Re: [PATCH] genirq/affinity: fix node generation from cpumask

Gavin Shan
2016-12-14T23:24:38Z

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 <gabriel@krisman.be>
>Signed-off-by: Guilherme G. Piccoli <gpiccoli@linux.vnet.ibm.com>
>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 <gwshan@linux.vnet.ibm.com>

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

> 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++;
> }
>- }
>+

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

> 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) {

Thanks,
Gavin