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

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Wed, Sep 09, 2026 at 09:19:12PM -0700, Nathan Chancellor 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        9.8s     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@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?

Nice, big simplification then :) fixed up for v2.

>
> 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

Yeah checked and not relevant :) I cc'd you on my
mostly-for-my-own-record-keeping reply to sashiko on that one :)

>
> --
> Cheers,
> Nathan
>

--
Cheers, Lorenzo