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From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date: Tue, 12 Mar 2024 15:42:41 -0700
Message-ID: <CAEf4Bzb6eA5Sz3hKCVKPZjGzCE1v5aGhieR4rOD_appnNDmBqw@mail.gmail.com>
Subject: Re: [PATCH] libbpf: ringbuf: allow to partially consume items
To: Andrea Righi <andrea.righi@canonical.com>
Cc: Andrii Nakryiko <andrii@kernel.org>, Eduard Zingerman <eddyz87@gmail.com>, 
	Alexei Starovoitov <ast@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>, 
	Martin KaFai Lau <martin.lau@linux.dev>, Song Liu <song@kernel.org>, 
	Yonghong Song <yonghong.song@linux.dev>, John Fastabend <john.fastabend@gmail.com>, 
	KP Singh <kpsingh@kernel.org>, Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>, 
	Jiri Olsa <jolsa@kernel.org>, David Vernet <void@manifault.com>, Tejun Heo <tj@kernel.org>, 
	bpf@vger.kernel.org, linux-kernel@vger.kernel.org
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On Mon, Mar 11, 2024 at 1:25=E2=80=AFPM Andrea Righi <andrea.righi@canonica=
l.com> wrote:
>
> On Mon, Mar 11, 2024 at 10:55:57AM -0700, Andrii Nakryiko wrote:
> > On Sun, Mar 10, 2024 at 8:47=E2=80=AFAM Andrea Righi <andrea.righi@cano=
nical.com> wrote:
> > >
> > > Instead of always consuming all items from a ring buffer in a greedy
> > > way, allow to stop when the callback returns a value > 0.
> > >
> > > This allows to distinguish between an error condition and an intentio=
nal
> > > stop condition by returning a non-negative non-zero value from the ri=
ng
> > > buffer callback.
> > >
> > > This can be useful, for example, to consume just a single item from t=
he
> > > ring buffer.
> > >
> > > Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
> > > ---
> > >  tools/lib/bpf/ringbuf.c | 8 ++++++++
> > >  1 file changed, 8 insertions(+)
> > >
> > > diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c
> > > index aacb64278a01..dd8908eb3204 100644
> > > --- a/tools/lib/bpf/ringbuf.c
> > > +++ b/tools/lib/bpf/ringbuf.c
> > > @@ -265,6 +265,14 @@ static int64_t ringbuf_process_ring(struct ring =
*r)
> > >                                         return err;
> > >                                 }
> > >                                 cnt++;
> > > +                               if (err > 0) {
> >
> > So libbpf already stops at any err < 0 (and sets correct consumer
> > pos). So you could already get desired behavior by just returning your
> > own error code. If you need count, you'd have to count it yourself
> > through custom context, that's a bit of inconvenience.
>
> Yep, that's exactly what I'm doing right now.
>
> To give more context, here's the code:
> https://github.com/sched-ext/scx/blob/b7c06b9ed9f72cad83c31e39e9c4e2cfd86=
83a55/rust/scx_rustland_core/src/bpf.rs#L217
>

cool, great that you at least have a work-around


> >
> > But on the other hand, currently if user callback returns anything > 0
> > they keep going and that return value is ignored. Your change will
> > break any such user pretty badly. So I'm a bit hesitant to do this.
>
> So, returning a value > 0 should have the same behavior as returning 0?

yes, that's what the contract is right now

> Why any user callback would return > 0 then?

this is not the code I can control and ringbuf API was like that for a
long time, which is why I'm saying I'm hesitant to make these changes

>
> >
> > Is there any reason you can't just return error code (libbpf doesn't
> > do anything with it, just passes it back, so it might as well be
> > `-cnt`, if you need that).
>
> Sure, I can keep using my special error code to stop. It won't be a
> problem for my particular use case.
>
> Actually, one thing that it would be nice to have is a way to consume up
> to a certain amount of items, let's say I need to copy multiple items
> from the ring buffer to a limited user buffer. But that would require a
> new API I guess, in order to pass the max counter... right?

Yes, definitely a new API, but that's not a big problem. Though I'm
wondering if ring_buffer__consume_one() would be a more flexible API,
where user would have more flexible control. Either way libbpf is
doing smp_store_release() after each consumed element, so I don't
think it will have any downsides in terms of performance.

So please consider contributing a patch for this new API.

>
> Thanks,
> -Andrea
>
> >
> > pw-bot: cr
> >
> > > +                                       /* update consumer pos and re=
turn the
> > > +                                        * total amount of items cons=
umed.
> > > +                                        */
> > > +                                       smp_store_release(r->consumer=
_pos,
> > > +                                                         cons_pos);
> > > +                                       goto done;
> > > +                               }
> > >                         }
> > >
> > >                         smp_store_release(r->consumer_pos, cons_pos);
> > > --
> > > 2.43.0
> > >
> > >