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Date:   Fri, 22 Jan 2021 11:47:45 +0000
To:     Eric Dumazet <eric.dumazet@gmail.com>
From:   Alexander Lobakin <alobakin@pm.me>
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>,
        =?utf-8?Q?Bj=C3=B6rn_T=C3=B6pel?= <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
Reply-To: Alexander Lobakin <alobakin@pm.me>
Subject: Re: [PATCH bpf-next v3 3/3] xsk: build skb by page
Message-ID: <20210122114729.1758-1-alobakin@pm.me>
In-Reply-To: <dcee4592-9fa9-adbb-55ca-58a962076e7a@gmail.com>
References: <cover.1611236588.git.xuanzhuo@linux.alibaba.com> <340f1dfa40416dd966a56e08507daba82d633088.1611236588.git.xuanzhuo@linux.alibaba.com> <dcee4592-9fa9-adbb-55ca-58a962076e7a@gmail.com>
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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.
> >=20
> > 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.
> >=20
> > ---------------- Performance Testing ------------
> >=20
> > The test environment is Aliyun ECS server.
> > Test cmd:
> > ```
> > xdpsock -i eth0 -t  -S -s <msg size>
> > ```
> >=20
> > Test result data:
> >=20
> > size    64      512     1024    1500
> > copy    1916747 1775988 1600203 1440054
> > page    1974058 1953655 1945463 1904478
> > percent 3.0%    10.0%   21.58%  32.3%
> >=20
> > 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(-)
> >=20
> > 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)
> >  =09sock_wfree(skb);
> >  }
> > =20
> > +static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
> > +=09=09=09=09=09      struct xdp_desc *desc)
> > +{
> > +=09u32 len, offset, copy, copied;
> > +=09struct sk_buff *skb;
> > +=09struct page *page;
> > +=09void *buffer;
> > +=09int err, i;
> > +=09u64 addr;
> > +
> > +=09skb =3D sock_alloc_send_skb(&xs->sk, 0, 1, &err);
> > +=09if (unlikely(!skb))
> > +=09=09return ERR_PTR(err);
> > +
> > +=09addr =3D desc->addr;
> > +=09len =3D desc->len;
> > +
> > +=09buffer =3D xsk_buff_raw_get_data(xs->pool, addr);
> > +=09offset =3D offset_in_page(buffer);
> > +=09addr =3D buffer - xs->pool->addrs;
> > +
> > +=09for (copied =3D 0, i =3D 0; copied < len; i++) {
> > +=09=09page =3D xs->pool->umem->pgs[addr >> PAGE_SHIFT];
> > +
> > +=09=09get_page(page);
> > +
> > +=09=09copy =3D min_t(u32, PAGE_SIZE - offset, len - copied);
> > +
> > +=09=09skb_fill_page_desc(skb, i, page, offset, copy);
> > +
> > +=09=09copied +=3D copy;
> > +=09=09addr +=3D copy;
> > +=09=09offset =3D 0;
> > +=09}
> > +
> > +=09skb->len +=3D len;
> > +=09skb->data_len +=3D len;
>=20
> > +=09skb->truesize +=3D len;
>=20
> This is not the truesize, unfortunately.
>=20
> We need to account for the number of pages, not number of bytes.

The easiest solution is:

=09skb->truesize +=3D PAGE_SIZE * i;

i would be equal to skb_shinfo(skb)->nr_frags after exiting the loop.

> > +
> > +=09refcount_add(len, &xs->sk.sk_wmem_alloc);
> > +
> > +=09return skb;
> > +}
> > +

Al