linux-kernel - Re: irq/affinity: Fix extra vecs calculation

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Date:   Wed, 19 Apr 2017 17:53:09 -0400
From:   Keith Busch <keith.busch@...el.com>
To:     Andrei Vagin <avagin@...tuozzo.com>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Xiaolong Ye <xiaolong.ye@...el.com>
Subject: Re: irq/affinity: Fix extra vecs calculation

On Wed, Apr 19, 2017 at 12:53:44PM -0700, Andrei Vagin wrote:
> On Wed, Apr 19, 2017 at 01:03:59PM -0400, Keith Busch wrote:
> > If it's a divide by 0 as your last link indicates, that must mean there
> > are possible nodes, but have no CPUs, and those should be skipped. If
> > that's the case, the following should fix it, but I'm going to do some
> > more qemu testing with various CPU topologies to confirm.
> 
> I printed variables from my test host, I think this can help to
> investigate the issue:
> 
> irq_create_affinity_masks:116: vecs_to_assign 0 ncpus 2 extra_vecs 2 vecs_per_node 0 affv 2 curvec 2 nodes 1

That explains a lot. This setup wants 2 "pre_vectors", but I didn't
know that was even a thing. This should fix it:

---
diff --git a/kernel/irq/affinity.c b/kernel/irq/affinity.c
index d052947..eb8b689 100644
--- a/kernel/irq/affinity.c
+++ b/kernel/irq/affinity.c
@@ -98,13 +98,16 @@ irq_create_affinity_masks(int nvecs, const struct irq_affinity *affd)
 		int ncpus, v, vecs_to_assign, vecs_per_node;
 
 		/* Spread the vectors per node */
-		vecs_per_node = (affv - curvec) / nodes;
+		vecs_per_node = (affv - (curvec - affd->pre_vectors)) / nodes;
 
 		/* Get the cpus on this node which are in the mask */
 		cpumask_and(nmsk, cpu_online_mask, cpumask_of_node(n));
--