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

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Date: Sat, 6 Apr 2024 10:13:37 +0200
From: Andrea Righi <andrea.righi@...onical.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Andrii Nakryiko <andrii@...nel.org>,
	Eduard Zingerman <eddyz87@...il.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>,
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	David Vernet <void@...ifault.com>, Tejun Heo <tj@...nel.org>,
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/2] libbpf: API to partially consume items from
 ringbuffer

On Fri, Apr 05, 2024 at 04:16:34PM -0700, Andrii Nakryiko wrote:
> On Fri, Apr 5, 2024 at 3:36 PM Andrea Righi <andrea.righi@...onical.com> wrote:
> >
> > 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@canonical.com/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
> 
> You also need to update Makefile, CI is failing right now ([0]).

Yep, I wasn't sure if should have bumped up the version as well
(in libbpf_version.h). I'll add that.

> 
> Please also add a simple test to BPF selftests using the new API.
> Other than that the changes look good.

Ok, will add a selftest.

Thanks,
-Andrea

> 
>   [0] https://github.com/kernel-patches/bpf/actions/runs/8576515495/job/23507610313#step:11:73
> 
> pw-bot: cr
> 
> >  - 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(-)