Linux Virtualization - Apr 2019 - [PATCH AUTOSEL 4.9 13/16] virtio-blk: limit number of hw queues by nr_cpu_ids

From: Dongli Zhang <dongli.zhang at oracle.com>

[ Upstream commit bf348f9b78d413e75bb079462751a1d86b6de36c ]

When tag_set->nr_maps is 1, the block layer limits the number of hw queues
by nr_cpu_ids. No matter how many hw queues are used by virtio-blk, as it
has (tag_set->nr_maps == 1), it can use at most nr_cpu_ids hw queues.

In addition, specifically for pci scenario, when the 'num-queues'
specified
by qemu is more than maxcpus, virtio-blk would not be able to allocate more
than maxcpus vectors in order to have a vector for each queue. As a result,
it falls back into MSI-X with one vector for config and one shared for
queues.

Considering above reasons, this patch limits the number of hw queues used
by virtio-blk by nr_cpu_ids.

Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
Signed-off-by: Dongli Zhang <dongli.zhang at oracle.com>
Signed-off-by: Jens Axboe <axboe at kernel.dk>
Signed-off-by: Sasha Levin <sashal at kernel.org>
---
 drivers/block/virtio_blk.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 10332c24f961..44ef1d66caa6 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -392,6 +392,8 @@ static int init_vq(struct virtio_blk *vblk)
 	if (err)
 		num_vqs = 1;
 
+	num_vqs = min_t(unsigned int, nr_cpu_ids, num_vqs);
+
 	vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
 	if (!vblk->vqs)
 		return -ENOMEM;
-- 
2.19.1