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.