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

[Alexander Lobakin]

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

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

>> 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@xxxxxxxxxxxxxxxxx>
>> Reviewed-by: Dust Li <dust.li@xxxxxxxxxxxxxxxxx>
>> ---
>>  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.

>>  };
>>  
>>  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