Re: [PATCH 1/2] libbpf: ringbuf: allow to consume up to a certain amount of items

[Andrii Nakryiko]

On Mon, Apr 1, 2024 at 12:32 AM Andrea Righi <andrea.righi@xxxxxxxxxxxxx> wrote:
>
> In some cases, instead of always consuming all items from ring buffers
> in a greedy way, we may want to consume up to a certain amount of items,
> for example when we need to copy items from the BPF ring buffer to a
> limited user buffer.
>
> This change allows to set an upper limit to the amount of items consumed
> from one or more ring buffers.
>
> Link: https://lore.kernel.org/lkml/20240310154726.734289-1-andrea.righi@xxxxxxxxxxxxx/T
> Signed-off-by: Andrea Righi <andrea.righi@xxxxxxxxxxxxx>
> ---
>  tools/lib/bpf/ringbuf.c | 29 +++++++++++++++++------------
>  1 file changed, 17 insertions(+), 12 deletions(-)
>
> diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c
> index aacb64278a01..81df535040d1 100644
> --- a/tools/lib/bpf/ringbuf.c
> +++ b/tools/lib/bpf/ringbuf.c
> @@ -231,7 +231,7 @@ static inline int roundup_len(__u32 len)
>         return (len + 7) / 8 * 8;
>  }
>
> -static int64_t ringbuf_process_ring(struct ring *r)
> +static int64_t ringbuf_process_ring(struct ring *r, int64_t max_items)
>  {
>         int *len_ptr, len, err;
>         /* 64-bit to avoid overflow in case of extreme application behavior */
> @@ -264,7 +264,14 @@ static int64_t ringbuf_process_ring(struct ring *r)
>                                                           cons_pos);
>                                         return err;
>                                 }
> -                               cnt++;
> +                               if (++cnt >= max_items) {
> +                                       /* update consumer pos and return the
> +                                        * total amount of items consumed.
> +                                        */
> +                                       smp_store_release(r->consumer_pos,
> +                                                         cons_pos);

Does this fit on a single line under 100 characters? If yes, please
keep it as a single line

but actually it seems cleaner to keep cnt++ intact, let
smp_store_release() below happen, and then check the exit condition.
Were you afraid to do unnecessary checks on discarded samples? I
wouldn't worry about that.

> +                                       goto done;
> +                               }
>                         }
>
>                         smp_store_release(r->consumer_pos, cons_pos);
> @@ -281,19 +288,18 @@ static int64_t ringbuf_process_ring(struct ring *r)
>   */
>  int ring_buffer__consume(struct ring_buffer *rb)
>  {
> -       int64_t err, res = 0;
> +       int64_t err, res = 0, max_items = INT_MAX;

I'm wondering if it might be better to have a convention that zero
means "no limit", which might allow the compiler to eliminate the
condition in ringbuf_process_ring altogether due to constant
propagation. WDYT?

>         int i;
>
>         for (i = 0; i < rb->ring_cnt; i++) {
>                 struct ring *ring = rb->rings[i];
>
> -               err = ringbuf_process_ring(ring);
> +               err = ringbuf_process_ring(ring, max_items);
>                 if (err < 0)
>                         return libbpf_err(err);
>                 res += err;
> +               max_items -= err;
>         }
> -       if (res > INT_MAX)
> -               return INT_MAX;
>         return res;
>  }

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