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

[Jiri Olsa <jolsa@redhat.com>]

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@redhat.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

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
> >
>