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

[Gavin Li <gavinl@nvidia.com>]

On 8/4/2022 3:24 PM, Jason Wang wrote:
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> 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
This is not a safe or straightforward way to determine that guest GSO is 
not supported, as big_packets just indicates the packets can be big, not 
specifically guest GSO.
>>> 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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