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Date:   Fri, 22 Jan 2021 18:45:33 +0000
To:     Magnus Karlsson <magnus.karlsson@gmail.com>
From:   Alexander Lobakin <alobakin@pm.me>
Cc:     Alexander Lobakin <alobakin@pm.me>,
        Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Bjorn Topel <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 <bpf@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        open list <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: <20210122184457.37812-1-alobakin@pm.me>
In-Reply-To: <CAJ8uoz310du+0qsdGWKtsrK3tBxGFTr=kWkT+GwY1GqN=A2ejQ@mail.gmail.com>
References: <1611329955.4913929-2-xuanzhuo@linux.alibaba.com> <20210122172534.9896-1-alobakin@pm.me> <CAJ8uoz310du+0qsdGWKtsrK3tBxGFTr=kWkT+GwY1GqN=A2ejQ@mail.gmail.com>
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From: Magnus Karlsson <magnus.karlsson@gmail.com>
Date: Fri, 22 Jan 2021 19:37:06 +0100

> On Fri, Jan 22, 2021 at 6:26 PM Alexander Lobakin <alobakin@pm.me> wrote:
> >
> > From: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > Date: Fri, 22 Jan 2021 23:39:15 +0800
> >
> > > On Fri, 22 Jan 2021 13:55:14 +0100, Magnus Karlsson <magnus.karlsson@=
gmail.com> wrote:
> > > > On Fri, Jan 22, 2021 at 1:39 PM Alexander Lobakin <alobakin@pm.me> =
wrote:
> > > > >
> > > > > From: Magnus Karlsson <magnus.karlsson@gmail.com>
> > > > > Date: Fri, 22 Jan 2021 13:18:47 +0100
> > > > >
> > > > > > On Fri, Jan 22, 2021 at 12:57 PM Alexander Lobakin <alobakin@pm=
.me> wrote:
> > > > > > >
> > > > > > > From: Alexander Lobakin <alobakin@pm.me>
> > > > > > > Date: Fri, 22 Jan 2021 11:47:45 +0000
> > > > > > >
> > > > > > > > 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 sa=
ve memory copy
> > > > > > > > > > overhead.
> > > > > > > > > >
> > > > > > > > > > This function is implemented based on IFF_TX_SKB_NO_LIN=
EAR. Only the
> > > > > > > > > > network card priv_flags supports IFF_TX_SKB_NO_LINEAR w=
ill use page to
> > > > > > > > > > directly construct skb. If this feature is not supporte=
d, 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(struc=
t sk_buff *skb)
> > > > > > > > > >   sock_wfree(skb);
> > > > > > > > > >  }
> > > > > > > > > >
> > > > > > > > > > +static struct sk_buff *xsk_build_skb_zerocopy(struct x=
dp_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 =3D sock_alloc_send_skb(&xs->sk, 0, 1, &err);
> > > > > > > > > > + if (unlikely(!skb))
> > > > > > > > > > +         return ERR_PTR(err);
> > > > > > > > > > +
> > > > > > > > > > + addr =3D desc->addr;
> > > > > > > > > > + len =3D desc->len;
> > > > > > > > > > +
> > > > > > > > > > + buffer =3D xsk_buff_raw_get_data(xs->pool, addr);
> > > > > > > > > > + offset =3D offset_in_page(buffer);
> > > > > > > > > > + addr =3D buffer - xs->pool->addrs;
> > > > > > > > > > +
> > > > > > > > > > + for (copied =3D 0, i =3D 0; copied < len; i++) {
> > > > > > > > > > +         page =3D xs->pool->umem->pgs[addr >> PAGE_SHI=
FT];
> > > > > > > > > > +
> > > > > > > > > > +         get_page(page);
> > > > > > > > > > +
> > > > > > > > > > +         copy =3D min_t(u32, PAGE_SIZE - offset, len -=
 copied);
> > > > > > > > > > +
> > > > > > > > > > +         skb_fill_page_desc(skb, i, page, offset, copy=
);
> > > > > > > > > > +
> > > > > > > > > > +         copied +=3D copy;
> > > > > > > > > > +         addr +=3D copy;
> > > > > > > > > > +         offset =3D 0;
> > > > > > > > > > + }
> > > > > > > > > > +
> > > > > > > > > > + skb->len +=3D len;
> > > > > > > > > > + skb->data_len +=3D len;
> > > > > > > > >
> > > > > > > > > > + skb->truesize +=3D 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 +=3D 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 +=3D PAGE_SIZE * npages;
> > > > > > >
> > > > > > > Right.
> > > > > >
> > > > > > There are two possible packet buffer (chunks) sizes in a umem, =
2K and
> > > > > > 4K on a system with a PAGE_SIZE of 4K. If I remember correctly,=
 and
> > > > > > please correct me if wrong, truesize is used for memory account=
ing.
> > > > > > But in this code, no kernel memory has been allocated (apart fr=
om the
> > > > > > skb). The page is just a part of the umem that has been already
> > > > > > allocated beforehand and by user-space in this case. So what sh=
ould
> > > > > > truesize be in this case? Do we add 0, chunk_size * i, or the
> > > > > > complicated case of counting exactly how many 4K pages that are=
 used
> > > > > > when the chunk_size is 2K, as two chunks could occupy the same =
page,
> > > > > > or just the upper bound of PAGE_SIZE * i that is likely a good
> > > > > > approximation in most cases? Just note that there might be othe=
r uses
> > > > > > of truesize that I am unaware of that could impact this choice.
> > > > >
> > > > > Truesize is "what amount of memory does this skb occupy with all =
its
> > > > > fragments, linear space and struct sk_buff itself". The closest i=
t
> > > > > will be to the actual value, the better.
> > > > > In this case, I think adding of chunk_size * i would be enough.
> > > >
> > > > Sounds like a good approximation to me.
> > > >
> > > > > (PAGE_SIZE * i can be overwhelming when chunk_size is 2K, especia=
lly
> > > > > for setups with PAGE_SIZE > SZ_4K)
> > > >
> > > > You are right. That would be quite horrible on a system with a page=
 size of 64K.
> > >
> > > Thank you everyone, I learned it.
> > >
> > > I also think it is appropriate to add a chunk size here, and there is=
 actually
> > > only one chunk here, so it's very simple
> > >
> > >       skb->truesize +=3D xs->pool->chunk_size;
> >
> > umem chunks can't cross page boundaries. So if you're sure that
> > there could be only one chunk, you don't need the loop at all,
> > if I'm not missing anything.
>=20
> In the default mode, this is true. But in the unaligned_chunk mode
> that can be set on the umem, the chunk may cross one page boundary, so
> we need the loop and the chunk_size * i in the assignment of truesize.
> So "i" can be 1 or 2, but nothing else.

