[Patch v2] genirq/affinity: Spread IRQs to all available NUMA nodes

[Patch v2] genirq/affinity: Spread IRQs to all available NUMA nodes

[Long Li]

From: Long Li <longli@microsoft.com>

On systems with large number of NUMA nodes, there may be more NUMA nodes than
the number of MSI/MSI-X interrupts that device requests for. The current code
always picks up the NUMA nodes starting from the node 0, up to the number of
interrupts requested. This may left some later NUMA nodes unused.

For example, if the system has 16 NUMA nodes, and the device reqeusts for 8
interrupts, NUMA node 0 to 7 are assigned for those interrupts, NUMA 8 to 15
are unused.

There are several problems with this approach:
1. Later, when those managed IRQs are allocated, they can not be assigned to
NUMA 8 to 15, this may create an IRQ concentration on NUMA 0 to 7.
2. Some upper layers assume affinity mask has a complete coverage over NUMA nodes.
For example, block layer use the affinity mask to decide how to map CPU queues to
hardware queues, missing NUMA nodes in the masks may result in an uneven mapping
of queues. For the above example of 16 NUMA nodes, CPU queues on NUMA node 0 to 7
are assigned to the hardware queues 0 to 7, respectively. But CPU queues on NUMA
node 8 to 15 are all assigned to the hardware queue 0.

Fix this problem by going over all NUMA nodes and assign them round-robin to
all IRQs.

Change in v2: Removed extra code for calculating "done". (Michael Kelley
<mikelley@microsoft.com>)

Signed-off-by: Long Li <longli@microsoft.com>
---
 kernel/irq/affinity.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/irq/affinity.c b/kernel/irq/affinity.c
index f4f29b9d90ee..e12cdf637c71 100644
--- a/kernel/irq/affinity.c
+++ b/kernel/irq/affinity.c
@@ -117,12 +117,11 @@ static int irq_build_affinity_masks(const struct irq_affinity *affd,
 	 */
 	if (numvecs <= nodes) {
 		for_each_node_mask(n, nodemsk) {
-			cpumask_copy(masks + curvec, node_to_cpumask[n]);
-			if (++done == numvecs)
-				break;
+			cpumask_or(masks + curvec, masks + curvec, node_to_cpumask[n]);
 			if (++curvec == last_affv)
 				curvec = affd->pre_vectors;
 		}
+		done = numvecs;
 		goto out;
 	}
 
-- 
2.14.1

[tip:irq/core] genirq/affinity: Spread IRQs to all available NUMA nodes

[tip-bot for Long Li]

Commit-ID:  b82592199032bf7c778f861b936287e37ebc9f62
Gitweb:     https://git.kernel.org/tip/b82592199032bf7c778f861b936287e37ebc9f62
Author:     Long Li <longli@microsoft.com>
AuthorDate: Fri, 2 Nov 2018 18:02:48 +0000
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Mon, 5 Nov 2018 12:16:26 +0100

genirq/affinity: Spread IRQs to all available NUMA nodes

If the number of NUMA nodes exceeds the number of MSI/MSI-X interrupts
which are allocated for a device, the interrupt affinity spreading code
fails to spread them across all nodes.

The reason is, that the spreading code starts from node 0 and continues up
to the number of interrupts requested for allocation. This leaves the nodes
past the last interrupt unused.

This results in interrupt concentration on the first nodes which violates
the assumption of the block layer that all nodes are covered evenly. As a
consequence the NUMA nodes above the number of interrupts are all assigned
to hardware queue 0 and therefore NUMA node 0, which results in bad
performance and has CPU hotplug implications, because queue 0 gets shut
down when the last CPU of node 0 is offlined.

Go over all NUMA nodes and assign them round-robin to all requested
interrupts to solve this.

[ tglx: Massaged changelog ]

Signed-off-by: Long Li <longli@microsoft.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Cc: Michael Kelley <mikelley@microsoft.com>
Link: https://lkml.kernel.org/r/20181102180248.13583-1-longli@linuxonhyperv.com

---
 kernel/irq/affinity.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/irq/affinity.c b/kernel/irq/affinity.c
index f4f29b9d90ee..e12cdf637c71 100644
--- a/kernel/irq/affinity.c
+++ b/kernel/irq/affinity.c
@@ -117,12 +117,11 @@ static int irq_build_affinity_masks(const struct irq_affinity *affd,
 	 */
 	if (numvecs <= nodes) {
 		for_each_node_mask(n, nodemsk) {
-			cpumask_copy(masks + curvec, node_to_cpumask[n]);
-			if (++done == numvecs)
-				break;
+			cpumask_or(masks + curvec, masks + curvec, node_to_cpumask[n]);
 			if (++curvec == last_affv)
 				curvec = affd->pre_vectors;
 		}
+		done = numvecs;
 		goto out;
 	}