vhost_net: correctly limit the max pending buffers

As Michael point out, We used to limit the max pending DMAs to get better cache
utilization. But it was not done correctly since it was one done when there's no
new buffers submitted from guest. Guest can easily exceeds the limitation by
keeping sending packets.

So this patch moves the check into main loop. Tests shows about 5%-10%
improvement on per cpu throughput for guest tx.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 11 deletions

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 8e9dc554b1ef..831eb4fd197d 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -363,6 +363,13 @@ static void handle_tx(struct vhost_net *net)
                 if (zcopy)
                         vhost_zerocopy_signal_used(net, vq);
 
+                /* If more outstanding DMAs, queue the work.
+                 * Handle upend_idx wrap around
+                 */
+                if (unlikely((nvq->upend_idx + vq->num - VHOST_MAX_PEND)
+                              % UIO_MAXIOV == nvq->done_idx))
+                        break;
+
                 head = vhost_get_vq_desc(&net->dev, vq, vq->iov,
                                          ARRAY_SIZE(vq->iov),
                                          &out, &in,
@@ -372,17 +379,6 @@ static void handle_tx(struct vhost_net *net)
                         break;
                 /* Nothing new?  Wait for eventfd to tell us they refilled. */
                 if (head == vq->num) {
-                        int num_pends;
-
-                        /* If more outstanding DMAs, queue the work.
-                         * Handle upend_idx wrap around
-                         */
-                        num_pends = likely(nvq->upend_idx >= nvq->done_idx) ?
-                                    (nvq->upend_idx - nvq->done_idx) :
-                                    (nvq->upend_idx + UIO_MAXIOV -
-                                     nvq->done_idx);
-                        if (unlikely(num_pends > VHOST_MAX_PEND))
-                                break;
                         if (unlikely(vhost_enable_notify(&net->dev, vq))) {
                                 vhost_disable_notify(&net->dev, vq);
                                 continue;