virtio-blk: base queue-depth on virtqueue ringsize or module param

Venkatash spake thus:

  virtio-blk set the default queue depth to 64 requests, which was
  insufficient for high-IOPS devices. Instead set the blk-queue depth to
  the device's virtqueue depth divided by two (each I/O requires at least
  two VQ entries).

But behold, Ted added a module parameter:

  Also allow the queue depth to be something which can be set at module
  load time or via a kernel boot-time parameter, for
  testing/benchmarking purposes.

And I rewrote it substantially, mainly to take
VIRTIO_RING_F_INDIRECT_DESC into account.

As QEMU sets the vq size for PCI to 128, Venkatash's patch wouldn't
have made a change.  This version does (since QEMU also offers
VIRTIO_RING_F_INDIRECT_DESC.

Inspired-by: "Theodore Ts'o" <tytso@mit.edu>
Based-on-the-true-story-of: Venkatesh Srinivas <venkateshs@google.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: virtio-dev@lists.oasis-open.org
Cc: virtualization@lists.linux-foundation.org
Cc: Frank Swiderski <fes@google.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 9 insertions, 1 deletions

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index a2db9ed288f2..196222271a50 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -491,10 +491,11 @@ static struct blk_mq_ops virtio_mq_ops = {
 static struct blk_mq_reg virtio_mq_reg = {
         .ops            = &virtio_mq_ops,
         .nr_hw_queues   = 1,
-        .queue_depth    = 64,
+        .queue_depth    = 0, /* Set in virtblk_probe */
         .numa_node      = NUMA_NO_NODE,
         .flags          = BLK_MQ_F_SHOULD_MERGE,
 };
+module_param_named(queue_depth, virtio_mq_reg.queue_depth, uint, 0444);
 
 static void virtblk_init_vbr(void *data, struct blk_mq_hw_ctx *hctx,
                              struct request *rq, unsigned int nr)
@@ -558,6 +559,13 @@ static int virtblk_probe(struct virtio_device *vdev)
                 goto out_free_vq;
         }
 
+        /* Default queue sizing is to fill the ring. */
+        if (!virtio_mq_reg.queue_depth) {
+                virtio_mq_reg.queue_depth = vblk->vq->num_free;
+                /* ... but without indirect descs, we use 2 descs per req */
+                if (!virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC))
+                        virtio_mq_reg.queue_depth /= 2;
+        }
         virtio_mq_reg.cmd_size =
                 sizeof(struct virtblk_req) +
                 sizeof(struct scatterlist) * sg_elems;