[PATCH 06/23] kbuild: only emit vmlinux relocations when required

[Nathan Chancellor]

> 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        9.8s     9.4s    -0.43s (-4%)
>   x86 defconfig, clean, gcc                28.6s    28.1s    -0.47s (-2%)
>   x86 allmodconfig, touch mm/vma.c, gcc    44.0s    42.7s     -1.3s (-3%)
> 
> 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 at kernel.org>
>
> diff --git a/Makefile b/Makefile
> index 9bd226726358..de095b11e20a 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1266,8 +1266,10 @@ LDFLAGS_vmlinux += --orphan-handling=$(CONFIG_LD_ORPHAN_WARN_LEVEL)
>  endif
>  
>  ifneq ($(CONFIG_ARCH_VMLINUX_NEEDS_RELOCS),)
> -LDFLAGS_vmlinux	+= --emit-relocs --discard-none
> +LDFLAGS_vmlinux	+= --discard-none
> +LDFLAGS_vmlinux_relocs := --emit-relocs
>  endif
> +export LDFLAGS_vmlinux_relocs
>  
>  # Align the architecture of userspace programs with the kernel
>  USERFLAGS_FROM_KERNEL := --target=%
> diff --git a/scripts/Makefile.vmlinux b/scripts/Makefile.vmlinux
> index fcae1e432d9a..4b54aaeca65d 100644
> --- a/scripts/Makefile.vmlinux
> +++ b/scripts/Makefile.vmlinux
> @@ -64,7 +64,8 @@ ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(SRCARCH)/Makefile.postlink)
>  
>  # Final link of vmlinux with optional arch pass after final link
>  cmd_link_vmlinux =							\
> -	$< "$(LD)" "$(KBUILD_LDFLAGS)" "$(LDFLAGS_vmlinux)" "$@";	\
> +	$< "$(LD)" "$(KBUILD_LDFLAGS)" "$(LDFLAGS_vmlinux)" "$@"	\
> +	   "$(LDFLAGS_vmlinux_relocs)";					\
>  	$(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true)
>  
>  targets += vmlinux.unstripped .vmlinux.export.o
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index 970ca10f8fa9..09c5222ccb94 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -32,6 +32,7 @@ LD="$1"
>  KBUILD_LDFLAGS="$2"
>  LDFLAGS_vmlinux="$3"
>  VMLINUX="$4"
> +LDFLAGS_vmlinux_relocs="$5"
>  
>  is_enabled() {
>  	grep -q "^$1=y" include/config/auto.conf
> @@ -96,6 +97,11 @@ vmlinux_link()
>  		ldflags="${ldflags} ${wl}--strip-debug"
>  	fi
>  
> +	# Only the final link actually requires the relocations.
> +	if [ "${output}" = "${VMLINUX}" ] && [ -n "${LDFLAGS_vmlinux_relocs}" ]; then
> +		ldflags="${ldflags} ${wl}${LDFLAGS_vmlinux_relocs}"
> +	fi
> +

We don't use LDFLAGS_vmlinux_relocs anywhere else, so do we even need
this whole dance to just end up with the equivalent of

  if [ "${output}" = "${VMLINUX}" ] && is_enabled CONFIG_ARCH_VMLINUX_NEEDS_RELOCS; then
    ldflags="${ldflags} ${wl}--emit-relocs"
  fi

which would make this diff much simpler?

That being said, Sashiko makes a comment that may be relevant, have not
verified though:

  https://sashiko.dev/#/message/20260908-build-speedup-v1-6-5dc1ac01672d@kernel.org

-- 
Cheers,
Nathan