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

[Jiri Olsa <jolsa@redhat.com>]

On Wed, Oct 27, 2021 at 10:53:55AM -0700, Andrii Nakryiko wrote:
> 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).

it failed for me on that hypot_type_id check where I did fix,
I thought it's the issue of multiple same struct in the kmod,
but now I see I might have confused cannon_id with cand_id ;-)
I'll check more on this

jirka