[PATCH] 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 <redacted>
Signed-off-by: Guilherme G. Piccoli <redacted>
Cc: stable@vger.kernel.org # v4.9+
Cc: Christoph Hellwig <hch@lst.de>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-pci@vger.kernel.org
---
 kernel/irq/affinity.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/irq/affinity.c b/kernel/irq/affinity.c
index 9be9bda..464eaf0 100644
--- a/kernel/irq/affinity.c
+++ b/kernel/irq/affinity.c

@@ -37,15 +37,15 @@ 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++;
 		}
-	}
+
 	return 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) {

-- 
2.1.0

On Wed, Dec 14, 2016 at 04:01:12PM -0200, Guilherme G. Piccoli wrote:

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 <redacted>
Signed-off-by: Guilherme G. Piccoli <redacted>
Cc: stable@vger.kernel.org # v4.9+
Cc: Christoph Hellwig <hch@lst.de>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-pci@vger.kernel.org
---

Reviewed-by: Gavin Shan <redacted>

There is one picky comment as below, but you don't have to fix it :)

It'd better to keep the brackets so that we needn't add them when adding
more code into the block next time.

Thanks,
Gavin

Looks fine:

Reviewed-by: Christoph Hellwig <hch@lst.de>

(but I agree with the bracing nitpick from Gavin)

On Thu, 15 Dec 2016, Gavin Shan wrote:

quoted

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++;
		}
-	}
+

It'd better to keep the brackets so that we needn't add them when adding
more code into the block next time.

Removing the brackets is outright wrong. See:
  https://marc.info/?l=linux-kernel&m=147351236615103

I'll fix that up when applying the patch.

Thanks,

	tglx

On 15/12/16 05:01, Guilherme G. Piccoli wrote:

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.

Very good catch! 

Acked-by: Balbir Singh <bsingharora@gmail.com>

On 12/15/2016 07:36 AM, Thomas Gleixner wrote:

On Thu, 15 Dec 2016, Gavin Shan wrote:

quoted

quoted

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++;
		}
-	}
+

It'd better to keep the brackets so that we needn't add them when adding
more code into the block next time.

Removing the brackets is outright wrong. See:
  https://marc.info/?l=linux-kernel&m=147351236615103

I'll fix that up when applying the patch.

Thanks,

	tglx

Thanks you all very much for the reviews and comments - lesson learned
about the brackets in multi-line if/for statements!

Thanks for fixing it Thomas.
Cheers,


Guilherme