netdev - Re: [PATCH net] virtio_net: Do not pull payload in skb->head

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Date:   Tue, 6 Apr 2021 10:03:22 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH net] virtio_net: Do not pull payload in skb->head


在 2021/4/2 下午9:26, Eric Dumazet 写道:
> From: Eric Dumazet <edumazet@...gle.com>
>
> 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@...ux.alibaba.com>
> Link: https://www.spinics.net/lists/netdev/msg731397.html
> Co-Developed-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: "Michael S. Tsirkin" <mst@...hat.com>
> Cc: Jason Wang <jasowang@...hat.com>
> Cc: virtualization@...ts.linux-foundation.org
> ---


Acked-by: Jason Wang <jasowang@...hat.com>


>   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;
>   		}
>   	}