Re: [PATCH] sched/deadline: Skip overflow check if 0 capacity

[Waiman Long]

On 11/8/24 8:25 AM, Juri Lelli wrote:
> On 07/11/24 23:29, Waiman Long wrote:
> > By properly setting up a 1-cpu sched domain (partition) with no
> > task, it was found that offlining that particular CPU failed because
> > dl_bw_check_overflow() in cpuset_cpu_inactive() returned -EBUSY. This
> > is due to the fact that dl_bw_capacity() return 0 as the sched domain
> > has no active CPU causing a false positive in the overflow check.
> >
> > Fix this corner case by skipping the __dl_overflow() check in
> > dl_bw_manage() when the returned capacity is 0.
> >
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > ---
> >   kernel/sched/deadline.c | 8 +++++++-
> >   1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index be1b917dc8ce..0195f350d6d3 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -3479,7 +3479,13 @@ static int dl_bw_manage(enum dl_bw_request req, int cpu, u64 dl_bw)
> >   	} else {
> >   		unsigned long cap = dl_bw_capacity(cpu);
> >   
> > -		overflow = __dl_overflow(dl_b, cap, 0, dl_bw);
> > +		/*
> > +		 * In the unlikely case of 0 capacity (e.g. a sched domain
> > +		 * with no active CPUs), skip the overflow check as it will
> > +		 * always return a false positive.
> > +		 */
> > +		if (likely(cap))
> > +			overflow = __dl_overflow(dl_b, cap, 0, dl_bw);
> The remaining total_bw that make this check fail should be the one
> relative to the dl_server on the cpu that is going offline. Wonder if we
> shouldn't rather clean that up (remove dl_server contribution) before we
> get to this point during an hotplug operation. Need to think about it a
> little more.
static inline bool
__dl_overflow(struct dl_bw *dl_b, unsigned long cap, u64 old_bw, u64 new_bw)
{
        return dl_b->bw != -1 &&
               cap_scale(dl_b->bw, cap) < dl_b->total_bw - old_bw + new_bw;
}

With a 0 cap, cap_scale(dl_b->bw, cap) will always be 0. As long as 
total_bw isn't 0 and bw isn't -1, the condition will be true.

Cheers,
Longman