Subject: Re: [PATCH] virtio_net: Eliminate OOO packets during switching

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 17 Jun 2024 09:25:10 -0400

On Fri, Jun 14, 2024 at 10:04:22PM +0000, Abhinav Jain wrote:
> Disable the network device & turn off carrier before modifying the
> number of queue pairs.
> Process all the in-flight packets and then turn on carrier, followed
> by waking up all the queues on the network device.

Did you test that there's a workload with OOO and
this patch actually prevents that?

> 
> Signed-off-by: Abhinav Jain <[email-masked]>

> ---
>  drivers/net/virtio_net.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 61a57d134544..d0a655a3b4c6 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3447,7 +3447,6 @@ static void virtnet_get_drvinfo(struct net_device *dev,
>  
>  }
>  
> -/* TODO: Eliminate OOO packets during switching */
>  static int virtnet_set_channels(struct net_device *dev,
>  				struct ethtool_channels *channels)
>  {
> @@ -3471,6 +3470,15 @@ static int virtnet_set_channels(struct net_device *dev,
>  	if (vi->rq[0].xdp_prog)
>  		return -EINVAL;
>  
> +	/* Disable network device to prevent packet processing during
> +	 * the switch.
> +	 */
> +	netif_tx_disable(dev);
> +	netif_carrier_off(dev);

Won't turning off carrier cause a lot of damage such as
changing IP and so on?

> +
> +	/* Make certain that all in-flight packets are processed. */
> +	synchronize_net();
> +

The comment seems to say what the code does not do.

Also, doing this under rtnl is a heavy weight operation.

>  	cpus_read_lock();
>  	err = virtnet_set_queues(vi, queue_pairs);
>  	if (err) {
> @@ -3482,7 +3490,12 @@ static int virtnet_set_channels(struct net_device *dev,
>  
>  	netif_set_real_num_tx_queues(dev, queue_pairs);
>  	netif_set_real_num_rx_queues(dev, queue_pairs);
> - err:
> +
> +	/* Restart the network device */
> +	netif_carrier_on(dev);
> +	netif_tx_wake_all_queues(dev);
> +
> +err:
>  	return err;
>  }
>  

Given the result is, presumably, improved performance with less
packet loss due to OOO, I'd like to see some actual testing results,
hopefully also measuring the effect on CPU load.

> -- 
> 2.34.1