Re: [PATCH v3] sched/fair: bring back select_idle_smt, but differently

[Peter Zijlstra <peterz@infradead.org>]

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.