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

[Gavin Li <gavinl@nvidia.com>]

On 8/4/2022 3:10 PM, Michael S. Tsirkin wrote:
> External email: Use caution opening links or attachments
>
>
> 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.
ACK.
>
>>> +       }
>>>
>>>          if (virtio_has_feature(vdev, VIRTIO_NET_F_MRG_RXBUF))
>>>                  vi->mergeable_rx_bufs = true;
>>> --
>>> 2.31.1
>>>
>>>
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