Re: [tip: sched/core] sched/fair: Multi-LLC select_idle_sibling()

Peter Zijlstra
2023-06-05T19:08:30Z

On Mon, Jun 05, 2023 at 05:25:30PM +0200, Marek Szyprowski wrote:

> nfortunately it causes
> regression on my ARM 64bit Exynos5433-based TM2e test board during the
> CPU hotplug tests.

Can you elucidate an ARM illiterate on the actual topology of that
machine?

> CPU: 6 PID: 43 Comm: cpuhp/6 Not tainted 6.4.0-rc1+ #13640
> Hardware name: Samsung TM2E board (DT)
> pstate: 000000c5 (nzcv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> pc : __bitmap_and+0x4c/0x78
> lr : select_idle_cpu+0x64/0x450

Btw, where is lr at? Is that perhaps per_cpu(sd_llc) being NULL or
something?

> > ---
> > kernel/sched/fair.c | 38 ++++++++++++++++++++++++++++++++++++++
> > kernel/sched/features.h | 1 +
> > 2 files changed, 39 insertions(+)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 48b6f0c..0172458 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -7028,6 +7028,38 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> > }
> >
> > /*
> > + * For the multiple-LLC per node case, make sure to try the other LLC's if the
> > + * local LLC comes up empty.
> > + */
> > +static int
> > +select_idle_node(struct task_struct *p, struct sched_domain *sd, int target)
> > +{
> > + struct sched_domain *parent = sd->parent;
> > + struct sched_group *sg;
> > +
> > + /* Make sure to not cross nodes. */
> > + if (!parent || parent->flags & SD_NUMA)
> > + return -1;
> > +
> > + sg = parent->groups;
> > + do {
> > + int cpu = cpumask_first(sched_group_span(sg));
> > + struct sched_domain *sd_child;
> > +
> > + sd_child = per_cpu(sd_llc, cpu);

IOW, sd_child end up NULL?

> > + if (sd_child != sd) {
> > + int i = select_idle_cpu(p, sd_child, test_idle_cores(cpu), cpu);
> > + if ((unsigned)i < nr_cpumask_bits)
> > + return i;
> > + }
> > +
> > + sg = sg->next;
> > + } while (sg != parent->groups);
> > +
> > + return -1;
> > +}