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

From: Alexander Lobakin <alobakin@pm.me>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: "Alexander Lobakin" <alobakin@pm.me>,
	"Xuan Zhuo" <xuanzhuo@linux.alibaba.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	"Jason Wang" <jasowang@redhat.com>,
	"David S. Miller" <davem@davemloft.net>,
	"Jakub Kicinski" <kuba@kernel.org>,
	"Björn Töpel" <bjorn@kernel.org>,
	"Magnus Karlsson" <magnus.karlsson@intel.com>,
	"Jonathan Lemon" <jonathan.lemon@gmail.com>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Jesper Dangaard Brouer" <hawk@kernel.org>,
	"John Fastabend" <john.fastabend@gmail.com>,
	"Andrii Nakryiko" <andrii@kernel.org>,
	"Martin KaFai Lau" <kafai@fb.com>,
	"Song Liu" <songliubraving@fb.com>, "Yonghong Song" <yhs@fb.com>,
	"KP Singh" <kpsingh@kernel.org>,
	virtualization@lists.linux-foundation.org, bpf@vger.kernel.org,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH bpf-next v3 3/3] xsk: build skb by page
Date: Fri, 22 Jan 2021 11:47:45 +0000	[thread overview]
Message-ID: <20210122114729.1758-1-alobakin@pm.me> (raw)
In-Reply-To: <dcee4592-9fa9-adbb-55ca-58a962076e7a@gmail.com>

From: Eric Dumazet <eric.dumazet@gmail.com>
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@linux.alibaba.com>
> > Reviewed-by: Dust Li <dust.li@linux.alibaba.com>
> > ---
> >  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);
> > +	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.

> > +
> > +	refcount_add(len, &xs->sk.sk_wmem_alloc);
> > +
> > +	return skb;
> > +}
> > +

Al