Re: [PATCH bpf-next] xsk: build skb by page

[Alexander Lobakin <alobakin@pm.me>]

From: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Date: Sat, 16 Jan 2021 10:44:53 +0800

> This patch is used to construct skb based on page to save memory copy
> overhead.
> 
> This has one problem:
> 
> We construct the skb by fill the data page as a frag into the skb. In
> this way, the linear space is empty, and the header information is also
> in the frag, not in the linear space, which is not allowed for some
> network cards. For example, Mellanox Technologies MT27710 Family
> [ConnectX-4 Lx] will get the following error message:
> 
>     mlx5_core 0000:3b:00.1 eth1: Error cqe on cqn 0x817, ci 0x8, qn 0x1dbb, opcode 0xd, syndrome 0x1, vendor syndrome 0x68
>     00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>     00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>     00000020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>     00000030: 00 00 00 00 60 10 68 01 0a 00 1d bb 00 0f 9f d2
>     WQE DUMP: WQ size 1024 WQ cur size 0, WQE index 0xf, len: 64
>     00000000: 00 00 0f 0a 00 1d bb 03 00 00 00 08 00 00 00 00
>     00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>     00000020: 00 00 00 2b 00 08 00 00 00 00 00 05 9e e3 08 00
>     00000030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>     mlx5_core 0000:3b:00.1 eth1: ERR CQE on SQ: 0x1dbb
> 
> I also tried to use build_skb to construct skb, but because of the
> existence of skb_shinfo, it must be behind the linear space, so this
> method is not working. We can't put skb_shinfo on desc->addr, it will be
> exposed to users, this is not safe.
> 
> Finally, I added a feature NETIF_F_SKB_NO_LINEAR to identify whether the
> network card supports the header information of the packet in the frag
> and not in the linear space.
> 
> ---------------- Performance Testing ------------
> 
> The test environment is Aliyun ECS server.
> Test cmd:
> ```
> xdpsock -i eth0 -t  -S -s <msg size>
> ```
> 
> Test result data:
> 
> size    64      512     1024    1500
> copy    1916747 1775988 1600203 1440054
> page    1974058 1953655 1945463 1904478
> percent 3.0%    10.0%   21.58%  32.3%
> 
> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> Reviewed-by: Dust Li <dust.li@linux.alibaba.com>
> ---
>  drivers/net/virtio_net.c        |   2 +-
>  include/linux/netdev_features.h |   5 +-
>  net/ethtool/common.c            |   1 +
>  net/xdp/xsk.c                   | 108 +++++++++++++++++++++++++++++++++-------
>  4 files changed, 97 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 4ecccb8..841a331 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2985,7 +2985,7 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	/* Set up network device as normal. */
>  	dev->priv_flags |= IFF_UNICAST_FLT | IFF_LIVE_ADDR_CHANGE;
>  	dev->netdev_ops = &virtnet_netdev;
> -	dev->features = NETIF_F_HIGHDMA;
> +	dev->features = NETIF_F_HIGHDMA | NETIF_F_SKB_NO_LINEAR;
>  
>  	dev->ethtool_ops = &virtnet_ethtool_ops;
>  	SET_NETDEV_DEV(dev, &vdev->dev);
> diff --git a/include/linux/netdev_features.h b/include/linux/netdev_features.h
> index 934de56..8dd28e2 100644
> --- a/include/linux/netdev_features.h
> +++ b/include/linux/netdev_features.h
> @@ -85,9 +85,11 @@ enum {
>  
>  	NETIF_F_HW_MACSEC_BIT,		/* Offload MACsec operations */
>  
> +	NETIF_F_SKB_NO_LINEAR_BIT,	/* Allow skb linear is empty */
> +
>  	/*
>  	 * Add your fresh new feature above and remember to update
> -	 * netdev_features_strings[] in net/core/ethtool.c and maybe
> +	 * netdev_features_strings[] in net/ethtool/common.c and maybe
>  	 * some feature mask #defines below. Please also describe it
>  	 * in Documentation/networking/netdev-features.rst.
>  	 */
> @@ -157,6 +159,7 @@ enum {
>  #define NETIF_F_GRO_FRAGLIST	__NETIF_F(GRO_FRAGLIST)
>  #define NETIF_F_GSO_FRAGLIST	__NETIF_F(GSO_FRAGLIST)
>  #define NETIF_F_HW_MACSEC	__NETIF_F(HW_MACSEC)
> +#define NETIF_F_SKB_NO_LINEAR	__NETIF_F(SKB_NO_LINEAR)
>  
>  /* Finds the next feature with the highest number of the range of start till 0.
>   */
> diff --git a/net/ethtool/common.c b/net/ethtool/common.c
> index 24036e3..2f3d309 100644
> --- a/net/ethtool/common.c
> +++ b/net/ethtool/common.c
> @@ -68,6 +68,7 @@
>  	[NETIF_F_HW_TLS_RX_BIT] =	 "tls-hw-rx-offload",
>  	[NETIF_F_GRO_FRAGLIST_BIT] =	 "rx-gro-list",
>  	[NETIF_F_HW_MACSEC_BIT] =	 "macsec-hw-offload",
> +	[NETIF_F_SKB_NO_LINEAR_BIT] =	 "skb-no-linear",
>  };
>  
>  const char

I think the best would be if you will split this patch into three:
 - the first one will introduce NETI_F_SKB_NO_LINEAR;
 - the second will add this feature to virtio_net;
 - the third will do the rest.

