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@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx>
> > > ---
> > > 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@xxxxxxxxx/
> >
> > 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? :)
