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

Alexander Lobakin
2021-01-18T12:39:34Z

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