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

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Oct 26, 2021 at 09:12:31PM -0700, Andrii Nakryiko wrote:
> On Tue, Oct 26, 2021 at 5:03 AM Jiri Olsa <jolsa@redhat.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@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
> >
> 
> 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)

ugh, that's with no fix applied, sry

I applied the first patch and uploaded new files

now when I compare the 'module' struct from vmlinux:

	[885] STRUCT 'module' size=1280 vlen=70

and same one from pnet.ko:

	[89323] STRUCT 'module' size=1280 vlen=70

they seem to completely match, all the fields
and yet it still appears in the kmod's BTF

thanks,
jirka

> 
> 
> > 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
> > > >
> > >
> >
>