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

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Message-ID: <aLhdvY1D_RZF_ahw@gpd4>
Date: Wed, 3 Sep 2025 17:24:45 +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 05:15:18PM +0200, Juri Lelli wrote:
> On 03/09/25 17:10, Andrea Righi wrote:
> > 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").
> 
> Please mind that Yuri's change is still under discussion! :))
> 
> I just wanted to mention it here as it might change how we account for
> dl-servers if we decide to go that way.

That's fine, I've already done a quick test. :)

It seems to work (more or less), meaning that in case of RT/sched_ext
contention the sched_ext tasks seem to get the right amount of CPU
bandwidth (5%), but the total_bw kselftest is quite broken and it's always
reporting a bw value of 0... in any case, even if we go this way there's no
major disruption apparently.

-Andrea