linux-kernel - RE: [PATCH bpf-next v4] libbpf: Expose API to consume one ring at a time

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Message-ID: <64c2bebadbbba_831d208b3@john.notmuch>
Date:   Thu, 27 Jul 2023 12:00:10 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Adam Sindelar <adam@...signal.io>, bpf@...r.kernel.org
Cc:     Adam Sindelar <ats@...com>, David Vernet <void@...ifault.com>,
        Brendan Jackman <jackmanb@...gle.com>,
        KP Singh <kpsingh@...omium.org>, linux-kernel@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Florent Revest <revest@...omium.org>
Subject: RE: [PATCH bpf-next v4] libbpf: Expose API to consume one ring at a
 time

Adam Sindelar wrote:
> We already provide ring_buffer__epoll_fd to enable use of external
> polling systems. However, the only API available to consume the ring
> buffer is ring_buffer__consume, which always checks all rings. When
> polling for many events, this can be wasteful.
> 
> Signed-off-by: Adam Sindelar <adam@...signal.io>
> ---
> v1->v2: Added entry to libbpf.map
> v2->v3: Correctly set errno and handle overflow
> v3->v4: Fixed an embarrasing typo from zealous autocomplete
> 
>  tools/lib/bpf/libbpf.h   |  1 +
>  tools/lib/bpf/libbpf.map |  1 +
>  tools/lib/bpf/ringbuf.c  | 22 ++++++++++++++++++++++
>  3 files changed, 24 insertions(+)
> 
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 55b97b2087540..20ccc65eb3f9d 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -1195,6 +1195,7 @@ 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_ring(struct ring_buffer *rb, uint32_t ring_id);
>  LIBBPF_API int ring_buffer__epoll_fd(const struct ring_buffer *rb);
>  
>  struct user_ring_buffer_opts {
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index 9c7538dd5835e..42dc418b4672f 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -398,4 +398,5 @@ LIBBPF_1.3.0 {
>  		bpf_prog_detach_opts;
>  		bpf_program__attach_netfilter;
>  		bpf_program__attach_tcx;
> +		ring_buffer__consume_ring;
>  } LIBBPF_1.2.0;
> diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c
> index 02199364db136..457469fc7d71e 100644
> --- a/tools/lib/bpf/ringbuf.c
> +++ b/tools/lib/bpf/ringbuf.c
> @@ -290,6 +290,28 @@ int ring_buffer__consume(struct ring_buffer *rb)
>  	return res;
>  }
>  
> +/* Consume available data from a single RINGBUF map identified by its ID.
> + * The ring ID is returned in epoll_data by epoll_wait when called with
> + * ring_buffer__epoll_fd.
> + */
> +int ring_buffer__consume_ring(struct ring_buffer *rb, uint32_t ring_id)
> +{
> +	struct ring *ring;
> +	int64_t res;
> +
> +	if (ring_id >= rb->ring_cnt)
> +		return libbpf_err(-EINVAL);
> +
> +	ring = &rb->rings[ring_id];
> +	res = ringbuf_process_ring(ring);
> +	if (res < 0)
> +		return libbpf_err(res);
> +
> +	if (res > INT_MAX)
> +		return INT_MAX;
> +	return res;

Why not just return int64_t here? Then skip the INT_MAX check? I would
just assume get the actual value if I was calling this.

> +}
> +
>  /* Poll for available data and consume records, if any are available.
>   * Returns number of records consumed (or INT_MAX, whichever is less), or
>   * negative number, if any of the registered callbacks returned error.
> -- 
> 2.39.2
> 
>