linux-kernel - Re: [PATCH] kbuild: reuse vmlinux.o in vmlinux

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Date:   Sat, 23 May 2020 20:06:30 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>
Subject: Re: [PATCH] kbuild: reuse vmlinux.o in vmlinux_link

Excerpts from Masahiro Yamada's message of May 23, 2020 3:44 am:
> + Michael, and PPC ML.
> 
> They may know something about the reason of failure.

Because the linker can't put branch stubs within object code sections, 
so when you incrementally link them too large, the linker can't resolve 
branches into other object files.

This is why we added incremental linking in the first place. I suppose 
it could be made conditional for platforms that can use this 
optimization.

What'd be really nice is if we could somehow build and link kallsyms 
without relinking everything twice, and if we could do section mismatch 
analysis without making that vmlinux.o as well. I had a few ideas but 
not enough time to do much work on it.

Thanks,
Nick

> 
> 
> On Sat, May 23, 2020 at 2:41 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
>>
>> On Fri, May 22, 2020 at 5:27 AM Sami Tolvanen <samitolvanen@...gle.com> wrote:
>> >
>> > Instead of linking all compilation units again each time vmlinux_link is
>> > called, reuse vmlinux.o from modpost_link.
>> >
>> > With x86_64 allyesconfig, vmlinux_link is called three times and reusing
>> > vmlinux.o reduces the build time ~38 seconds on my system (59% reduction
>> > in the time spent in vmlinux_link).
>> >
>> > Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
>> > ---
>> >  scripts/link-vmlinux.sh | 5 +----
>> >  1 file changed, 1 insertion(+), 4 deletions(-)
>> >
>> > diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
>> > index d09ab4afbda4..c6cc4305950c 100755
>> > --- a/scripts/link-vmlinux.sh
>> > +++ b/scripts/link-vmlinux.sh
>> > @@ -77,11 +77,8 @@ vmlinux_link()
>> >
>> >         if [ "${SRCARCH}" != "um" ]; then
>> >                 objects="--whole-archive                        \
>> > -                       ${KBUILD_VMLINUX_OBJS}                  \
>> > +                       vmlinux.o                               \
>> >                         --no-whole-archive                      \
>> > -                       --start-group                           \
>> > -                       ${KBUILD_VMLINUX_LIBS}                  \
>> > -                       --end-group                             \
>> >                         ${@}"
>> >
>> >                 ${LD} ${KBUILD_LDFLAGS} ${LDFLAGS_vmlinux}      \
>> >
>> > base-commit: b85051e755b0e9d6dd8f17ef1da083851b83287d
>> > --
>> > 2.27.0.rc0.183.gde8f92d652-goog
>> >
>>
>>
>> I like this patch irrespective of CLANG_LTO, but
>> unfortunately, my build test failed.
>>
>>
>> ARCH=powerpc failed to build as follows:
>>
>>
>>
>>   MODPOST vmlinux.o
>>   MODINFO modules.builtin.modinfo
>>   GEN     modules.builtin
>>   LD      .tmp_vmlinux.kallsyms1
>> vmlinux.o:(__ftr_alt_97+0x20): relocation truncated to fit:
>> R_PPC64_REL14 against `.text'+4b1c
>> vmlinux.o:(__ftr_alt_97+0x164): relocation truncated to fit:
>> R_PPC64_REL14 against `.text'+1cf78
>> vmlinux.o:(__ftr_alt_97+0x288): relocation truncated to fit:
>> R_PPC64_REL14 against `.text'+1dac4
>> vmlinux.o:(__ftr_alt_97+0x2f0): relocation truncated to fit:
>> R_PPC64_REL14 against `.text'+1e254
>> make: *** [Makefile:1125: vmlinux] Error 1
>>
>>
>>
>> I used powerpc-linux-gcc
>> available at
>> https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/9.2.0/
>>
>>
>> Build command:
>>
>> make -j24 ARCH=powerpc  CROSS_COMPILE=powerpc-linux-  defconfig all
>>
>>
>> Could you check it please?
>>
>>
>>
>> I will apply it to my test branch.
>> Perhaps, 0-day bot may find more failure cases.
>>
>>
>> --
>> Best Regards
>> Masahiro Yamada
> 
> 
> 
> -- 
> Best Regards
> Masahiro Yamada
>