Re: [PATCH v5 bpf-next 6/6] xsk: build skb by page (aka generic zerocopy xmit)

[Alexander Lobakin]

From: Alexander Lobakin <alobakin@xxxxx>
Date: Tue, 16 Feb 2021 14:35:02 +0000

> From: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> 
> This patch is used to construct skb based on page to save memory copy
> overhead.
> 
> This function is implemented based on IFF_TX_SKB_NO_LINEAR. Only the
> network card priv_flags supports IFF_TX_SKB_NO_LINEAR will use page to
> directly construct skb. If this feature is not supported, it is still
> necessary to copy data to construct skb.
> 
> ---------------- 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>
> [ alobakin:
>  - expand subject to make it clearer;
>  - improve skb->truesize calculation;
>  - reserve some headroom in skb for drivers;
>  - tailroom is not needed as skb is non-linear ]
> Signed-off-by: Alexander Lobakin <alobakin@xxxxx>
> Acked-by: Magnus Karlsson <magnus.karlsson@xxxxxxxxx>
> ---
>  net/xdp/xsk.c | 119 ++++++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 95 insertions(+), 24 deletions(-)
> 
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 143979ea4165..ff7bd06e1241 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -445,6 +445,96 @@ 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)
> +{
> +	struct xsk_buff_pool *pool = xs->pool;
> +	u32 hr, len, offset, copy, copied;
> +	struct sk_buff *skb;
> +	struct page *page;
> +	void *buffer;
> +	int err, i;
> +	u64 addr;
> +
> +	hr = max(NET_SKB_PAD, L1_CACHE_ALIGN(xs->dev->needed_headroom));
> +
> +	skb = sock_alloc_send_skb(&xs->sk, hr, 1, &err);
> +	if (unlikely(!skb))
> +		return ERR_PTR(err);
> +
> +	skb_reserve(skb, hr);
> +
> +	addr = desc->addr;
> +	len = desc->len;
> +
> +	buffer = xsk_buff_raw_get_data(pool, addr);
> +	offset = offset_in_page(buffer);
> +	addr = buffer - pool->addrs;
> +
> +	for (copied = 0, i = 0; copied < len; i++) {
> +		page = pool->umem->pgs[addr >> PAGE_SHIFT];
> +		get_page(page);
> +
> +		copy = min_t(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 += pool->unaligned ? len : pool->chunk_size;
> +
> +	refcount_add(skb->truesize, &xs->sk.sk_wmem_alloc);

Meh, there's a refcount leak here I accidentally introduced in v4.
Sorry for that, I'll upload v6 in just a moment.

> +	return skb;
> +}
> +
> +static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
> +				     struct xdp_desc *desc)
> +{
> +	struct net_device *dev = xs->dev;
> +	struct sk_buff *skb;
> +
> +	if (dev->priv_flags & IFF_TX_SKB_NO_LINEAR) {
> +		skb = xsk_build_skb_zerocopy(xs, desc);
> +		if (IS_ERR(skb))
> +			return skb;
> +	} else {
> +		u32 hr, tr, len;
> +		void *buffer;
> +		int err;
> +
> +		hr = max(NET_SKB_PAD, L1_CACHE_ALIGN(dev->needed_headroom));
> +		tr = dev->needed_tailroom;
> +		len = desc->len;
> +
> +		skb = sock_alloc_send_skb(&xs->sk, hr + len + tr, 1, &err);
> +		if (unlikely(!skb))
> +			return ERR_PTR(err);
> +
> +		skb_reserve(skb, hr);
> +		skb_put(skb, len);
> +
> +		buffer = xsk_buff_raw_get_data(xs->pool, desc->addr);
> +		err = skb_store_bits(skb, 0, buffer, len);
> +		if (unlikely(err)) {
> +			kfree_skb(skb);
> +			return ERR_PTR(err);
> +		}
> +	}
> +
> +	skb->dev = 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);
> @@ -454,56 +544,37 @@ static int xsk_generic_xmit(struct sock *sk)
>  	struct sk_buff *skb;
>  	unsigned long flags;
>  	int err = 0;
> -	u32 hr, tr;
>  
>  	mutex_lock(&xs->mutex);
>  
>  	if (xs->queue_id >= xs->dev->real_num_tx_queues)
>  		goto out;
>  
> -	hr = max(NET_SKB_PAD, L1_CACHE_ALIGN(xs->dev->needed_headroom));
> -	tr = xs->dev->needed_tailroom;
> -
>  	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, hr + len + tr, 1, &err);
> -		if (unlikely(!skb))
> +		skb = xsk_build_skb(xs, &desc);
> +		if (IS_ERR(skb)) {
> +			err = PTR_ERR(skb);
>  			goto out;
> +		}
>  
> -		skb_reserve(skb, hr);
> -		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 */
> -- 
> 2.30.1

Al