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

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Message-ID: <CAEf4BzbaD60KFsUB4VkTAH2v3+GFkRvRbY_O-bNSpNG0=8pJ0Q@mail.gmail.com>
Date:   Mon, 25 Oct 2021 21:54:48 -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 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?

> 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
>