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

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Jiri Olsa <jolsa@redhat.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@chromium.org>
Subject: Re: [RFC bpf-next 0/2] bpf: Fix BTF data for modules
Date: Wed, 27 Oct 2021 10:53:55 -0700	[thread overview]
Message-ID: <CAEf4BzYP8eK0qxF+1UK7=TZ+vFRVMfmnm9AN=B2JHROoDwaHeg@mail.gmail.com> (raw)
In-Reply-To: <YXkTihiRKKJIc9M6@krava>

On Wed, Oct 27, 2021 at 1:53 AM Jiri Olsa <jolsa@redhat.com> wrote:
>
> 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
>

Ok, now struct module is identical down to the types referenced from
the fields, which means it should have been deduplicated completely.
This will require a more time-consuming debugging, though, so I'll put
it on my TODO list for now. If you get to this earlier, see where the
equivalence check fails in btf_dedup (sprinkle debug outputs around to
see what's going on).

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