Re: libbpf: pinning multiple progs from the same section

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Feb 9, 2021 at 3:03 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Gilad Reti <gilad.reti@gmail.com> writes:
>
> >> > I didn't get this last comment. What I meant is that I want something
> >> > like the bpf_object__pin_maps but that doesn't pin the maps, just
> >> > exposing its naming part.
> >>
> >> Right, OK. Why, though? I can kinda see how it could be convenient to
> >> (basically )make libbpf behave as if all maps has the 'pinning'
> >> attribute set, for map reuse. But I'm not sure I can think any concrete
> >> use cases where this would be needed. What's yours?
> >>
> >
> > I am using the same bpf objects (more specifically, the new skeleton
> > feature) in two different processes that need access to the same
> > maps/programs (for example, they both need access to shared maps).
> > Thus, I want to reuse the entire object in both. Since we already have
> > a way to pin an entire bpf object, I thought it would be convenient to
> > have a way of reusing it entirely (though I am fine with pinning and
> > reusing each one manually).
> > (I cannot set the __uint(pinning, LIBBPF_PIN_BY_NAME) on each since I
> > want to share the bss map too)
>
> Ah, see, now *this* could go under the "missing API" header: having a
> way to make libbpf pin (and reuse) the auto-generated maps, like you can
> do with the 'pinning' attribute.
>
> Andrii, WDYT?

I think that the whole pinning handling in libbpf feels a bit ad-hoc.
It would be good for someone to sit and think through this end-to-end.
Unfortunately I never had a need for pinning, so I'm not the best
person to do this.

In this case, you can still do bpf_map__set_pin_path() on internal
maps (.bss, .data, etc), but you'll need to specify pin path
explicitly, which is different from what you get with
LIBBPF_PIN_BY_NAME.

So I think pinning support is not complete, but I'd also like to avoid
adding new APIs in an ad-hoc manner without a holistic view of how
pinning should work with libbpf.

>
> -Toke
>