Re: [PATCH bpf-next] bpf: warn against BPF_RB_NO_WAKEUP in bpf_ringbuf_discard()

[Andrii Nakryiko]

On Mon, Mar 30, 2026 at 5:18 AM Eyal Birger <eyal.birger@xxxxxxxxx> wrote:
>
> Document that BPF_RB_NO_WAKEUP is not recommended for
> bpf_ringbuf_discard().
>
> A discard done with BPF_RB_NO_WAKEUP can suppress a later adaptive
> wakeup from a valid record, leaving an epoll-based userspace consumer
> asleep even though data is available in the ring buffer.
>
> Scenario:
>
> epoll_wait(rb_fd);                     // blocks
>
> rec = bpf_ringbuf_reserve(&rb, ...);
> bpf_ringbuf_discard(rec, BPF_RB_NO_WAKEUP);
>
> rec = bpf_ringbuf_reserve(&rb, ...);
> bpf_ringbuf_submit(rec, 0);           // valid record, but no wakeup
>
> This behavior is surprising in the context of bpf_ringbuf_discard()
> as it seems natural not to want to wake userspace.

why? discarded record still takes space in ringbuf and needs to be
consumed by user-space. So all the same concerns apply here whether
it's discarded or submitted ringbuf record.

>
> Reported-by: Shmulik Ladkani <shmulik.ladkani@xxxxxxxxx>
> Signed-off-by: Eyal Birger <eyal.birger@xxxxxxxxx>
> ---
>  include/uapi/linux/bpf.h       | 3 ++-
>  tools/include/uapi/linux/bpf.h | 3 ++-
>  2 files changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index c8d400b7680a..c46b06d45904 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -4645,7 +4645,8 @@ union bpf_attr {
>   *     Description
>   *             Discard reserved ring buffer sample, pointed to by *data*.
>   *             If **BPF_RB_NO_WAKEUP** is specified in *flags*, no notification
> - *             of new data availability is sent.
> + *             of new data availability is sent, which is not recommended as
> + *             it can suppress a later adaptive wakeup from a subsequent submit.

NAK, it's not "not recommended". If you are using NO_WAKEUP you
normally would use it with FORCE_WAKEUP as well. It's not a rule, but
that's the most practical combination. But if you are fancy and know
what you are doing, you are free to combine NO_WAKEUP, FORCE_WAKEUP
and "adaptive wakeup" in any form or shape you want.


If anything, instead, it might maybe help to emphasize that
bpf_ringbuf_discard() does live data in the ringbuf that needs to be
consumed by user-space code (normally transparently in libbpf or other
BPF libraries), and the only distinction between submitted and
discarded record is whether application's user code sees that record
or it's silently skipped.

>   *             If **BPF_RB_FORCE_WAKEUP** is specified in *flags*, notification
>   *             of new data availability is sent unconditionally.
>   *             If **0** is specified in *flags*, an adaptive notification
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index 5e38b4887de6..96de37c3b896 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -4645,7 +4645,8 @@ union bpf_attr {
>   *     Description
>   *             Discard reserved ring buffer sample, pointed to by *data*.
>   *             If **BPF_RB_NO_WAKEUP** is specified in *flags*, no notification
> - *             of new data availability is sent.
> + *             of new data availability is sent, which is not recommended as
> + *             it can suppress a later adaptive wakeup from a subsequent submit.
>   *             If **BPF_RB_FORCE_WAKEUP** is specified in *flags*, notification
>   *             of new data availability is sent unconditionally.
>   *             If **0** is specified in *flags*, an adaptive notification
> --
> 2.43.0
>