Linux Virtualization - Apr 2017 - [PATCH net-next v2 5/5] virtio-net: keep tx interrupts disabled unless kick

From: Willem de Bruijn <willemb at google.com>

Tx napi mode increases the rate of transmit interrupts. Suppress some
by masking interrupts while more packets are expected. The interrupts
will be reenabled before the last packet is sent.

This optimization reduces the througput drop with tx napi for
unidirectional flows such as UDP_STREAM that do not benefit from
cleaning tx completions in the the receive napi handler.

Signed-off-by: Willem de Bruijn <willemb at google.com>
---
 drivers/net/virtio_net.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index b14c82ce0032..b107ae011632 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1196,8 +1196,14 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct
net_device *dev)
 	bool use_napi = sq->napi.weight;
 
 	/* Free up any pending old buffers before queueing new ones. */
-	if (!use_napi)
+	if (use_napi) {
+		if (kick)
+			virtqueue_enable_cb_delayed(sq->vq);
+		else
+			virtqueue_disable_cb(sq->vq);
+	} else {
 		free_old_xmit_skbs(sq);
+	}
 
 	/* timestamp packet in software */
 	skb_tx_timestamp(skb);
-- 
2.12.2.816.g2cccc81164-goog

>> -       if (!use_napi)
>> +       if (use_napi) {
>> +               if (kick)
>> +                       virtqueue_enable_cb_delayed(sq->vq);
>> +               else
>> +                       virtqueue_disable_cb(sq->vq);
>
>
> Since virtqueue_disable_cb() do nothing for event idx. I wonder whether or
> not just calling enable_cb_dealyed() is ok here.
Good point.
> Btw, it does not disable interrupt at all, I propose a patch in the past
> which can do more than this:
>
> https://patchwork.kernel.org/patch/6472601/
Interesting. Yes, let me evaluate that variant.

Thanks for reviewing,

  Willem