Re: [PATCH] sched/topology: fix the issue groups don't span domain->span for NUMA diameter > 2

[Valentin Schneider <valentin.schneider@arm.com>]

Hi,

On 01/02/21 16:38, Barry Song wrote:
> A tricky thing is that we shouldn't use the sgc of the 1st CPU of node2
> for the sched_group generated by grandchild, otherwise, when this cpu
> becomes the balance_cpu of another sched_group of cpus other than node0,
> our sched_group generated by grandchild will access the same sgc with
> the sched_group generated by child of another CPU.
>
> So in init_overlap_sched_group(), sgc's capacity be overwritten:
>         build_balance_mask(sd, sg, mask);
>         cpu = cpumask_first_and(sched_group_span(sg), mask);
>
>         sg->sgc = *per_cpu_ptr(sdd->sgc, cpu);
>
> And WARN_ON_ONCE(!cpumask_equal(group_balance_mask(sg), mask)) will
> also be triggered:
> static void init_overlap_sched_group(struct sched_domain *sd,
>                                      struct sched_group *sg)
> {
>         if (atomic_inc_return(&sg->sgc->ref) == 1)
>                 cpumask_copy(group_balance_mask(sg), mask);
>         else
>                 WARN_ON_ONCE(!cpumask_equal(group_balance_mask(sg), mask));
> }
>
> So here move to use the sgc of the 2nd cpu. For the corner case, if NUMA
> has only one CPU, we will still trigger this WARN_ON_ONCE. But It is
> really unlikely to be a real case for one NUMA to have one CPU only.
>

Well, it's trivial to boot this with QEMU, and it's actually the example
the comment atop that WARN_ONCE() is based on. Also, you could end up with
a single CPU on a node during hotplug operations...

I am not entirely sure whether having more than one CPU per node is a
sufficient condition. I'm starting to *think* it is, but I'm not entirely
convinced yet - and now I need a new notebook.