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

[Jason Wang <jasowang@redhat.com>]

On Thu, Aug 4, 2022 at 3:23 PM Jason Wang <jasowang@redhat.com> wrote:
>
> On Thu, Aug 4, 2022 at 3:10 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > 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.
>
> Yes.
>
> Thanks

I wonder if it's better to introduce the boolean here:

        /* TODO: size buffers correctly in this case. */
                if (dev->mtu > ETH_DATA_LEN)
                        vi->big_packets = true;

Thanks

>
> > 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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> > > >
> >