[tip:irq/urgent] genirq/affinity: Fix node generation from cpumask

[tip-bot for Guilherme G. Piccoli]

Commit-ID:  c0af52437254fda8b0cdbaae5a9b6d9327f1fcd5
Gitweb:     http://git.kernel.org/tip/c0af52437254fda8b0cdbaae5a9b6d9327f1fcd5
Author:     Guilherme G. Piccoli <gpiccoli@xxxxxxxxxxxxxxxxxx>
AuthorDate: Wed, 14 Dec 2016 16:01:12 -0200
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Thu, 15 Dec 2016 12:32:35 +0100

genirq/affinity: Fix node generation from cpumask

Commit 34c3d9819fda ("genirq/affinity: Provide smarter irq spreading
infrastructure") introduced a better IRQ spreading mechanism, taking
account of the available NUMA nodes in the machine.

Problem is that the algorithm of retrieving the nodemask iterates
"linearly" based on the number of online nodes - some architectures
present non-linear node distribution among the nodemask, like PowerPC.
If this is the case, the algorithm lead to a wrong node count number
and therefore to a bad/incomplete IRQ affinity distribution.

For example, this problem were found in a machine with 128 CPUs and two
nodes, namely nodes 0 and 8 (instead of 0 and 1, if it was linearly
distributed). This led to a wrong affinity distribution which then led to
a bad mq allocation for nvme driver.

Finally, we take the opportunity to fix a comment regarding the affinity
distribution when we have _more_ nodes than vectors.

Fixes: 34c3d9819fda ("genirq/affinity: Provide smarter irq spreading infrastructure")
Reported-by: Gabriel Krisman Bertazi <gabriel@xxxxxxxxxx>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Reviewed-by: Gabriel Krisman Bertazi <gabriel@xxxxxxxxxx>
Reviewed-by: Gavin Shan <gwshan@xxxxxxxxxxxxxxxxxx>
Cc: linux-pci@xxxxxxxxxxxxxxx
Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
Cc: hch@xxxxxx
Link: http://lkml.kernel.org/r/1481738472-2671-1-git-send-email-gpiccoli@xxxxxxxxxxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

---
 kernel/irq/affinity.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/irq/affinity.c b/kernel/irq/affinity.c
index 9be9bda..4544b11 100644
--- a/kernel/irq/affinity.c
+++ b/kernel/irq/affinity.c
@@ -37,10 +37,10 @@ static void irq_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
 
 static int get_nodes_in_cpumask(const struct cpumask *mask, nodemask_t *nodemsk)
 {
-	int n, nodes;
+	int n, nodes = 0;
 
 	/* Calculate the number of nodes in the supplied affinity mask */
-	for (n = 0, nodes = 0; n < num_online_nodes(); n++) {
+	for_each_online_node(n) {
 		if (cpumask_intersects(mask, cpumask_of_node(n))) {
 			node_set(n, *nodemsk);
 			nodes++;
@@ -82,7 +82,7 @@ irq_create_affinity_masks(int nvecs, const struct irq_affinity *affd)
 	nodes = get_nodes_in_cpumask(cpu_online_mask, &nodemsk);
 
 	/*
-	 * If the number of nodes in the mask is less than or equal the
+	 * If the number of nodes in the mask is greater than or equal the
 	 * number of vectors we just spread the vectors across the nodes.
 	 */
 	if (affv <= nodes) {