Linux Virtualization - Oct 2014 - [RFC PATCH net-next 6/6] virtio-net: enable tx interrupt only for the final skb in the chain

With the help of xmit_more and virtqueue_enable_cb_avail(), this patch
enable tx interrupt only for the final skb in the chain if
needed. This will help to mitigate tx interrupts.

Cc: Rusty Russell <rusty at rustcorp.com.au>
Cc: Michael S. Tsirkin <mst at redhat.com>
Signed-off-by: Jason Wang <jasowang at redhat.com>
---
 drivers/net/virtio_net.c |   10 +++++++---
 1 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 2afc2e2..5943f3f 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -993,12 +993,16 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct
net_device *dev)
 				virtqueue_disable_cb(sq->vq);
 			}
 		}
-	} else if (virtqueue_enable_cb(sq->vq)) {
-		free_old_xmit_skbs(sq, qsize);
 	}
 
-	if (__netif_subqueue_stopped(dev, qnum) || !skb->xmit_more)
+	if (__netif_subqueue_stopped(dev, qnum) || !skb->xmit_more) {
 		virtqueue_kick(sq->vq);
+		if (sq->vq->num_free >= 2 +MAX_SKB_FRAGS &&
+		    unlikely(virtqueue_enable_cb_avail(sq->vq))) {
+			free_old_xmit_skbs(sq, qsize);
+			virtqueue_disable_cb(sq->vq);
+		}
+	}
 
 	return NETDEV_TX_OK;
 }
-- 
1.7.1

On 10/15/2014 06:22 PM, Michael S. Tsirkin wrote:
> On Wed, Oct 15, 2014 at 03:25:30PM +0800, Jason Wang wrote:
>> With the help of xmit_more and virtqueue_enable_cb_avail(), this patch
>> enable tx interrupt only for the final skb in the chain if
>> needed. This will help to mitigate tx interrupts.
>>
>> Cc: Rusty Russell <rusty at rustcorp.com.au>
>> Cc: Michael S. Tsirkin <mst at redhat.com>
>> Signed-off-by: Jason Wang <jasowang at redhat.com>
> I think you should split xmit_more stuff out.
No much difference but if you prefer I will do this.