Subject: Re: [PATCH net-next v2 3/3] xsk: build skb by page

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 20 Jan 2021 03:11:52 -0500

On Wed, Jan 20, 2021 at 03:11:04AM -0500, Michael S. Tsirkin wrote:
> On Wed, Jan 20, 2021 at 03:50:01PM +0800, Xuan Zhuo wrote:
> > 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 <[email-masked]>
> > Reviewed-by: Dust Li <[email-masked]>
> 
> I can't see the cover letter or 1/3 in this series - was probably
> threaded incorrectly?

Hmm looked again and now I do see them. My mistake pls ignore.

> 
> > ---
> >  net/xdp/xsk.c | 104 ++++++++++++++++++++++++++++++++++++++++++++++++----------
> >  1 file changed, 86 insertions(+), 18 deletions(-)
> > 
> > diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> > index 8037b04..817a3a5 100644
> > --- a/net/xdp/xsk.c
> > +++ b/net/xdp/xsk.c
> > @@ -430,6 +430,87 @@ 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;
> 
> Actually, make this void *, this way you will not need
> casts down the road. I know this is from xsk_generic_xmit -
> I don't know why it's char * there, either.
> 
> > +	int err, i;
> > +	u64 addr;
> > +
> > +	skb = sock_alloc_send_skb(&xs->sk, 0, 1, &err);
> > +	if (unlikely(!skb))
> > +		return ERR_PTR(err);
> > +
> > +	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_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 += 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)
> > +{
> > +	struct sk_buff *skb = NULL;
> > +
> > +	if (xs->dev->priv_flags & IFF_TX_SKB_NO_LINEAR) {
> > +		skb = xsk_build_skb_zerocopy(xs, desc);
> > +		if (IS_ERR(skb))
> > +			return skb;
> > +	} else {
> > +		char *buffer;
> > +		u32 len;
> > +		int err;
> > +
> > +		len = desc->len;
> > +		skb = sock_alloc_send_skb(&xs->sk, len, 1, &err);
> > +		if (unlikely(!skb))
> > +			return ERR_PTR(err);
> > +
> > +		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 = 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 +527,30 @@ 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);
> > -		if (unlikely(!skb))
> > +		skb = xsk_build_skb(xs, &desc);
> > +		if (IS_ERR(skb)) {
> > +			err = PTR_ERR(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