Re: Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue available early during boot)

Hello, Michael.

On Mon, Oct 17, 2016 at 11:24:34PM +1100, Michael Ellerman wrote:

The bad case (where we hit the BUG_ON I added above) is where we are
creating a wq for node 1.

In wq_calc_node_cpumask() we do:

	cpumask_and(cpumask, attrs->cpumask, wq_numa_possible_cpumask[node]);
	return !cpumask_equal(cpumask, attrs->cpumask);

Which with the arguments inserted is:

	cpumask_and(tmp_attrs->cpumask, new_attrs->cpumask, wq_numa_possible_cpumask[1]);
	return !cpumask_equal(tmp_attrs->cpumask, new_attrs->cpumask);

And that results in tmp_attrs->cpumask being empty, because
wq_numa_possible_cpumask[1] is an empty cpumask.

Ah, should have read this before replying to the previous mail, so
it's the numa mask, not the cpu_possible_mask.

The reason wq_numa_possible_cpumask[1] is an empty mask is because in
wq_numa_init() we did:

	for_each_possible_cpu(cpu) {
		node = cpu_to_node(cpu);
		if (WARN_ON(node == NUMA_NO_NODE)) {
			pr_warn("workqueue: NUMA node mapping not available for cpu%d, disabling NUMA support\n", cpu);
			/* happens iff arch is bonkers, let's just proceed */
			return;
		}
		cpumask_set_cpu(cpu, tbl[node]);
	}

And cpu_to_node() returned node 0 for every CPU in the system, despite there
being multiple nodes.

That happened because we haven't yet called set_cpu_numa_node() for the non-boot
cpus, because that happens in smp_prepare_cpus(), and
workqueue_init_early() is called much earlier than that.

This doesn't trigger on x86 because it does set_cpu_numa_node() in
setup_per_cpu_areas(), which is called prior to workqueue_init_early().

We can (should) probably do the same on powerpc, I'll look at that
tomorrow. But other arches may have a similar problem, and at the very
least we need to document that workqueue_init_early() relies on
cpu_to_node() working.

I should be able to move the numa part of initialization to the later
init function.  Working on it.

Thanks.

-- 
tejun