Re: [PATCH 04/14] sched/deadline: Return EBUSY if dl_bw_cpus is zero

[Andrea Righi <arighi@nvidia.com>]

On Mon, Oct 20, 2025 at 11:49:51AM +0200, Juri Lelli wrote:
> Hi!
> 
> On 17/10/25 11:25, 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 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. :)

Yeah, this comment is actually incorrect, we're not "retrying later"
anymore (we used to do that in a previous version), now the params are
applied via:

  ext.c:handle_hotplug() -> dl_server_on() -> dl_server_apply_params()

Or via scx_enable() when an scx scheduler is loaded. So, I'm wondering if
this condition is still needed. Will do some tests.

Thanks!
-Andrea