On Wed, Apr 07, 2021 at 12:15:13PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 07, 2021 at 10:41:06AM +0100, Mel Gorman wrote:
>
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -6112,6 +6112,27 @@ static int select_idle_core(struct task_
> > > return -1;
> > > }
> > >
> > > +/*
> > > + * Scan the local SMT mask for idle CPUs.
> > > + */
> > > +static int select_idle_smt(struct task_struct *p, struct sched_domain
> > > *sd, int target)
> > > +{
> > > + int cpu;
> > > +
> > > + if (!static_branch_likely(&sched_smt_present))
> > > + return -1;
> > > +
> > > + for_each_cpu(cpu, cpu_smt_mask(target)) {
> > > + if (!cpumask_test_cpu(cpu, p->cpus_ptr) ||
> > > + !cpumask_test_cpu(cpu, sched_domain_span(sd)))
> > > + continue;
> >
> > While I know that !cpumask_test_cpu(cpu, sched_domain_span(sd)) was
> > done previously, I found it hard to believe that the test matters. If
> > target/prev share a the LLC domain, why would the SMT siblings *not*
> > share a LLC?
>
> I think the reason for it is that a cpuset might have split the siblings
> apart and disabled load-balancing across them or something.
>
> Then the affinity mask can still cross the partition, but we shouldn't
> ever move into it through balancing.
Ok, cpusets do split domains. I can't imagine the logic of splitting SMT
siblings across cpusets but if it's possible, it has to be checked and
protecting that with cpusets_enabled() would be a little overkill and
possibly miss some other corner case :(
Thanks.
--
Mel Gorman
SUSE Labs