Also, it would be nice if you'll mention (in the cover letter or
in the third patch) that in order to get a nice boost on non-ZC
XSK xmit developers can add a support for completely non-linear
skbs and advertise this new feature in their drivers. I think
there'll be enough folks wanting to do this.

> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 8037b04..94d17dc 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -430,6 +430,95 @@ static void xsk_destruct_skb(struct sk_buff *skb)
>  	sock_wfree(skb);
>  }
>  
> +static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
> +					      struct xdp_desc *desc)
> +{
> +	u32 len, offset, copy, copied;
> +	struct sk_buff *skb;
> +	struct page *page;
> +	char *buffer;
> +	int err, i;
> +	u64 addr;
> +
> +	skb = sock_alloc_send_skb(&xs->sk, 0, 1, &err);
> +	if (unlikely(!skb))
> +		return NULL;
> +
> +	addr = desc->addr;
> +	len = desc->len;
> +
> +	buffer = xsk_buff_raw_get_data(xs->pool, addr);
> +	offset = offset_in_page(buffer);
> +	addr = buffer - (char *)xs->pool->addrs;
> +
> +	for (copied = 0, i = 0; copied < len; ++i) {
> +		page = xs->pool->umem->pgs[addr >> PAGE_SHIFT];
> +
> +		get_page(page);
> +
> +		copy = min((u32)(PAGE_SIZE - offset), len - copied);
> +
> +		skb_fill_page_desc(skb, i, page, offset, copy);
> +
> +		copied += copy;
> +		addr += copy;
> +		offset = 0;
> +	}
> +
> +	skb->len += len;
> +	skb->data_len += len;
> +	skb->truesize += len;
> +
> +	refcount_add(len, &xs->sk.sk_wmem_alloc);
> +
> +	return skb;
> +}
> +
> +static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
> +				     struct xdp_desc *desc, int *err)

As the others said, just use ERR_PTR() and PTR_ERR().
You also should have received the letters from kernel test robot
that the current version is non-compilable at all.

> +	struct sk_buff *skb;
> +
> +	if (xs->dev->features & NETIF_F_SKB_NO_LINEAR) {
> +		skb = xsk_build_skb_zerocopy(xs, desc);
> +		if (unlikely(!skb)) {
> +			*err = -ENOMEM;
> +			return NULL;
> +		}
> +	} else {
> +		char *buffer;
> +		u64 addr;
> +		u32 len;
> +		int err;
> +
> +		len = desc->len;
> +		skb = sock_alloc_send_skb(&xs->sk, len, 1, &err);
> +		if (unlikely(!skb)) {
> +			*err = -ENOMEM;
> +			return NULL;
> +		}
> +
> +		skb_put(skb, len);
> +		addr = desc->addr;
> +		buffer = xsk_buff_raw_get_data(xs->pool, desc->addr);
> +		err = skb_store_bits(skb, 0, buffer, len);
> +
> +		if (unlikely(err)) {
> +			kfree_skb(skb);
> +			*err = -EINVAL;
> +			return NULL;
> +		}
> +	}
> +
> +	skb->dev = xs->dev;
> +	skb->priority = xs->sk.sk_priority;
> +	skb->mark = xs->sk.sk_mark;
> +	skb_shinfo(skb)->destructor_arg = (void *)(long)desc->addr;
> +	skb->destructor = xsk_destruct_skb;
> +
> +	return skb;
> +}
> +
>  static int xsk_generic_xmit(struct sock *sk)
>  {
>  	struct xdp_sock *xs = xdp_sk(sk);
> @@ -446,43 +535,28 @@ static int xsk_generic_xmit(struct sock *sk)
>  		goto out;
>  
>  	while (xskq_cons_peek_desc(xs->tx, &desc, xs->pool)) {
> -		char *buffer;
> -		u64 addr;
> -		u32 len;
> -
>  		if (max_batch-- == 0) {
>  			err = -EAGAIN;
>  			goto out;
>  		}
>  
> -		len = desc.len;
> -		skb = sock_alloc_send_skb(sk, len, 1, &err);
> +		skb = xsk_build_skb(xs, &desc, &err);
>  		if (unlikely(!skb))
>  			goto out;
>  
> -		skb_put(skb, len);
> -		addr = desc.addr;
> -		buffer = xsk_buff_raw_get_data(xs->pool, addr);
> -		err = skb_store_bits(skb, 0, buffer, len);
>  		/* This is the backpressure mechanism for the Tx path.
>  		 * Reserve space in the completion queue and only proceed
>  		 * if there is space in it. This avoids having to implement
>  		 * any buffering in the Tx path.
>  		 */
>  		spin_lock_irqsave(&xs->pool->cq_lock, flags);
> -		if (unlikely(err) || xskq_prod_reserve(xs->pool->cq)) {
> +		if (xskq_prod_reserve(xs->pool->cq)) {
>  			spin_unlock_irqrestore(&xs->pool->cq_lock, flags);
>  			kfree_skb(skb);
>  			goto out;
>  		}
>  		spin_unlock_irqrestore(&xs->pool->cq_lock, flags);
>  
> -		skb->dev = xs->dev;
> -		skb->priority = sk->sk_priority;
> -		skb->mark = sk->sk_mark;
> -		skb_shinfo(skb)->destructor_arg = (void *)(long)desc.addr;
> -		skb->destructor = xsk_destruct_skb;
> -
>  		err = __dev_direct_xmit(skb, xs->queue_id);
>  		if  (err == NETDEV_TX_BUSY) {
>  			/* Tell user-space to retry the send */
> -- 
> 1.8.3.1