Subject: Re: [PATCH RESEND v4 2/2] virtio-net: use mtu size as buffer length for big packets

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 31 Aug 2022 09:11:53 -0400

On Wed, Aug 31, 2022 at 04:05:41PM +0300, Gavin Li wrote:
> Currently add_recvbuf_big() allocates MAX_SKB_FRAGS segments for big
> packets even when GUEST_* offloads are not present on the device.
> However, if guest GSO is not supported, it would be sufficient to
> allocate segments to cover just up the MTU size and no further.
> Allocating the maximum amount of segments results in a large waste of
> buffer space in the queue, which limits the number of packets that can
> be buffered and can result in reduced performance.
> 
> Therefore, if guest GSO is not supported, use the MTU to calculate the
> optimal amount of segments required.
> 
> When guest offload is enabled at runtime, RQ already has packets of bytes
> less than 64K. So when packet of 64KB arrives, all the packets of such
> size will be dropped. and RQ is now not usable.
> 
> So this means that during set_guest_offloads() phase, RQs have to be
> destroyed and recreated, which requires almost driver reload.
> 
> If VIRTIO_NET_F_CTRL_GUEST_OFFLOADS has been negotiated, then it should
> always treat them as GSO enabled.

a better way to put this last sentence:

Accordingly, for now we assume that if GSO has been negotiated
then it has been enabled, even if it's actually been disabled
at runtime through VIRTIO_NET_F_CTRL_GUEST_OFFLOADS.

> Below is the iperf TCP test results over a Mellanox NIC, using vDPA for
> 1 VQ, queue size 1024, before and after the change, with the iperf
> server running over the virtio-net interface.
> 
> MTU(Bytes)/Bandwidth (Gbit/s)
>              Before   After
>   1500        22.5     22.4
>   9000        12.8     25.9
> 
> Signed-off-by: Gavin Li <[email-masked]>
> Reviewed-by: Gavi Teitz <[email-masked]>
> Reviewed-by: Parav Pandit <[email-masked]>
> Reviewed-by: Xuan Zhuo <[email-masked]>
> Reviewed-by: Si-Wei Liu <[email-masked]>
> ---
> changelog:
> v3->v4
> - Addressed comments from Si-Wei
> - Rename big_packets_sg_num with big_packets_num_skbfrags
> v2->v3
> - Addressed comments from Si-Wei
> - Simplify the condition check to enable the optimization
> v1->v2
> - Addressed comments from Jason, Michael, Si-Wei.
> - Remove the flag of guest GSO support, set sg_num for big packets and
>   use it directly
> - Recalculate sg_num for big packets in virtnet_set_guest_offloads
> - Replace the round up algorithm with DIV_ROUND_UP
> ---
>  drivers/net/virtio_net.c | 37 ++++++++++++++++++++++++-------------
>  1 file changed, 24 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index e1904877d461..ba2852b41795 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -225,6 +225,9 @@ struct virtnet_info {
>  	/* I like... big packets and I cannot lie! */
>  	bool big_packets;
>  
> +	/* number of sg entries allocated for big packets */
> +	unsigned int big_packets_num_skbfrags;
> +
>  	/* Host will merge rx buffers for big packets (shake it! shake it!) */
>  	bool mergeable_rx_bufs;
>  
> @@ -1331,10 +1334,10 @@ static int add_recvbuf_big(struct virtnet_info *vi, struct receive_queue *rq,
>  	char *p;
>  	int i, err, offset;
>  
> -	sg_init_table(rq->sg, MAX_SKB_FRAGS + 2);
> +	sg_init_table(rq->sg, vi->big_packets_num_skbfrags + 2);
>  
> -	/* page in rq->sg[MAX_SKB_FRAGS + 1] is list tail */
> -	for (i = MAX_SKB_FRAGS + 1; i > 1; --i) {
> +	/* page in rq->sg[vi->big_packets_num_skbfrags + 1] is list tail */
> +	for (i = vi->big_packets_num_skbfrags + 1; i > 1; --i) {
>  		first = get_a_page(rq, gfp);
>  		if (!first) {
>  			if (list)
> @@ -1365,7 +1368,7 @@ static int add_recvbuf_big(struct virtnet_info *vi, struct receive_queue *rq,
>  
>  	/* chain first in list head */
>  	first->private = (unsigned long)list;
> -	err = virtqueue_add_inbuf(rq->vq, rq->sg, MAX_SKB_FRAGS + 2,
> +	err = virtqueue_add_inbuf(rq->vq, rq->sg, vi->big_packets_num_skbfrags + 2,
>  				  first, gfp);
>  	if (err < 0)
>  		give_pages(rq, first);
> @@ -3690,13 +3693,27 @@ static bool virtnet_check_guest_gso(const struct virtnet_info *vi)
>  		virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_UFO));
>  }
>  
> +static void virtnet_set_big_packets_fields(struct virtnet_info *vi, const int mtu)
> +{
> +	bool guest_gso = virtnet_check_guest_gso(vi);
> +
> +	/* If device can receive ANY guest GSO packets, regardless of mtu,
> +	 * allocate packets of maximum size, otherwise limit it to only
> +	 * mtu size worth only.
> +	 */
> +	if (mtu > ETH_DATA_LEN || guest_gso) {
> +		vi->big_packets = true;
> +		vi->big_packets_num_skbfrags = guest_gso ? MAX_SKB_FRAGS : DIV_ROUND_UP(mtu, PAGE_SIZE);
> +	}
> +}
> +
>  static int virtnet_probe(struct virtio_device *vdev)
>  {
>  	int i, err = -ENOMEM;
>  	struct net_device *dev;
>  	struct virtnet_info *vi;
>  	u16 max_queue_pairs;
> -	int mtu;
> +	int mtu = 0;
>  
>  	/* Find if host supports multiqueue/rss virtio_net device */
>  	max_queue_pairs = 1;
> @@ -3784,10 +3801,6 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	INIT_WORK(&vi->config_work, virtnet_config_changed_work);
>  	spin_lock_init(&vi->refill_lock);
>  
> -	/* If we can receive ANY GSO packets, we must allocate large ones. */
> -	if (virtnet_check_guest_gso(vi))
> -		vi->big_packets = true;
> -
>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_MRG_RXBUF))
>  		vi->mergeable_rx_bufs = true;
>  
> @@ -3853,12 +3866,10 @@ static int virtnet_probe(struct virtio_device *vdev)
>  
>  		dev->mtu = mtu;
>  		dev->max_mtu = mtu;
> -
> -		/* TODO: size buffers correctly in this case. */
> -		if (dev->mtu > ETH_DATA_LEN)
> -			vi->big_packets = true;
>  	}
>  
> +	virtnet_set_big_packets_fields(vi, mtu);
> +
>  	if (vi->any_header_sg)
>  		dev->needed_headroom = vi->hdr_len;
>  
> -- 
> 2.31.1