RE: [PATCH v5 2/2] virtio-net: use mtu size as buffer length for big packets

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> From: Michael S. Tsirkin <mst@xxxxxxxxxx>
> Sent: Thursday, September 22, 2022 5:35 AM
> 
> On Thu, Sep 01, 2022 at 05:10:38AM +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.
> >
> > Accordingly, for now the assumption is that if guest 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 <gavinl@xxxxxxxxxx>
> > Reviewed-by: Gavi Teitz <gavi@xxxxxxxxxx>
> > Reviewed-by: Parav Pandit <parav@xxxxxxxxxx>
> > Reviewed-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> > Reviewed-by: Si-Wei Liu <si-wei.liu@xxxxxxxxxx>
> 
> OK I think the logic is correct, it's just a bit harder to read than necessary.
> Small improvement suggestions:
> 
> 
> > ---
> > changelog:
> > v4->v5
> > - Addressed comments from Michael S. Tsirkin
> > - Improve commit message
> > 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
> > f831a0290998..dbffd5f56fb8 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;
> >
> 
> big_packets_num_skbfrags -> big_packet_num_skbfrags
> 
> > @@ -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;
> >
> 
> I think it's better to drop this and instead just put the code
> where we already know the config. So:
> 
> >  	/* 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;
> 
>     /* Size buffers to fit mtu. */
>     if (mtu > ETH_DATA_LEN) {
>                     vi->big_packets = true;
>                     vi->big_packets_num_skbfrags = DIV_ROUND_UP(mtu,
> PAGE_SIZE);
>     }
> 
How doing things twice is better i.e. when mtu is > ETH_DATA_LEN and gso is offered?
It calculates big_packets variable twice.

It also easier to read the code at single place where big_packets decision is taken.

> >  	}
> >
> > +	virtnet_set_big_packets_fields(vi, mtu);
> > +
> 
> and here:
>         /* If device can receive guest GSO packets, allocate buffers for
>          * packets of maximum size, regardless of mtu.
> 	 */
> 
> 	if (virtnet_check_guest_gso(vi)) {
> 		vi->big_packets = true;
> 		vi->big_packets_num_skbfrags = MAX_SKB_FRAGS;
>         }
> 
> 
> >  	if (vi->any_header_sg)
> >  		dev->needed_headroom = vi->hdr_len;
> >
> > --
> > 2.31.1

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