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

From: Alexander Lobakin <redacted>
Date: Mon, 18 Jan 2021 13:00:17 +0000

From: Yunsheng Lin <redacted>
Date: Mon, 18 Jan 2021 20:40:52 +0800

quoted

On 2021/1/16 10:44, Xuan Zhuo wrote:

quoted

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

Does it make sense to use ETHTOOL_TX_COPYBREAK tunable in ethtool to
configure if the data is copied or not?

As far as I can grep, only mlx4 supports this, and it has a different
meaning in that driver.
So I guess a new netdev_feature would be a better solution.

quoted

quoted

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",

I completely forgot to add that you'd better to mention in both
enumeration/feature and its Ethtool string that the feature applies
to Tx path.
Smth like:

NETIF_F_SKB_TX_NO_LINEAR{,_BIT}, "skb-tx-no-linear"
or
NETIF_F_TX_SKB_NO_LINEAR{,_BIT}, "tx-skb-no-linear"

Otherwise, it may be confusing for users and developers.

OR, I think we may tight the feature with the new approach to build
skbs by page as it makes no sense for anything else.
So, if we define something like:

NETIF_F_XSK_TX_GENERIC_ZC{,_BIT}, "xsk-tx-generic-zerocopy",

then user can toggle your new XSK Tx path on/off via Ethtool for
drivers that will support it (don't forget to add it to hw_features
for virtio_net then).

quoted

quoted

 };
 
 const char

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)
+{
+	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 */

Al

Al