Subject: Re: [PATCH net-next v3 5/5] virtio-net: keep tx interrupts disabled unless kick

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 13 Apr 2021 01:06:09 -0400

On Mon, Apr 24, 2017 at 01:49:30PM -0400, Willem de Bruijn wrote:
> From: Willem de Bruijn <[email-masked]>
> 
> 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 <[email-masked]>
> ---
>  drivers/net/virtio_net.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 9dd978f34c1f..003143835766 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1200,6 +1200,9 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>  	/* Free up any pending old buffers before queueing new ones. */
>  	free_old_xmit_skbs(sq);
>  
> +	if (use_napi && kick)
> +		virtqueue_enable_cb_delayed(sq->vq);
> +
>  	/* timestamp packet in software */
>  	skb_tx_timestamp(skb);

I have been poking at this code today and I noticed that is
actually does enable cb where the commit log says masking interrupts.
I think the reason is that with even index previously disable cb
actually did nothing while virtqueue_enable_cb_delayed pushed
the event index out some more.
And this likely explains why it does not work well for packed,
where virtqueue_enable_cb_delayed is same as virtqueue_enable_cb.

Right? Or did I miss something?

> -- 
> 2.12.2.816.g2cccc81164-goog