linux-kernel - Re: [PATCH bpf-next] libbpf: Expose libbpf ringbufer epoll

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Date:   Mon, 14 Dec 2020 11:29:36 +0000
From:   Brendan Jackman <jackmanb@...gle.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        KP Singh <kpsingh@...omium.org>,
        Florent Revest <revest@...omium.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] libbpf: Expose libbpf ringbufer epoll_fd

On Fri, Dec 11, 2020 at 11:44:41AM -0800, Andrii Nakryiko wrote:
> On Fri, Dec 11, 2020 at 10:58 AM Brendan Jackman <jackmanb@...gle.com> wrote:
> >
> > This allows the user to do their own manual polling in more
> > complicated setups.
> >
> > Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
> > ---
> 
> perf_buffer has it, so it's good for consistency. In practice, though,
> I'd expect anyone who needs more complicated polling to use ring buf's
> map FD directly on their instance of epoll. Doesn't that work for you?

Yep, thanks - on closer inspection I think that would be a better
eventual solution.  However this API provides a convenient migration
path. I suspect it's a similar situation to what motivated
perf_buffer__epoll_fd in commit dca5612f8eb9d.

> Regardless, though, you need to add the API into libbpf.map file first.

Ack, will send a v2. I guess this falls into Linus description of 'happy
sending it in this upcoming week' for the 5.10 window so I'll put it in
libbpf 0.3.0.

> >  tools/lib/bpf/libbpf.h  | 1 +
> >  tools/lib/bpf/ringbuf.c | 6 ++++++
> >  2 files changed, 7 insertions(+)
> >
> > diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> > index 6909ee81113a..cde07f64771e 100644
> > --- a/tools/lib/bpf/libbpf.h
> > +++ b/tools/lib/bpf/libbpf.h
> > @@ -536,6 +536,7 @@ LIBBPF_API int ring_buffer__add(struct ring_buffer *rb, int map_fd,
> >                                 ring_buffer_sample_fn sample_cb, void *ctx);
> >  LIBBPF_API int ring_buffer__poll(struct ring_buffer *rb, int timeout_ms);
> >  LIBBPF_API int ring_buffer__consume(struct ring_buffer *rb);
> > +LIBBPF_API int ring_buffer__epoll_fd(struct ring_buffer *rb);
> >
> >  /* Perf buffer APIs */
> >  struct perf_buffer;
> > diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c
> > index 5c6522c89af1..45a36648b403 100644
> > --- a/tools/lib/bpf/ringbuf.c
> > +++ b/tools/lib/bpf/ringbuf.c
> > @@ -282,3 +282,9 @@ int ring_buffer__poll(struct ring_buffer *rb, int timeout_ms)
> >         }
> >         return cnt < 0 ? -errno : res;
> >  }
> > +
> > +/* Get an fd that can be used to sleep until data is available in the ring(s) */
> > +int ring_buffer__epoll_fd(struct ring_buffer *rb)
> > +{
> > +       return rb->epoll_fd;
> > +}
> >
> > base-commit: b4fe9fec51ef48011f11c2da4099f0b530449c92
> > --
> > 2.29.2.576.ga3fc446d84-goog
> >