[PATCH v2] virtio_blk: add use_irq_affinity module parameter

["Liu, Changcheng" <jerrliu@nvidia.com>]

When many virtio-blk devices are present, managed IRQ reservations
can exhaust the per-CPU interrupt vector space and cause probe to
fail with -ENOSPC:

  virtio_blk virtioNNN: probe with driver virtio_blk failed with error -28

On x86, the affinity masks of the managed request-queue interrupts
collectively cover the possible CPUs. irq_matrix_reserve_managed()
therefore reserves one vector slot per possible CPU for each device,
even when the device has far fewer queues than CPUs. Once a CPU's
vector space is exhausted, further managed reservations fail.

Add use_irq_affinity, defaulting to true to keep existing behavior.
Setting it to false makes the request-queue interrupts unmanaged, so
each interrupt consumes a vector on only one CPU. The vector allocator
places each unmanaged interrupt on the least-loaded CPU, allowing the
interrupts to be spread across CPUs.

For example, the reported 64-CPU system used one request queue per
device, so each device needs two interrupts (config-change and
request queue). With managed affinity disabled, 800 devices consume
about 800 * 2 / 64 = 25 vector slots per CPU. This stays well below
the per-CPU vector limit and allows more devices to probe.

Signed-off-by: Liu, Changcheng <jerrliu@nvidia.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 32bf3ba07a9d..184f1c1f4485 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -41,6 +41,11 @@ static unsigned int poll_queues;
 module_param(poll_queues, uint, 0644);
 MODULE_PARM_DESC(poll_queues, "The number of dedicated virtqueues for polling I/O");
 
+static bool use_irq_affinity = true;
+module_param(use_irq_affinity, bool, 0644);
+MODULE_PARM_DESC(use_irq_affinity,
+		 "Use managed IRQ affinity for virtqueues (default: true)");
+
 static int major;
 static DEFINE_IDA(vd_index_ida);
 
@@ -1016,7 +1021,8 @@ static int init_vq(struct virtio_blk *vblk)
 	}
 
 	/* Discover virtqueues and write information to configuration.  */
-	err = virtio_find_vqs(vdev, num_vqs, vqs, vqs_info, &desc);
+	err = virtio_find_vqs(vdev, num_vqs, vqs, vqs_info,
+			      use_irq_affinity ? &desc : NULL);
 	if (err)
 		goto out;
 
-- 
2.43.7