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

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Message-ID: <aLhafcdtmW6s-ydD@gpd4>
Date: Wed, 3 Sep 2025 17:10:53 +0200
From: Andrea Righi <arighi@...dia.com>
To: Juri Lelli <juri.lelli@...hat.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

On Wed, Sep 03, 2025 at 04:53:59PM +0200, Juri Lelli wrote:
> 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'll run some tests with Yuri's patch applied and dropping this one (and we
may also need to drop "[PATCH 10/16] sched/deadline: Account ext server
bandwidth").

Thanks,
-Andrea