netdev - Re: [RFC bpf-next 0/2] bpf: Fix BTF data for modules

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Date:   Tue, 26 Oct 2021 21:12:31 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <jolsa@...hat.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>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [RFC bpf-next 0/2] bpf: Fix BTF data for modules

On Tue, Oct 26, 2021 at 5:03 AM Jiri Olsa <jolsa@...hat.com> wrote:
>
> On Mon, Oct 25, 2021 at 09:54:48PM -0700, Andrii Nakryiko wrote:
> > On Sat, Oct 23, 2021 at 5:05 AM Jiri Olsa <jolsa@...hat.com> wrote:
> > >
> > > hi,
> > > I'm trying to enable BTF for kernel module in fedora,
> > > and I'm getting big increase on modules sizes on s390x arch.
> > >
> > > Size of modules in total - kernel dir under /lib/modules/VER/
> > > from kernel-core and kernel-module packages:
> > >
> > >                current   new
> > >       aarch64      60M   76M
> > >       ppc64le      53M   66M
> > >       s390x        21M   41M
> > >       x86_64       64M   79M
> > >
> > > The reason for higher increase on s390x was that dedup algorithm
> > > did not detect some of the big kernel structs like 'struct module',
> > > so they are duplicated in the kernel module BTF data. The s390x
> > > has many small modules that increased significantly in size because
> > > of that even after compression.
> > >
> > > First issues was that the '--btf_gen_floats' option is not passed
> > > to pahole for kernel module BTF generation.
> > >
> > > The other problem is more tricky and is the reason why this patchset
> > > is RFC ;-)
> > >
> > > The s390x compiler generates multiple definitions of the same struct
> > > and dedup algorithm does not seem to handle this at the moment.
> > >
> > > I put the debuginfo and btf dump of the s390x pnet.ko module in here:
> > >   http://people.redhat.com/~jolsa/kmodbtf/
> > >
> > > Please let me know if you'd like to see other info/files.
> > >
> >
> > Hard to tell what's going on without vmlinux itself. Can you upload a
> > corresponding kernel image with BTF in it?
>
> sure, uploaded
>

vmlinux.btfdump:

[174] FLOAT 'float' size=4
[175] FLOAT 'double' size=8

VS

pnet.btfdump:

[89318] INT 'float' size=4 bits_offset=0 nr_bits=32 encoding=(none)
[89319] INT 'double' size=8 bits_offset=0 nr_bits=64 encoding=(none)


> jirka
>
> >
> > > I found code in dedup that seems to handle such situation for arrays,
> > > and added 'some' fix for structs. With that change I can no longer
> > > see vmlinux's structs in kernel module BTF data, but I have no idea
> > > if that breaks anything else.
> > >
> > > thoughts? thanks,
> > > jirka
> > >
> > >
> > > ---
> > > Jiri Olsa (2):
> > >       kbuild: Unify options for BTF generation for vmlinux and modules
> > >       bpf: Add support to detect and dedup instances of same structs
> > >
> > >  Makefile                  |  3 +++
> > >  scripts/Makefile.modfinal |  2 +-
> > >  scripts/link-vmlinux.sh   | 11 +----------
> > >  scripts/pahole-flags.sh   | 20 ++++++++++++++++++++
> > >  tools/lib/bpf/btf.c       | 12 ++++++++++--
> > >  5 files changed, 35 insertions(+), 13 deletions(-)
> > >  create mode 100755 scripts/pahole-flags.sh
> > >
> >
>