Re: [PATCH 2/2] libbpf: Add ring__consume_max / ring_buffer__consume_max

[Andrii Nakryiko]

On Mon, Apr 1, 2024 at 12:32 AM Andrea Righi <andrea.righi@xxxxxxxxxxxxx> wrote:
>
> Introduce a new API to consume items from a ring buffer, limited to a
> specified amount, and return to the caller the actual number of items
> consumed.
>
> Link: https://lore.kernel.org/lkml/20240310154726.734289-1-andrea.righi@xxxxxxxxxxxxx/T
> Signed-off-by: Andrea Righi <andrea.righi@xxxxxxxxxxxxx>
> ---
>  tools/lib/bpf/libbpf.h   | 13 +++++++++++++
>  tools/lib/bpf/libbpf.map |  2 ++
>  tools/lib/bpf/ringbuf.c  | 35 +++++++++++++++++++++++++++++++++--
>  3 files changed, 48 insertions(+), 2 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index f88ab50c0229..85161889efd4 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -1293,6 +1293,8 @@ LIBBPF_API int ring_buffer__add(struct ring_buffer *rb, int map_fd,
>                                 ring_buffer_sample_fn sample_cb, void *ctx);
>  LIBBPF_API int ring_buffer__poll(struct ring_buffer *rb, int timeout_ms);
>  LIBBPF_API int ring_buffer__consume(struct ring_buffer *rb);
> +LIBBPF_API int ring_buffer__consume_max(struct ring_buffer *rb,
> +                                       size_t max_items);
>  LIBBPF_API int ring_buffer__epoll_fd(const struct ring_buffer *rb);
>
>  /**
> @@ -1367,6 +1369,17 @@ LIBBPF_API int ring__map_fd(const struct ring *r);
>   */
>  LIBBPF_API int ring__consume(struct ring *r);
>
> +/**
> + * @brief **ring__consume_max()** consumes up to a certain amount of items from

nit: certain -> requested ?

> + * a ringbuffer without event polling.
> + *
> + * @param r A ringbuffer object.
> + * @param max_items Maximum amount of items to consume.
> + * @return The number of items consumed (or max_items, whichever is less), or a

if we reach max_items, we did consume max_items, so I think "number of
items consumed" is right and I'd drop the part in parenthesis

> + * negative number if any of the callbacks return an error.
> + */
> +LIBBPF_API int ring__consume_max(struct ring *r, size_t max_items);

I'm bikeshedding here, of course, but I prefer `ring__consume_n` and
max_items -> n.

> +
>  struct user_ring_buffer_opts {
>         size_t sz; /* size of this struct, for forward/backward compatibility */
>  };
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index 51732ecb1385..bb3ed905119d 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -415,4 +415,6 @@ LIBBPF_1.4.0 {
>                 bpf_token_create;
>                 btf__new_split;
>                 btf_ext__raw_data;
> +               ring__consume_max;
> +               ring_buffer__consume_max;

we are in 1.5 cycle now, please add another section that inherits from
LIBBPF_1.4.0

>  } LIBBPF_1.3.0;
> diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c
> index 81df535040d1..c8123e326b1a 100644
> --- a/tools/lib/bpf/ringbuf.c
> +++ b/tools/lib/bpf/ringbuf.c
> @@ -281,6 +281,32 @@ static int64_t ringbuf_process_ring(struct ring *r, int64_t max_items)
>         return cnt;
>  }
>
> +/* Consume available ring buffer(s) data without event polling up to max_items.
> + *
> + * Returns number of records consumed across all registered ring buffers (or
> + * max_items, whichever is less), or negative number if any of the callbacks
> + * return error.
> + */
> +int ring_buffer__consume_max(struct ring_buffer *rb, size_t max_items)
> +{
> +       int64_t err, res = 0;
> +       int i;
> +
> +       for (i = 0; i < rb->ring_cnt; i++) {
> +               struct ring *ring = rb->rings[i];
> +
> +               err = ringbuf_process_ring(ring, max_items);
> +               if (err < 0)
> +                       return libbpf_err(err);
> +               res += err;
> +               max_items -= err;
> +
> +               if (!max_items)

please use `== 0`, it's not a bool and not a pointer

> +                       break;
> +       }
> +       return res;
> +}
> +
>  /* Consume available ring buffer(s) data without event polling.
>   * Returns number of records consumed across all registered ring buffers (or
>   * INT_MAX, whichever is less), or negative number if any of the callbacks
> @@ -376,17 +402,22 @@ int ring__map_fd(const struct ring *r)
>         return r->map_fd;
>  }
>
> -int ring__consume(struct ring *r)
> +int ring__consume_max(struct ring *r, size_t max_items)
>  {
>         int64_t res;

hm.. we can probably change this to int, can you do that as part of
your changes?

>
> -       res = ringbuf_process_ring(r, INT_MAX);
> +       res = ringbuf_process_ring(r, max_items);
>         if (res < 0)
>                 return libbpf_err(res);
>
>         return res;
>  }
>
> +int ring__consume(struct ring *r)
> +{
> +       return ring__consume_max(r, INT_MAX);
> +}
> +
>  static void user_ringbuf_unmap_ring(struct user_ring_buffer *rb)
>  {
>         if (rb->consumer_pos) {
> --
> 2.43.0
>