Re: [PATCH bpf-next v3 3/3] xsk: build skb by page

[Alexander Lobakin]

From: Alexander Lobakin <alobakin@xxxxx>
Date: Fri, 22 Jan 2021 11:55:35 +0000

> From: Alexander Lobakin <alobakin@xxxxx>
> Date: Fri, 22 Jan 2021 11:47:45 +0000
> 
> > From: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> > Date: Thu, 21 Jan 2021 16:41:33 +0100
> > 
> > > On 1/21/21 2:47 PM, 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 <xuanzhuo@xxxxxxxxxxxxxxxxx>
> > > > Reviewed-by: Dust Li <dust.li@xxxxxxxxxxxxxxxxx>
> > > > ---
> > > >  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 4a83117..38af7f1 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;
> > > > +	void *buffer;
> > > > +	int err, i;
> > > > +	u64 addr;
> > > > +
> > > > +	skb = sock_alloc_send_skb(&xs->sk, 0, 1, &err);

Also,
maybe we should allocate it with NET_SKB_PAD so NIC drivers could
use some reserved space?

		skb = sock_alloc_send_skb(&xs->sk, NET_SKB_PAD, 1, &err);
		...
		skb_reserve(skb, NET_SKB_PAD);

Eric, what do you think?

> > > > +	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 - 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;
> > > 
> > > This is not the truesize, unfortunately.
> > > 
> > > We need to account for the number of pages, not number of bytes.
> > 
> > The easiest solution is:
> > 
> > 	skb->truesize += PAGE_SIZE * i;
> > 
> > i would be equal to skb_shinfo(skb)->nr_frags after exiting the loop.
> 
> Oops, pls ignore this. I forgot that XSK buffers are not
> "one per page".
> We need to count the number of pages manually and then do
> 
> 	skb->truesize += PAGE_SIZE * npages;
> 
> Right.
> 
> > > > +
> > > > +	refcount_add(len, &xs->sk.sk_wmem_alloc);
> > > > +
> > > > +	return skb;
> > > > +}
> > > > +
> > 
> > Al
> 
> Thanks,
> Al

Al