> 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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