Re: libbpf: pinning multiple progs from the same section

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Gilad Reti <gilad.reti@gmail.com> writes:

> On Mon, Feb 8, 2021 at 5:09 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>>
>> Gilad Reti <gilad.reti@gmail.com> writes:
>>
>> > On Mon, Feb 8, 2021 at 4:28 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>> >>
>> >> Gilad Reti <gilad.reti@gmail.com> writes:
>> >>
>> >> > On Mon, Feb 8, 2021 at 1:42 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>> >> >>
>> >> >> Gilad Reti <gilad.reti@gmail.com> writes:
>> >> >>
>> >> >> > Also, is there a way to set the pin path to all maps/programs at once?
>> >> >> > For example, bpf_object__pin_maps pins all maps at a specific path,
>> >> >> > but as far as I was able to find there is no similar function to set
>> >> >> > the pin path for all maps only (without pinning) so that at loading
>> >> >> > time libbpf will try to reuse all maps. The only way to achieve a
>> >> >> > complete reuse of all maps that I could find is to "reverse engineer"
>> >> >> > libbpf's pin path generation algorithm (i.e. <path>/<map_name>) and
>> >> >> > set the pin path on each map before load.
>> >> >>
>> >> >> You can set the 'pinning' attribute in the map definition - add
>> >> >> '__uint(pinning, LIBBPF_PIN_BY_NAME);' to the map struct. By default
>> >> >> this will pin beneath /sys/fs/bpf, but you can customise that by setting
>> >> >> the pin_root_path attribute in bpf_object_open_opts.
>> >> >
>> >> > Yes, I am familiar with that feature, but it has some downsides:
>> >> > 1. I need to set it manually on every map (and in cases that I have
>> >> > only the compiled object file that would be hard).
>> >> > 2. It only works for bpf maps and not bpf programs.
>> >> > 3. It only works for bpf maps that are defined explicitly in the bpf
>> >> > code and not for implicit (inner) bpf maps (bss, rodata, etc).
>> >>
>> >> Ah, right. Well, other than that I don't think there's a way to set pin
>> >> paths in bulk, other than by manually iterating and setting them one at
>> >> a time. But, erm, can't you just do that? :)
>> >>
>> >
>> > Sure, I can, but I think we should avoid that. As I said this forces
>> > the user to know libbpf's pin path naming algorithm, which is not part
>> > of the libbpf api afaik.
>>
>> Why? If you set the pin path from your application, libbpf will also try
>> to reuse the map from that path. So you don't need to know libbpf's
>> algorithm if you just override it with your own paths?
>>
>
> If I do bpf_object__pin_maps then libbpf decides where it wants to pin
> them. I can set each path by my own, but then why do we need this
> function?

Erm, what do you mean, "libbpf decides". bpf_object__pin_maps(obj, path)
does exactly what you're asking for: If you supply the path, all maps
are going to be pinned by name underneath that directory...

> All It does is pinning all the maps at paths that are not part of the
> api. In this libbpf version it is here, in the next it is there, and
> user code will need to change accordingly.

Why would you assume the paths would change? That would be an API break?

> For example, libbpf today uses <path>/<map_name> as the pin path, but
> it is also doing sanitize_pin_path on each path. This means that after
> if use bpf_object__pin_maps I also need to know how libbpf sanitizes
> its paths and mimic that behavior on my side.

The paths are sanitised so the kernel will accept them. If you're using
invalid paths your pinning is not going to work at all. If you just want
the paths that the maps are pinned under, use bpf_map__get_pin_path().

>> > I think that if we have a method to pin all maps at a specific path
>> > there should also be a method for reusing them all from this path,
>> > either by exposing the function that builds the pin path, or a
>> > function that sets all the paths from a root path.
>>
>> What you're asking for is basically a function
>> bpf_object__set_all_pin_paths(obj, path)
>>
>> instead of having to do
>>
>> bpf_object__for_each_map(map, obj) {
>>   sprintf(path, "path/%s", bpf_map__name(map));
>>   bpf_map__set_pin_path(map, path);
>> }
>>
>> or? Is that really needed?
>>
>
> Yes, that is what I am asking for. Either that or a
> bpf_map__build_pin_path(path, map) That will return a pin path that is
> compatible with libbpf's one, and then I can iterate over all maps.

See above; this is what bpf_object__pin_maps() does today?

-Toke