Re: [virtio-dev] [PATCH] virtio-net: use mtu size as buffer length for big packets

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Aug 04, 2022 at 01:00:46PM +0800, Jason Wang wrote:
> On Tue, Aug 2, 2022 at 12:47 PM Gavin Li <gavinl@nvidia.com> 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 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 GSO is not supported, use the MTU to calculate the
> > optimal amount of segments required.
> >
> > 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>
> > ---
> >  drivers/net/virtio_net.c | 20 ++++++++++++++++----
> >  1 file changed, 16 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index ec8e1b3108c3..d36918c1809d 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -222,6 +222,9 @@ struct virtnet_info {
> >         /* I like... big packets and I cannot lie! */
> >         bool big_packets;
> >
> > +       /* Indicates GSO support */
> > +       bool gso_is_supported;
> > +
> >         /* Host will merge rx buffers for big packets (shake it! shake it!) */
> >         bool mergeable_rx_bufs;
> >
> > @@ -1312,14 +1315,21 @@ static int add_recvbuf_small(struct virtnet_info *vi, struct receive_queue *rq,
> >  static int add_recvbuf_big(struct virtnet_info *vi, struct receive_queue *rq,
> >                            gfp_t gfp)
> >  {
> > +       unsigned int sg_num = MAX_SKB_FRAGS;
> >         struct page *first, *list = NULL;
> >         char *p;
> >         int i, err, offset;
> >
> > -       sg_init_table(rq->sg, MAX_SKB_FRAGS + 2);
> > +       if (!vi->gso_is_supported) {
> > +               unsigned int mtu = vi->dev->mtu;
> > +
> > +               sg_num = (mtu % PAGE_SIZE) ? mtu / PAGE_SIZE + 1 : mtu / PAGE_SIZE;
> > +       }
> > +
> > +       sg_init_table(rq->sg, sg_num + 2);
> >
> >         /* page in rq->sg[MAX_SKB_FRAGS + 1] is list tail */
> > -       for (i = MAX_SKB_FRAGS + 1; i > 1; --i) {
> > +       for (i = sg_num + 1; i > 1; --i) {
> >                 first = get_a_page(rq, gfp);
> >                 if (!first) {
> >                         if (list)
> > @@ -1350,7 +1360,7 @@ 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, sg_num + 2,
> >                                   first, gfp);
> >         if (err < 0)
> >                 give_pages(rq, first);
> > @@ -3571,8 +3581,10 @@ static int virtnet_probe(struct virtio_device *vdev)
> >         if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO4) ||
> >             virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO6) ||
> >             virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_ECN) ||
> > -           virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_UFO))
> > +           virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_UFO)) {
> >                 vi->big_packets = true;
> > +               vi->gso_is_supported = true;
> 
> Why not simply re-use big_packets here?
> 
> Thanks

I don't get this question. The patch does use big_packets, it wants
to figure out guest GSO is off so MTU limits the size. 
The name "gso_is_supported" is confusing, should be e.g. guest_gso.


> > +       }
> >
> >         if (virtio_has_feature(vdev, VIRTIO_NET_F_MRG_RXBUF))
> >                 vi->mergeable_rx_bufs = true;
> > --
> > 2.31.1
> >
> >
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