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

[Pedro Tammela]

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?