linux-kernel - Re: [PATCH 05/16] sched/deadline: Return EBUSY if dl_bw

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Message-ID: <aLhWh9_bJ5oKlQ3O@jlelli-thinkpadt14gen4.remote.csb>
Date: Wed, 3 Sep 2025 16:53:59 +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,
	Luca Abeni <luca.abeni@...tannapisa.it>,
	Yuri Andriaccio <yurand2000@...il.com>
Subject: Re: [PATCH 05/16] sched/deadline: Return EBUSY if dl_bw_cpus is zero

Hi,

On 03/09/25 11:33, Andrea Righi wrote:
> From: Joel Fernandes <joelagnelf@...dia.com>
> 
> Hotplugged CPUs coming online do an enqueue but are not a part of any
> root domain containing cpu_active() CPUs. So in this case, don't mess
> with accounting and we can retry later. Without this patch, we see
> crashes with sched_ext selftest's hotplug test due to divide by zero.
> 
> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
> ---
>  kernel/sched/deadline.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 3c478a1b2890d..753e50b1e86fc 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1689,7 +1689,12 @@ int dl_server_apply_params(struct sched_dl_entity *dl_se, u64 runtime, u64 perio
>  	cpus = dl_bw_cpus(cpu);
>  	cap = dl_bw_capacity(cpu);
>  
> -	if (__dl_overflow(dl_b, cap, old_bw, new_bw))
> +	/*
> +	 * Hotplugged CPUs coming online do an enqueue but are not a part of any
> +	 * root domain containing cpu_active() CPUs. So in this case, don't mess
> +	 * with accounting and we can retry later.
> +	 */
> +	if (!cpus || __dl_overflow(dl_b, cap, old_bw, new_bw))
>  		return -EBUSY;
>  
>  	if (init) {

Yuri is proposing to ignore dl-servers bandwidth contribution from
admission control (as they essentially operate on the remaining
bandwidth portion not available to RT/DEADLINE tasks):

https://lore.kernel.org/lkml/20250903114448.664452-1-yurand2000@gmail.com/

His patch should make this patch not required. Would you be able and
willing to test this assumption?

I don't believe Peter already expressed his opinion on what Yuri is
proposing, so this might be moot. But if we go that way all dl-servers
should share that non-RT portion of bandwidth I would guess. And we will
need to probably add checks and subdivide among active dl-servers, don't
we? Peter, others, what do you think?

Thanks,
Juri