Date:   Fri, 1 Oct 2021 16:27:40 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Quentin Monnet <quentin@...valent.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 0/9] install libbpf headers when using the library

On Fri, Oct 1, 2021 at 4:05 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Fri, Oct 1, 2021 at 4:09 AM Quentin Monnet <quentin@...valent.com> wrote:
> >
> > Libbpf is used at several locations in the repository. Most of the time,
> > the tools relying on it build the library in its own directory, and include
> > the headers from there. This works, but this is not the cleanest approach.
> > It generates objects outside of the directory of the tool which is being
> > built, and it also increases the risk that developers include a header file
> > internal to libbpf, which is not supposed to be exposed to user
> > applications.
> >
> > This set adjusts all involved Makefiles to make sure that libbpf is built
> > locally (with respect to the tool's directory or provided build directory),
> > and by ensuring that "make install_headers" is run from libbpf's Makefile
> > to export user headers properly.
> >
> > This comes at a cost: given that the libbpf was so far mostly compiled in
> > its own directory by the different components using it, compiling it once
> > would be enough for all those components. With the new approach, each
> > component compiles its own version. To mitigate this cost, efforts were
> > made to reuse the compiled library when possible:
> >
> > - Make the bpftool version in samples/bpf reuse the library previously
> >   compiled for the selftests.
> > - Make the bpftool version in BPF selftests reuse the library previously
> >   compiled for the selftests.
> > - Similarly, make resolve_btfids in BPF selftests reuse the same compiled
> >   library.
> > - Similarly, make runqslower in BPF selftests reuse the same compiled
> >   library; and make it rely on the bpftool version also compiled from the
> >   selftests (instead of compiling its own version).
> > - runqslower, when compiled independently, needs its own version of
> >   bpftool: make them share the same compiled libbpf.
> >
> > As a result:
> >
> > - Compiling the samples/bpf should compile libbpf just once.
> > - Compiling the BPF selftests should compile libbpf just once.
> > - Compiling the kernel (with BTF support) should now lead to compiling
> >   libbpf twice: one for resolve_btfids, one for kernel/bpf/preload.
> > - Compiling runqslower individually should compile libbpf just once. Same
> >   thing for bpftool, resolve_btfids, and kernel/bpf/preload/iterators.
>
> The whole sharing of libbpf build artifacts is great, I just want to
> point out that it's also dangerous if those multiple Makefiles aren't
> ordered properly. E.g., if you build runqslower and the rest of
> selftests in parallel without making sure that libbpf already
> completed its build, you might end up building libbpf in parallel in
> two independent make instances and subsequently corrupting generated
> object files. I haven't looked through all the changes (and I'll
> confess that it's super hard to reason about dependencies and ordering
> in Makefile) and I'll keep this in mind, but wanted to bring this up.
> I suspect you already thought about that, but would be worth to call
> out this explicitly.
>

Ok, I looked through the patches. It all looks reasonable to me, but
as I mentioned, Makefile is pure magic and human brain can't keep
track of all those dependencies and their ordering. I tried these
changes locally and so far all the builds I usually do work fine. But
it would be great if a few more folks try these changes locally for
their normal workflows to help test this. If not, we can land this
early next week, so that we have a whole week in front of us to fix
whatever fallout there will be due to these changes.

Sounds good?


> >
> > (Not accounting for the boostrap version of libbpf required by bpftool,
> > which was already placed under a dedicated .../boostrap/libbpf/ directory,
> > and for which the count remains unchanged.)
> >
> > A few commits in the series also contain drive-by clean-up changes for
> > bpftool includes, samples/bpf/.gitignore, or test_bpftool_build.sh. Please
> > refer to individual commit logs for details.
> >
> > v2: Declare an additional dependency on libbpf's headers for
> >     iterators/iterators.o in kernel/preload/Makefile to make sure that
> >     these headers are exported before we compile the object file (and not
> >     just before we link it).
> >
> > Quentin Monnet (9):
> >   tools: bpftool: remove unused includes to <bpf/bpf_gen_internal.h>
> >   tools: bpftool: install libbpf headers instead of including the dir
> >   tools: resolve_btfids: install libbpf headers when building
> >   tools: runqslower: install libbpf headers when building
> >   bpf: preload: install libbpf headers when building
> >   bpf: iterators: install libbpf headers when building
> >   samples/bpf: install libbpf headers when building
> >   samples/bpf: update .gitignore
> >   selftests/bpf: better clean up for runqslower in test_bpftool_build.sh
> >
> >  kernel/bpf/preload/Makefile                   | 25 ++++++++++---
> >  kernel/bpf/preload/iterators/Makefile         | 18 ++++++----
> >  samples/bpf/.gitignore                        |  3 ++
> >  samples/bpf/Makefile                          | 36 +++++++++++++------
> >  tools/bpf/bpftool/Makefile                    | 27 ++++++++------
> >  tools/bpf/bpftool/gen.c                       |  1 -
> >  tools/bpf/bpftool/prog.c                      |  1 -
> >  tools/bpf/resolve_btfids/Makefile             | 17 ++++++---
> >  tools/bpf/resolve_btfids/main.c               |  4 +--
> >  tools/bpf/runqslower/Makefile                 | 12 ++++---
> >  tools/testing/selftests/bpf/Makefile          | 22 ++++++++----
> >  .../selftests/bpf/test_bpftool_build.sh       |  4 +++
> >  12 files changed, 116 insertions(+), 54 deletions(-)
> >
> > --
> > 2.30.2
> >

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