Re: [PATCH 2/2] sched/fair: Adjust the allowed NUMA imbalance when SD_NUMA spans multiple LLCs

[Mel Gorman <mgorman@techsingularity.net>]

On Fri, Dec 03, 2021 at 09:15:15PM +1300, Barry Song wrote:
> > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > index d201a7052a29..fee2930745ab 100644
> > --- a/kernel/sched/topology.c
> > +++ b/kernel/sched/topology.c
> > @@ -2242,6 +2242,26 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
> >                 }
> >         }
> >
> > +       /* Calculate allowed NUMA imbalance */
> > +       for_each_cpu(i, cpu_map) {
> > +               int imb_numa_nr = 0;
> > +
> > +               for (sd = *per_cpu_ptr(d.sd, i); sd; sd = sd->parent) {
> > +                       struct sched_domain *child = sd->child;
> > +
> > +                       if (!(sd->flags & SD_SHARE_PKG_RESOURCES) && child &&
> > +                           (child->flags & SD_SHARE_PKG_RESOURCES)) {
> > +                               int nr_groups;
> > +
> > +                               nr_groups = sd->span_weight / child->span_weight;
> > +                               imb_numa_nr = max(1U, ((child->span_weight) >> 1) /
> > +                                               (nr_groups * num_online_nodes()));
> 
> Hi Mel, you used to have 25% * numa_weight if node has only one LLC.
> for a system with 4 numa,  In case sd has 2 nodes, child is 1 numa node,
> then  nr_groups=2, num_online_nodes()=4,  imb_numa_nr will be
> child->span_weight/2/2/4?
> 
> Does this patch change the behaviour for machines whose numa equals LLC?
> 

Yes, it changes behaviour. Instead of a flat 25%, it takes into account
the number of LLCs per node and the number of nodes overall.

-- 
Mel Gorman
SUSE Labs