Re: [PATCH bpf-next v3 04/10] xsk: check need wakeup flag in sendmsg()

[Magnus Karlsson <magnus.karlsson@gmail.com>]

On Thu, Nov 19, 2020 at 9:33 AM Björn Töpel <bjorn.topel@gmail.com> wrote:
>
> From: Björn Töpel <bjorn.topel@intel.com>
>
> Add a check for need wake up in sendmsg(), so that if a user calls
> sendmsg() when no wakeup is needed, do not trigger a wakeup.
>
> To simplify the need wakeup check in the syscall, unconditionally
> enable the need wakeup flag for Tx. This has a side-effect for poll();
> If poll() is called for a socket without enabled need wakeup, a Tx
> wakeup is unconditionally performed.
>
> The wakeup matrix for AF_XDP now looks like:
>
> need wakeup | poll()       | sendmsg()   | recvmsg()
> ------------+--------------+-------------+------------
> disabled    | wake Tx      | wake Tx     | nop
> enabled     | check flag;  | check flag; | check flag;
>             |   wake Tx/Rx |   wake Tx   |   wake Rx
>
> Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
> ---
>  net/xdp/xsk.c           |  6 +++++-
>  net/xdp/xsk_buff_pool.c | 13 ++++++-------
>  2 files changed, 11 insertions(+), 8 deletions(-)

Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>

> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 56a52ec75696..bf0f5c34af6c 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -522,13 +522,17 @@ static int xsk_sendmsg(struct socket *sock, struct msghdr *m, size_t total_len)
>         bool need_wait = !(m->msg_flags & MSG_DONTWAIT);
>         struct sock *sk = sock->sk;
>         struct xdp_sock *xs = xdp_sk(sk);
> +       struct xsk_buff_pool *pool;
>
>         if (unlikely(!xsk_is_bound(xs)))
>                 return -ENXIO;
>         if (unlikely(need_wait))
>                 return -EOPNOTSUPP;
>
> -       return __xsk_sendmsg(sk);
> +       pool = xs->pool;
> +       if (pool->cached_need_wakeup & XDP_WAKEUP_TX)
> +               return __xsk_sendmsg(sk);
> +       return 0;
>  }
>
>  static int xsk_recvmsg(struct socket *sock, struct msghdr *m, size_t len, int flags)
> diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c
> index 8a3bf4e1318e..96bb607853ad 100644
> --- a/net/xdp/xsk_buff_pool.c
> +++ b/net/xdp/xsk_buff_pool.c
> @@ -144,14 +144,13 @@ static int __xp_assign_dev(struct xsk_buff_pool *pool,
>         if (err)
>                 return err;
>
> -       if (flags & XDP_USE_NEED_WAKEUP) {
> +       if (flags & XDP_USE_NEED_WAKEUP)
>                 pool->uses_need_wakeup = true;
> -               /* Tx needs to be explicitly woken up the first time.
> -                * Also for supporting drivers that do not implement this
> -                * feature. They will always have to call sendto().
> -                */
> -               pool->cached_need_wakeup = XDP_WAKEUP_TX;
> -       }
> +       /* Tx needs to be explicitly woken up the first time.  Also
> +        * for supporting drivers that do not implement this
> +        * feature. They will always have to call sendto() or poll().
> +        */
> +       pool->cached_need_wakeup = XDP_WAKEUP_TX;
>
>         dev_hold(netdev);
>
> --
> 2.27.0
>