Re: [PATCHv2 RFC 0/4] virtio and vhost-net capacity handling

On Thu, Jun 02, 2011 at 06:42:35PM +0300, Michael S. Tsirkin wrote:

OK, here's a new attempt to use the new capacity api.  I also added more
comments to clarify the logic.  Hope this is more readable.  Let me know
pls.

This is on top of the patches applied by Rusty.

Warning: untested. Posting now to give people chance to
comment on the API.

Changes from v1:
- fix comment in patch 2 to correct confusion noted by Rusty
- rewrite patch 3 along the lines suggested by Rusty
  note: it's not exactly the same but I hope it's close
  enough, the main difference is that mine does limited
  polling even in the unlikely xmit failure case.
- added a patch to not return capacity from add_buf
  it always looked like a weird hack

Michael S. Tsirkin (4):
  virtio_ring: add capacity check API
  virtio_net: fix tx capacity checks using new API
  virtio_net: limit xmit polling
  Revert "virtio: make add_buf return capacity remaining:

 drivers/net/virtio_net.c     |  111 ++++++++++++++++++++++++++----------------
 drivers/virtio/virtio_ring.c |   10 +++-
 include/linux/virtio.h       |    7 ++-
 3 files changed, 84 insertions(+), 44 deletions(-)

-- 
1.7.5.53.gc233e

And just FYI, here's a patch (on top) that I considered but
decided against. This does a single get_buf before
xmit. I thought it's not really needed as the capacity
check in add_buf is relatively cheap, and we removed
the kick in xmit_skb. But the point is that the loop
will now be easy to move around if someone manages
to show this benefits speed (which I doubt).

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index b25db1c..75af5be 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -590,6 +590,9 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 	struct virtnet_info *vi = netdev_priv(dev);
 	int ret, i;
 
+	/* Try to pop an skb, to increase the chance we can add this one. */
+	free_old_xmit_skb(v);
+
 	/* We normally do have space in the ring, so try to queue the skb as
 	 * fast as possible. */
 	ret = xmit_skb(vi, skb);

@@ -627,8 +630,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 	 * This is so that we don't hog the skb memory unnecessarily. *
 	 * Doing this after kick means there's a chance we'll free
 	 * the skb we have just sent, which is hot in cache. */
-	for (i = 0; i < 2; i++)
-		free_old_xmit_skb(v);
+	free_old_xmit_skb(v);
 
 	if (likely(free_xmit_capacity(vi)))
 		return NETDEV_TX_OK;