: Andrea Righi <arighi@...dia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
> ---
>  kernel/sched/core.c     |  3 +++
>  kernel/sched/deadline.c |  2 +-
>  kernel/sched/ext.c      | 51 +++++++++++++++++++++++++++++++++++++++--
>  kernel/sched/sched.h    |  2 ++
>  4 files changed, 55 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 096e8d03d85e7..31a9c9381c63f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8679,6 +8679,9 @@ void __init sched_init(void)
>  		hrtick_rq_init(rq);
>  		atomic_set(&rq->nr_iowait, 0);
>  		fair_server_init(rq);
> +#ifdef CONFIG_SCHED_CLASS_EXT
> +		ext_server_init(rq);
> +#endif
>  
>  #ifdef CONFIG_SCHED_CORE
>  		rq->core = rq;
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 0680e0186577a..3c1fd2190949e 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1504,7 +1504,7 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64
>  	 * The fair server (sole dl_server) does not account for real-time

Fair server is not alone anymore. :))

Please update the comment as well.

>  	 * workload because it is running fair work.
>  	 */
> -	if (dl_se == &rq->fair_server)
> +	if (dl_se->dl_server)
>  		return;
>  
>  #ifdef CONFIG_RT_GROUP_SCHED

...

> @@ -1487,6 +1499,11 @@ static bool dequeue_task_scx(struct rq *rq, struct task_struct *p, int deq_flags
>  	sub_nr_running(rq, 1);
>  
>  	dispatch_dequeue(rq, p);
> +
> +	/* Stop the server if this was the last task */
> +	if (rq->scx.nr_running == 0)
> +		dl_server_stop(&rq->ext_server);
> +

Do we want to use the delayed stop behavior for scx-server as we have
for fair-server? Wonder if it's a matter of removing this explicit stop
and wait for a full period to elapse as we do for fair. It should reduce
timer reprogramming overhead for scx as well.

Thanks,
Juri