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

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Message-ID: <aLk9BNnFYZ3bhVAE@jlelli-thinkpadt14gen4.remote.csb>
Date: Thu, 4 Sep 2025 09:17:24 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Andrea Righi <arighi@...dia.com>, Ingo Molnar <mingo@...hat.com>,
	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 03/09/25 22:05, Peter Zijlstra 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 don't believe Peter already expressed his opinion on what Yuri is
> > proposing, so this might be moot. 
> 
> Urgh, yeah, I don't like that at all. That reasoning makes no sense what
> so ever. That 5% is not lost time, that 5% is being very optimistic and
> 'models' otherwise unaccountable time like IRQ and random overheads.

But, wait. For dealing with IRQs and random overheads we usually say
'inflate your reservations', e.g. add a 3-5% to your runtime so that it
is sound against reality. And that gets included already in the 95%
default max cap and schedulability tests.

I believe what Yuri is saying is that dl-servers are different, because
they are only a safety net and don't provide any guarantees. With RT
throttling we used to run non-RT on the remaining 5% (from 95%) and with
Yuri's change we are going to go back at doing the same, but with
dl-server(s). If we don't do that we are somewhat going to pay overheads
twice, first we must inflate real reservations or your tasks gets
prematurely throttled, second we remove 5% of overall bandwidth if
dl-servers are accounted for with the rest of real reservation.

What do you think? :)