Commit 0f6925b3 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

virtio_net: Do not pull payload in skb->head 



Xuan Zhuo reported that commit 3226b158 ("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 3226b158 ("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: 3226b158 ("net: avoid 32 x truesize under-estimation for tiny skbs")
Reported-by: default avatarXuan Zhuo <xuanzhuo@linux.alibaba.com>
Link: https://www.spinics.net/lists/netdev/msg731397.html


Co-Developed-by: default avatarXuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: default avatarXuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Acked-by: default avatarJason Wang <jasowang@redhat.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent b25b343d

drivers/net/virtio_net.c

+7 −3
Original line number Diff line number Diff line
@@ -406,9 +406,13 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi, 
	offset += hdr_padded_len;

	p += hdr_padded_len;



	/* 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;

	if (copy > skb_tailroom(skb))

		copy = skb_tailroom(skb);

	else

		copy = ETH_HLEN + metasize;

	skb_put_data(skb, p, copy);



	if (metasize) {

include/linux/virtio_net.h

+9 −5
Original line number Diff line number Diff line
@@ -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;

		}

	}