Re: [PATCH v3 05/20] kbuild: only emit vmlinux relocations when required

[Nicolas Schier]

On Thu, Sep 17, 2026 at 05:06:15PM +0100, Lorenzo Stoakes (ARM) wrote:
> A kernel build consists of more than one linking pass on vmlinux.o and
> vmlinux, at minimum two, and with CONFIG_KALLSYMS and BTF enabled on x86-64
> for example there are 5 such stages.
> 
> For architectures that build their own relocation tables (x86, riscv, mips,
> s390), vmlinux is linked with the --emit-relocs parameter specified.
> 
> However, this is only required on the final vmlinux link.
> 
> Symbol tables of trial links preceding it don't need it because they
> already check that System.map matches kallsyms symbols on each build.
> 
> GNU ld is slow at emitting relocation tables, so this results in a
> reduction in build time.
> 
> Whole build, 128-thread Threadripper 9980X, best of N runs:
> 
>                                          before   after     delta
>                                          -------------------------------
>   x86 defconfig, touch mm/vma.c, gcc        9.5s     9.1s    -0.42s (-4%)
>   x86 defconfig, clean, gcc                30.1s    29.5s    -0.59s (-2%)
>   x86 allmodconfig, touch mm/vma.c, gcc    37.2s    35.8s     -1.4s (-4%)
> 
> Note that this has little impact on LLVM ld which performs this operation
> more efficiently.
> 
> Assisted-by: LLM
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> ---
>  Makefile                | 2 +-
>  scripts/link-vmlinux.sh | 5 +++++
>  2 files changed, 6 insertions(+), 1 deletion(-)
> 
}

Reviewed-by: Nicolas Schier <n.schier@xxxxxxxxx>

-- 
Nicolas