vhost-net: skip head management if no outstanding

For short packets zerocopy mode adds overhead
of managing heads which isn't necessary: we
could simly update used ring directly
same as with zerocopy disabled.

Things seem to run a bit faster if we detect
and bypass head management when zcopy isn't used.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 6a86deb39a7246f6dd2f7418e7fd3745f04277cd..aa76ca72606a6cded4932aea5c8864f1ae2bba05 100644 (file)
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -241,7 +241,7 @@ static void handle_tx(struct vhost_net *net)
        size_t hdr_size;
        struct socket *sock;
        struct vhost_ubuf_ref *uninitialized_var(ubufs);
-       bool zcopy;
+       bool zcopy, zcopy_used;
 
        /* TODO: check that we are running from vhost_worker? */
        sock = rcu_dereference_check(vq->private_data, 1);
@@ -319,8 +319,11 @@ static void handle_tx(struct vhost_net *net)
                               iov_length(vq->hdr, s), hdr_size);
                        break;
                }
+               zcopy_used = zcopy && (len >= VHOST_GOODCOPY_LEN ||
+                                      vq->upend_idx != vq->done_idx);
+
                /* use msg_control to pass vhost zerocopy ubuf info to skb */
-               if (zcopy) {
+               if (zcopy_used) {
                        vq->heads[vq->upend_idx].id = head;
                        if (!vhost_net_tx_select_zcopy(net) ||
                            len < VHOST_GOODCOPY_LEN) {
@@ -348,7 +351,7 @@ static void handle_tx(struct vhost_net *net)
                /* TODO: Check specific error and bomb out unless ENOBUFS? */
                err = sock->ops->sendmsg(NULL, sock, &msg, len);
                if (unlikely(err < 0)) {
-                       if (zcopy) {
+                       if (zcopy_used) {
                                if (ubufs)
                                        vhost_ubuf_put(ubufs);
                                vq->upend_idx = ((unsigned)vq->upend_idx - 1) %
@@ -362,7 +365,7 @@ static void handle_tx(struct vhost_net *net)
                if (err != len)
                        pr_debug("Truncated TX packet: "
                                 " len %d != %zd\n", err, len);
-               if (!zcopy)
+               if (!zcopy_used)
                        vhost_add_used_and_signal(&net->dev, vq, head, 0);
                else
                        vhost_zerocopy_signal_used(net, vq);