Virtualization - Dec 2022 - [PATCH] virtio_net: send notification coalescing command only if value changed

On Wed, Dec 21, 2022 at 02:06:18PM +0200, Alvaro Karsz
wrote:
> Don't send a VIRTIO_NET_CTRL_NOTF_COAL_TX_SET or
> VIRTIO_NET_CTRL_NOTF_COAL_RX_SET command if the coalescing parameters
> haven't changed.
> 
> Signed-off-by: Alvaro Karsz <alvaro.karsz at solid-run.com>
Why do we bother? Resending needs more code and helps
reliability ...
> ---
>  drivers/net/virtio_net.c | 48 ++++++++++++++++++++++------------------
>  1 file changed, 27 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 7723b2a49d8..1d7118de62a 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2760,31 +2760,37 @@ static int virtnet_send_notf_coal_cmds(struct
virtnet_info *vi,
>  	struct virtio_net_ctrl_coal_tx coal_tx;
>  	struct virtio_net_ctrl_coal_rx coal_rx;
>  
> -	coal_tx.tx_usecs = cpu_to_le32(ec->tx_coalesce_usecs);
> -	coal_tx.tx_max_packets = cpu_to_le32(ec->tx_max_coalesced_frames);
> -	sg_init_one(&sgs_tx, &coal_tx, sizeof(coal_tx));
> -
> -	if (!virtnet_send_command(vi, VIRTIO_NET_CTRL_NOTF_COAL,
> -				  VIRTIO_NET_CTRL_NOTF_COAL_TX_SET,
> -				  &sgs_tx))
> -		return -EINVAL;
> +	if (ec->tx_coalesce_usecs != vi->tx_usecs ||
> +	    ec->tx_max_coalesced_frames != vi->tx_max_packets) {
> +		coal_tx.tx_usecs = cpu_to_le32(ec->tx_coalesce_usecs);
> +		coal_tx.tx_max_packets = cpu_to_le32(ec->tx_max_coalesced_frames);
> +		sg_init_one(&sgs_tx, &coal_tx, sizeof(coal_tx));
> +
> +		if (!virtnet_send_command(vi, VIRTIO_NET_CTRL_NOTF_COAL,
> +					  VIRTIO_NET_CTRL_NOTF_COAL_TX_SET,
> +					  &sgs_tx))
> +			return -EINVAL;
>  
> -	/* Save parameters */
> -	vi->tx_usecs = ec->tx_coalesce_usecs;
> -	vi->tx_max_packets = ec->tx_max_coalesced_frames;
> +		/* Save parameters */
> +		vi->tx_usecs = ec->tx_coalesce_usecs;
> +		vi->tx_max_packets = ec->tx_max_coalesced_frames;
> +	}
>  
> -	coal_rx.rx_usecs = cpu_to_le32(ec->rx_coalesce_usecs);
> -	coal_rx.rx_max_packets = cpu_to_le32(ec->rx_max_coalesced_frames);
> -	sg_init_one(&sgs_rx, &coal_rx, sizeof(coal_rx));
> +	if (ec->rx_coalesce_usecs != vi->rx_usecs ||
> +	    ec->rx_max_coalesced_frames != vi->rx_max_packets) {
> +		coal_rx.rx_usecs = cpu_to_le32(ec->rx_coalesce_usecs);
> +		coal_rx.rx_max_packets = cpu_to_le32(ec->rx_max_coalesced_frames);
> +		sg_init_one(&sgs_rx, &coal_rx, sizeof(coal_rx));
>  
> -	if (!virtnet_send_command(vi, VIRTIO_NET_CTRL_NOTF_COAL,
> -				  VIRTIO_NET_CTRL_NOTF_COAL_RX_SET,
> -				  &sgs_rx))
> -		return -EINVAL;
> +		if (!virtnet_send_command(vi, VIRTIO_NET_CTRL_NOTF_COAL,
> +					  VIRTIO_NET_CTRL_NOTF_COAL_RX_SET,
> +					  &sgs_rx))
> +			return -EINVAL;
>  
> -	/* Save parameters */
> -	vi->rx_usecs = ec->rx_coalesce_usecs;
> -	vi->rx_max_packets = ec->rx_max_coalesced_frames;
> +		/* Save parameters */
> +		vi->rx_usecs = ec->rx_coalesce_usecs;
> +		vi->rx_max_packets = ec->rx_max_coalesced_frames;
> +	}
>  
>  	return 0;
>  }
> -- 
> 2.32.0

> Why do we bother? Resending needs more code and helps
> reliability ...
It just seems unnecessary.
If a user changes just one parameter:
$ ethtool -C <iface> tx-usecs 30
It will trigger 2 commands, including
VIRTIO_NET_CTRL_NOTF_COAL_RX_SET, even though no rx parameter changed.

If we'll add more ethtool coalescing parameters, changing one of the
new parameter will trigger meaningless
VIRTIO_NET_CTRL_NOTF_COAL_RX_SET and VIRTIO_NET_CTRL_NOTF_COAL_TX_SET
commands.

Alvaro