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

From: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: "Toke Høiland-Jørgensen" <toke@redhat.com>,
	"Jiri Olsa" <jolsa@kernel.org>,
	lkml <linux-kernel@vger.kernel.org>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	"Ingo Molnar" <mingo@kernel.org>,
	"Namhyung Kim" <namhyung@kernel.org>,
	"Alexander Shishkin" <alexander.shishkin@linux.intel.com>,
	"Peter Zijlstra" <a.p.zijlstra@chello.nl>,
	"Michael Petlan" <mpetlan@redhat.com>,
	"Jesper Dangaard Brouer" <brouer@redhat.com>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Alexei Starovoitov" <alexei.starovoitov@gmail.com>,
	"Martin KaFai Lau" <kafai@fb.com>,
	"Song Liu" <songliubraving@fb.com>, "Yonghong Song" <yhs@fb.com>,
	"Andrii Nakryiko" <andriin@fb.com>
Subject: Re: [PATCH 0/3] perf/bpftool: Allow to link libbpf dynamically
Date: Mon, 2 Dec 2019 15:54:34 -0300	[thread overview]
Message-ID: <20191202185434.GG4063@kernel.org> (raw)
In-Reply-To: <CAEf4BzYoJUttk=o+p=NHK8K_aS3z2LdLiqzRni7PwyDaOxu68A@mail.gmail.com>

Em Mon, Dec 02, 2019 at 10:42:53AM -0800, Andrii Nakryiko escreveu:
> On Mon, Dec 2, 2019 at 10:09 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> > Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:
> > > On Wed, Nov 27, 2019 at 1:49 AM Jiri Olsa <jolsa@kernel.org> wrote:
> > >> 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

> > > I wonder what's the motivation behind these changes, though? Why is
> > > linking bpftool dynamically with libbpf is necessary and important?
> > > They are both developed tightly within kernel repo, so I fail to see
> > > what are the huge advantages one can get from linking them
> > > dynamically.

> > Well, all the regular reasons for using dynamic linking (memory usage,
> > binary size, etc).

> bpftool is 327KB with statically linked libbpf. Hardly a huge problem
> for either binary size or memory usage. CPU instruction cache usage is
> also hardly a concern for bpftool specifically.

> > But in particular, the ability to update the libbpf
> > package if there's a serious bug, and have that be picked up by all
> > utilities making use of it.

> I agree, and that works only for utilities linking with libbpf
> dynamically. For tools that build statically, you'd have to update
> tools anyways. And if you can update libbpf, you can as well update
> bpftool at the same time, so I don't think linking bpftool statically
> with libbpf causes any new problems.

> > No reason why bpftool should be special in that respect.

> But I think bpftool is special and we actually want it to be special
> and tightly coupled to libbpf with sometimes very intimate knowledge
> of libbpf and access to "hidden" APIs. That allows us to experiment
> with new stuff that requires use of bpftool (e.g., code generation for
> BPF programs), without having to expose and seal public APIs. And I
> don't think it's a problem from the point of code maintenance, because
> both live in the same repository and are updated "atomically" when new
> features are added or changed.

> Beyond superficial binary size worries, I don't see any good reason
> why we should add more complexity and variables to libbpf and bpftool
> build processes just to have a "nice to have" option of linking
> bpftool dynamically with libbpf.

s/bpftool/perf/g
s/libbpf/libperf/g

And I would also agree 8-)

- Arnaldo