Re: [PATCH 05/16] sched/deadline: Return EBUSY if dl_bw_cpus is zero

[Peter Zijlstra <peterz@infradead.org>]

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@nvidia.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@nvidia.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.

Thinking you can give out 100% CPU time to a bandwidth limited group of
tasks is delusional.

Explicitly not accounting things that you *can* is just plain wrong. So
no, Yuri's thing is not going to go anywhere.