Ah, unaligned mode, sure. Thanks for explanation!
Right, we need the loop and skb->truesize +=3D xs->pool->chunk_size * i
then.

> > > In addition, I actually borrowed from the tcp code:
> > >
> > >    tcp_build_frag:
> > >    --------------
> > >
> > >       if (can_coalesce) {
> > >               skb_frag_size_add(&skb_shinfo(skb)->frags[i - 1], copy)=
;
> > >       } else {
> > >               get_page(page);
> > >               skb_fill_page_desc(skb, i, page, offset, copy);
> > >       }
> > >
> > >       if (!(flags & MSG_NO_SHARED_FRAGS))
> > >               skb_shinfo(skb)->flags |=3D SKBFL_SHARED_FRAG;
> > >
> > >       skb->len +=3D copy;
> > >       skb->data_len +=3D copy;
> > >       skb->truesize +=3D copy;
> > >
> > > So, here is one bug?
> >
> > skb_frag_t is an alias to struct bvec. It doesn't contain info about
> > real memory consumption, so there's no other option buf just to add
> > "copy" to truesize.
> > XSK is different in this term, as it operates with chunks of a known
> > size.
> >
> > > Thanks.
> > >
> > > >
> > > > > > > > > > +
> > > > > > > > > > + refcount_add(len, &xs->sk.sk_wmem_alloc);
> > > > > > > > > > +
> > > > > > > > > > + return skb;
> > > > > > > > > > +}
> > > > > > > > > > +
> > > > > > > >
> > > > > > > > Al
> > > > > > >
> > > > > > > Thanks,
> > > > > > > Al
> > > > >
> > > > > Al