[PATCH v3 0/2] libbpf: API to partially consume items from ringbuffer

[Andrea Righi]

Introduce ring__consume_n() and ring_buffer__consume_n() API to
partially consume items from one (or more) ringbuffer(s).

This can be useful, for example, to consume just a single item or when
we need to copy multiple items to a limited user-space buffer from the
ringbuffer callback.

Practical example (where this API can be used):
https://github.com/sched-ext/scx/blob/b7c06b9ed9f72cad83c31e39e9c4e2cfd8683a55/rust/scx_rustland_core/src/bpf.rs#L217

See also:
https://lore.kernel.org/lkml/20240310154726.734289-1-andrea.righi@xxxxxxxxxxxxx/T/#u

[ Note: I haven't implemented the special case of n == 0 as "no limit",
because we still need to add a bunch of extra checks for INT_MAX, making
the code less readable and, from a performance perspective, it seems
that we may get more downsides than benefits, but I can try to look more
at this if you think it's worth it ]

v3:
 - rename ring__consume_max() -> ring__consume_n() and
   ring_buffer__consume_max() -> ring_buffer__consume_n()
 - add new API to a new 1.5.0 cycle
 - fixed minor nits / comments

v2:
 - introduce a new API instead of changing the callback's retcode
   behavior

Andrea Righi (2):
      libbpf: ringbuf: allow to consume up to a certain amount of items
      libbpf: Add ring__consume_n / ring_buffer__consume_n

 tools/lib/bpf/libbpf.h   | 12 ++++++++++
 tools/lib/bpf/libbpf.map |  6 +++++
 tools/lib/bpf/ringbuf.c  | 59 ++++++++++++++++++++++++++++++++++++++++--------
 3 files changed, 67 insertions(+), 10 deletions(-)