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From:   Magnus Karlsson <magnus.karlsson@gmail.com>
Date:   Tue, 24 Nov 2020 12:36:39 +0100
Message-ID: <CAJ8uoz1S1brwy+2u48Y9jn3ys6QEHQjtw3OQDj3wrgxCf7Or3w@mail.gmail.com>
Subject: Re: [PATCH 1/3] xsk: replace datagram_poll by sock_poll_wait
To:     Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc:     =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn.topel@intel.com>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        Network Development <netdev@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>, linux-kernel@vger.kernel.org
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On Mon, Nov 23, 2020 at 3:11 PM Magnus Karlsson
<magnus.karlsson@gmail.com> wrote:
>
> On Wed, Nov 18, 2020 at 9:26 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> >
> > datagram_poll will judge the current socket status (EPOLLIN, EPOLLOUT)
> > based on the traditional socket information (eg: sk_wmem_alloc), but
> > this does not apply to xsk. So this patch uses sock_poll_wait instead of
> > datagram_poll, and the mask is calculated by xsk_poll.
> >
> > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > ---
> >  net/xdp/xsk.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> > index cfbec39..7f0353e 100644
> > --- a/net/xdp/xsk.c
> > +++ b/net/xdp/xsk.c
> > @@ -477,11 +477,13 @@ static int xsk_sendmsg(struct socket *sock, struct msghdr *m, size_t total_len)
> >  static __poll_t xsk_poll(struct file *file, struct socket *sock,
> >                              struct poll_table_struct *wait)
> >  {
> > -       __poll_t mask = datagram_poll(file, sock, wait);
> > +       __poll_t mask = 0;
>
> It would indeed be nice to not execute a number of tests in
> datagram_poll that will never be triggered. It will speed up things
> for sure. But we need to make sure that removing those flags that
> datagram_poll sets do not have any bad effects in the code above this.
> But let us tentatively keep this patch for the next version of the
> patch set. Just need to figure out how to solve your problem in a nice
> way first. See discussion in patch 0/3.
>
> >         struct sock *sk = sock->sk;
> >         struct xdp_sock *xs = xdp_sk(sk);
> >         struct xsk_buff_pool *pool;
> >
> > +       sock_poll_wait(file, sock, wait);
> > +
> >         if (unlikely(!xsk_is_bound(xs)))
> >                 return mask;
> >
> > --
> > 1.8.3.1
> >

The fix looks correct and it will speed things up too as a bonus.
Please include this patch in the v2 as outlined in my answer to 0/3.

Thanks!