linux-kernel - Re: [PATCH 04/14] sched/deadline: Return EBUSY if dl_bw

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Message-ID: <aPYFv6YcxqWez8aK@jlelli-thinkpadt14gen4.remote.csb>
Date: Mon, 20 Oct 2025 11:49:51 +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
Subject: Re: [PATCH 04/14] sched/deadline: Return EBUSY if dl_bw_cpus is zero

Hi!

On 17/10/25 11:25, 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 4aefb34a1d38b..f2f5b1aea8e2b 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1665,7 +1665,12 @@ 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.

Later when? It seems a little vague. :)

Thanks,
Juri