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Subject: Re: [PATCH v5 2/2] virtio-net: use mtu size as buffer length for big packets
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@nvidia.com>
> Reviewed-by: Gavi Teitz <gavi@nvidia.com>
> Reviewed-by: Parav Pandit <parav@nvidia.com>
> Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> Reviewed-by: Si-Wei Liu <si-wei.liu@oracle.com>
Which configurations were tested?
Did you test devices without VIRTIO_NET_F_MTU ?
> ---
> 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;
>
> @@ -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)
I'd rename this to just virtnet_set_big_packets.
> +{
> + 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 VIRTIO_NET_F_MTU is off, then mtu is uninitialized.
You should move it to within if () above to fix.
> if (vi->any_header_sg)
> dev->needed_headroom = vi->hdr_len;
>
> --
> 2.31.1
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