linux-kernel - Re: [PATCH 0/3] perf/bpftool: Allow to link libbpf dynamically

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Date:   Wed, 27 Nov 2019 12:24:19 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Michael Petlan <mpetlan@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>
Subject: Re: [PATCH 0/3] perf/bpftool: Allow to link libbpf dynamically

On Wed, Nov 27, 2019 at 8:38 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Wed, Nov 27, 2019 at 1:48 AM Jiri Olsa <jolsa@...nel.org> wrote:
> >
> > hi,
> > adding support to link bpftool with libbpf dynamically,
> > and config change for perf.
> >
> > It's now possible to use:
> >   $ make -C tools/bpf/bpftool/ LIBBPF_DYNAMIC=1
> >
> > which will detect libbpf devel package with needed version,
> > and if found, link it with bpftool.
> >
> > It's possible to use arbitrary installed libbpf:
> >   $ make -C tools/bpf/bpftool/ LIBBPF_DYNAMIC=1 LIBBPF_DIR=/tmp/libbpf/
> >
> > I based this change on top of Arnaldo's perf/core, because
> > it contains libbpf feature detection code as dependency.
> > It's now also synced with latest bpf-next, so Toke's change
> > applies correctly.
>
> I don't like it.
> Especially Toke's patch to expose netlink as public and stable libbpf api.
> bpftools needs to stay tightly coupled with libbpf (and statically
> linked for that reason).
> Otherwise libbpf will grow a ton of public api that would have to be stable
> and will quickly become a burden.

I second that. I'm currently working on adding few more APIs that I'd
like to keep unstable for a while, until we have enough real-world
usage (and feedback) accumulated, before we stabilize them. With
LIBBPF_API and a promise of stable API, we are going to over-stress
and over-design APIs, potentially making them either too generic and
bloated, or too limited (and thus become deprecated almost at
inception time). I'd like to take that pressure off for a super-new
and in flux APIs and not hamper the progress.

I'm thinking of splitting off those non-stable, sort-of-internal APIs
into separate libbpf-experimental.h (or whatever name makes sense),
and let those be used only by tools like bpftool, which are only ever
statically link against libbpf and are ok with occasional changes to
those APIs (which we'll obviously fix in bpftool as well). Pahole
seems like another candidate that fits this bill and we might expose
some stuff early on to it, if it provides tangible benefits (e.g., BTF
dedup speeds ups, etc).

Then as APIs mature, we might decide to move them into libbpf.h with
LIBBPF_API slapped onto them. Any objections?