Re: [PATCH bpf-next] libbpf: clarify flags in ringbuf helpers

[Andrii Nakryiko]

On Wed, Apr 7, 2021 at 1:10 PM Pedro Tammela <pctammela@xxxxxxxxx> wrote:
>
> Em qua., 7 de abr. de 2021 às 16:58, Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> escreveu:
> >
> > On Wed, Apr 7, 2021 at 11:43 AM Joe Stringer <joe@xxxxxxxxx> wrote:
> > >
> > > Hi Pedro,
> > >
> > > On Tue, Apr 6, 2021 at 11:58 AM Pedro Tammela <pctammela@xxxxxxxxx> wrote:
> > > >
> > > > In 'bpf_ringbuf_reserve()' we require the flag to '0' at the moment.
> > > >
> > > > For 'bpf_ringbuf_{discard,submit,output}' a flag of '0' might send a
> > > > notification to the process if needed.
> > > >
> > > > Signed-off-by: Pedro Tammela <pctammela@xxxxxxxxxxxx>
> > > > ---
> > > >  include/uapi/linux/bpf.h       | 7 +++++++
> > > >  tools/include/uapi/linux/bpf.h | 7 +++++++
> > > >  2 files changed, 14 insertions(+)
> > > >
> > > > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > > > index 49371eba98ba..8c5c7a893b87 100644
> > > > --- a/include/uapi/linux/bpf.h
> > > > +++ b/include/uapi/linux/bpf.h
> > > > @@ -4061,12 +4061,15 @@ union bpf_attr {
> > > >   *             of new data availability is sent.
> > > >   *             If **BPF_RB_FORCE_WAKEUP** is specified in *flags*, notification
> > > >   *             of new data availability is sent unconditionally.
> > > > + *             If **0** is specified in *flags*, notification
> > > > + *             of new data availability is sent if needed.
> > >
> > > Maybe a trivial question, but what does "if needed" mean? Does that
> > > mean "when the buffer is full"?
> >
> > I used to call it ns "adaptive notification", so maybe let's use that
> > term instead of "if needed"? It means that in kernel BPF ringbuf code
> > will check if the user-space consumer has caught up and consumed all
> > the available data. In that case user-space might be waiting
> > (sleeping) in epoll_wait() already and not processing samples
> > actively. That means that we have to send notification, otherwise
> > user-space might never wake up. But if the kernel sees that user-space
> > is still processing previous record (consumer position < producer
> > position), then we can bypass sending another notification, because
> > user-space consumer protocol dictates that it needs to consume all the
> > record until consumer position == producer position. So no
> > notification is necessary for the newly submitted sample, as
> > user-space will eventually see it without notification.
> >
> > Of course there is careful writes and memory ordering involved to make
> > sure that we never miss notification.
> >
> > Does someone want to try to condense it into a succinct description? ;)
>
> OK.
>
> I can try to condense this and perhaps add it as code in the comment?

Sure, though there is already a brief comment to that effect. But
having high-level explanation in uapi/linux/bpf.h would be great for
users, though.