On Fri, Apr 02, 2021 at 06:26:02AM -0700, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@xxxxxxxxxx>
>
> Xuan Zhuo reported that commit 3226b158e67c ("net: avoid 32 x truesize
> under-estimation for tiny skbs") brought a ~10% performance drop.
>
> The reason for the performance drop was that GRO was forced
> to chain sk_buff (using skb_shinfo(skb)->frag_list), which
> uses more memory but also cause packet consumers to go over
> a lot of overhead handling all the tiny skbs.
>
> It turns out that virtio_net page_to_skb() has a wrong strategy :
> It allocates skbs with GOOD_COPY_LEN (128) bytes in skb->head, then
> copies 128 bytes from the page, before feeding the packet to GRO stack.
>
> This was suboptimal before commit 3226b158e67c ("net: avoid 32 x truesize
> under-estimation for tiny skbs") because GRO was using 2 frags per MSS,
> meaning we were not packing MSS with 100% efficiency.
>
> Fix is to pull only the ethernet header in page_to_skb()
>
> Then, we change virtio_net_hdr_to_skb() to pull the missing
> headers, instead of assuming they were already pulled by callers.
>
> This fixes the performance regression, but could also allow virtio_net
> to accept packets with more than 128bytes of headers.
>
> Many thanks to Xuan Zhuo for his report, and his tests/help.
>
> Fixes: 3226b158e67c ("net: avoid 32 x truesize under-estimation for tiny skbs")
> Reported-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> Link: https://www.spinics.net/lists/netdev/msg731397.html
> Co-Developed-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
> Cc: Jason Wang <jasowang@xxxxxxxxxx>
> Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Thanks a lot for looking into this, Eric and Xuan Zhuo !
> ---
> drivers/net/virtio_net.c | 10 +++++++---
> include/linux/virtio_net.h | 14 +++++++++-----
> 2 files changed, 16 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 82e520d2cb1229a0c7b9fd0def3e4a7135536478..0824e6999e49957f7aaf7c990f6259792d42f32b 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -406,9 +406,13 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
> offset += hdr_padded_len;
> p += hdr_padded_len;
>
> - copy = len;
> - if (copy > skb_tailroom(skb))
> - copy = skb_tailroom(skb);
> + /* Copy all frame if it fits skb->head, otherwise
> + * we let virtio_net_hdr_to_skb() and GRO pull headers as needed.
> + */
> + if (len <= skb_tailroom(skb))
> + copy = len;
> + else
> + copy = ETH_HLEN + metasize;
> skb_put_data(skb, p, copy);
>
> if (metasize) {
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index 98775d7fa69632e2c2da30b581a666f7fbb94b64..b465f8f3e554f27ced45c35f54f113cf6dce1f07 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -65,14 +65,18 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
> skb_reset_mac_header(skb);
>
> if (hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) {
> - u16 start = __virtio16_to_cpu(little_endian, hdr->csum_start);
> - u16 off = __virtio16_to_cpu(little_endian, hdr->csum_offset);
> + u32 start = __virtio16_to_cpu(little_endian, hdr->csum_start);
> + u32 off = __virtio16_to_cpu(little_endian, hdr->csum_offset);
> + u32 needed = start + max_t(u32, thlen, off + sizeof(__sum16));
> +
> + if (!pskb_may_pull(skb, needed))
> + return -EINVAL;
>
> if (!skb_partial_csum_set(skb, start, off))
> return -EINVAL;
>
> p_off = skb_transport_offset(skb) + thlen;
> - if (p_off > skb_headlen(skb))
> + if (!pskb_may_pull(skb, p_off))
> return -EINVAL;
> } else {
> /* gso packets without NEEDS_CSUM do not set transport_offset.
> @@ -102,14 +106,14 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
> }
>
> p_off = keys.control.thoff + thlen;
> - if (p_off > skb_headlen(skb) ||
> + if (!pskb_may_pull(skb, p_off) ||
> keys.basic.ip_proto != ip_proto)
> return -EINVAL;
>
> skb_set_transport_header(skb, keys.control.thoff);
> } else if (gso_type) {
> p_off = thlen;
> - if (p_off > skb_headlen(skb))
> + if (!pskb_may_pull(skb, p_off))
> return -EINVAL;
> }
> }
> --
> 2.31.0.208.g409f899ff0-goog
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