Re: [PATCH bpf-next v2] libbpf: add xsk_ring_prod__nb_free() function

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

On Wed, Jun 26, 2019 at 10:33 AM Eelco Chaudron <echaudro@redhat.com> wrote:
>
> When an AF_XDP application received X packets, it does not mean X
> frames can be stuffed into the producer ring. To make it easier for
> AF_XDP applications this API allows them to check how many frames can
> be added into the ring.
>
> Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
> ---
>
> v1 -> v2
>  - Renamed xsk_ring_prod__free() to xsk_ring_prod__nb_free()
>  - Add caching so it will only touch global state when needed
>
>  tools/lib/bpf/xsk.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/tools/lib/bpf/xsk.h b/tools/lib/bpf/xsk.h
> index 82ea71a0f3ec..6acb81102346 100644
> --- a/tools/lib/bpf/xsk.h
> +++ b/tools/lib/bpf/xsk.h
> @@ -76,11 +76,11 @@ xsk_ring_cons__rx_desc(const struct xsk_ring_cons *rx, __u32 idx)
>         return &descs[idx & rx->mask];
>  }
>
> -static inline __u32 xsk_prod_nb_free(struct xsk_ring_prod *r, __u32 nb)
> +static inline __u32 xsk_prod__nb_free(struct xsk_ring_prod *r, __u32 nb)
>  {
>         __u32 free_entries = r->cached_cons - r->cached_prod;
>
> -       if (free_entries >= nb)
> +       if (free_entries >= nb && nb != 0)
>                 return free_entries;

Thanks Eelco for the patch. Is the test nb != 0 introduced here so
that the function will continue with the refresh from the global state
when nb is set to 0? If so, could a user not instead just set the nb
parameter to the size of the ring? This would always trigger a
refresh, except when the number of free entries is equal to the size
of the ring, but then we do not need the refresh anyway. This would
eliminate the nb != 0 test that you introduced from the fast path.

/Magnus

>         /* Refresh the local tail pointer.
> @@ -110,7 +110,7 @@ static inline __u32 xsk_cons_nb_avail(struct xsk_ring_cons *r, __u32 nb)
>  static inline size_t xsk_ring_prod__reserve(struct xsk_ring_prod *prod,
>                                             size_t nb, __u32 *idx)
>  {
> -       if (xsk_prod_nb_free(prod, nb) < nb)
> +       if (xsk_prod__nb_free(prod, nb) < nb)
>                 return 0;
>
>         *idx = prod->cached_prod;
> --
> 2.20.1
>