[PATCH 0/3] kbuild: reduce single-build dependencies

[PATCH 0/3] kbuild: reduce single-build dependencies

[Vincent Mailhol]

Building a single .o file depends on the prepare target which contains a
lot of dependencies. Some of those preparation dependencies are not
needed for a single .o build.

This is a small annoyance because a modification of the compilation
flags causes several prepare dependencies to rebuild. Consequently,
single-build incurs an artificial penalty.

Move the generic preparation checks and scripts/mod out of the
single-build preparation path. The first patch is a preparation change
to make rq-offsets-file an explicit prepare dependency. The second
addresses missing-syscalls check and atomic-checks. The third and last
patch addresses scripts/mod.

== Illustration ==

These are the build steps triggered when changing the compilation
flags (here: adding W=1).

Before this series...

  $ time make W=1 init/main.o
    CC      scripts/mod/empty.o
    MKELF   scripts/mod/elfconfig.h
    HOSTCC  scripts/mod/modpost.o
    CC      scripts/mod/devicetable-offsets.s
    HOSTCC  scripts/mod/file2alias.o
    HOSTCC  scripts/mod/sumversion.o
    HOSTCC  scripts/mod/symsearch.o
    HOSTLD  scripts/mod/modpost
    CC      kernel/bounds.s
    CC      arch/x86/kernel/asm-offsets.s
    CC      kernel/sched/rq-offsets.s
    CALL    scripts/checksyscalls.sh 
    DESCEND objtool
    CC      init/main.o
  
  real	0m3.603s
  user	0m2.839s
  sys	0m0.959s

... and after:

  $ time make W=1 init/main.o
    CC      kernel/bounds.s
    CC      arch/x86/kernel/asm-offsets.s
    CC      kernel/sched/rq-offsets.s
    DESCEND objtool
    CC      init/main.o
  
  real	0m2.060s
  user	0m1.505s
  sys	0m0.732s

(done on an x86_64 cpu).

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
Vincent Mailhol (3):
      kbuild: make rq-offsets-file an explicit preparation dependency
      kbuild: move preparation checks out of single-build
      kbuild: move scripts/mod target out of single-build

 Kbuild   |  7 +++++--
 Makefile | 11 +++++++----
 2 files changed, 12 insertions(+), 6 deletions(-)
---
base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
change-id: 20261111-reduce-single-build-dependencies-966646f25a0c

Best regards,
--  
Vincent Mailhol <mailhol@kernel.org>

[PATCH 1/3] kbuild: make rq-offsets-file an explicit preparation dependency

[Vincent Mailhol]

The generated rq-offsets.h header is included by linux/sched.h and must
exist before compiling ordinary objects. Currently that header is
prepared indirectly through the missing-syscalls target.

Make prepare depend directly on rq-offsets-file. Because rq-offsets-file
depends on offsets-file, that other dependency can be removed.

Keep the rq-offsets-file dependency in missing-syscalls so that the
check can still run independently.

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 Kbuild | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Kbuild b/Kbuild
index a6a0192dea08..5a65be2d51b0 100644
--- a/Kbuild
+++ b/Kbuild
@@ -87,7 +87,7 @@ $(atomic-checks): $(obj)/.checked-%: include/linux/atomic/%  FORCE
 # A phony target that depends on all the preparation targets
 
 PHONY += prepare
-prepare: $(offsets-file) missing-syscalls $(atomic-checks)
+prepare: $(rq-offsets-file) missing-syscalls $(atomic-checks)
 	@:
 
 # Ordinary directory descending

-- 
2.56.1

[PATCH 2/3] kbuild: move preparation checks out of single-build

[Vincent Mailhol]

The missing-syscalls check and the atomic-checks are currently set as
preparation dependencies. However, these are not needed to compile, for
example, a single object.

Add a new prepare_checks target to group those checks and call it
conditionally only if we are not doing a single-build so that the
prepare target does not trigger that dependency anymore.

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 Kbuild   | 7 +++++--
 Makefile | 2 +-
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/Kbuild b/Kbuild
index 5a65be2d51b0..5151e32985c6 100644
--- a/Kbuild
+++ b/Kbuild
@@ -86,8 +86,11 @@ $(atomic-checks): $(obj)/.checked-%: include/linux/atomic/%  FORCE
 
 # A phony target that depends on all the preparation targets
 
-PHONY += prepare
-prepare: $(rq-offsets-file) missing-syscalls $(atomic-checks)
+PHONY += prepare prepare_checks
+prepare: $(rq-offsets-file)
+	@:
+
+prepare_checks: missing-syscalls $(atomic-checks)
 	@:
 
 # Ordinary directory descending
diff --git a/Makefile b/Makefile
index f895752d4b05..2c1289a44c56 100644
--- a/Makefile
+++ b/Makefile
@@ -1413,8 +1413,8 @@ archprepare: outputmakefile archheaders archscripts scripts include/config/kerne
 	include/generated/rustc_cfg remove-stale-files
 
 prepare0: archprepare
+	$(Q)$(MAKE) $(build)=. prepare $(if $(single-build),,prepare_checks)
 	$(Q)$(MAKE) $(build)=scripts/mod
-	$(Q)$(MAKE) $(build)=. prepare
 
 # All the preparing..
 prepare: prepare0

-- 
2.56.1

[PATCH 3/3] kbuild: move scripts/mod target out of single-build

[Vincent Mailhol]

The scripts/mod target is currently set as a preparation dependency.
However, this is not needed to compile, for example, a single object.

Add a new modpost_prepare target for scripts/mod and make in-tree
modpost and modules_prepare depend on it so that the prepare target does
not trigger that dependency anymore.

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 Makefile | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/Makefile b/Makefile
index 2c1289a44c56..93236f310f08 100644
--- a/Makefile
+++ b/Makefile
@@ -1405,7 +1405,7 @@ scripts: scripts_basic scripts_dtc
 # archprepare is used in arch Makefiles and when processed asm symlink,
 # version.h and scripts_basic is processed / created.
 
-PHONY += prepare archprepare
+PHONY += prepare archprepare modpost_prepare
 
 archprepare: outputmakefile archheaders archscripts scripts include/config/kernel.release \
 	asm-generic $(version_h) include/generated/utsrelease.h \
@@ -1414,6 +1414,8 @@ archprepare: outputmakefile archheaders archscripts scripts include/config/kerne
 
 prepare0: archprepare
 	$(Q)$(MAKE) $(build)=. prepare $(if $(single-build),,prepare_checks)
+
+modpost_prepare: archprepare
 	$(Q)$(MAKE) $(build)=scripts/mod
 
 # All the preparing..
@@ -1765,7 +1767,7 @@ endif
 modules: modules_prepare
 
 # Target to prepare building external modules
-modules_prepare: prepare
+modules_prepare: prepare modpost_prepare
 	$(Q)$(MAKE) $(build)=scripts scripts/module.lds
 
 endif # CONFIG_MODULES
@@ -2189,7 +2191,8 @@ KBUILD_MODULES :=
 endif # CONFIG_MODULES
 
 PHONY += modpost
-modpost: $(if $(single-build),, $(if $(KBUILD_BUILTIN), vmlinux.o)) \
+modpost: $(if $(KBUILD_EXTMOD),,modpost_prepare) \
+	 $(if $(single-build),, $(if $(KBUILD_BUILTIN), vmlinux.o)) \
 	 $(if $(KBUILD_MODULES), modules_check)
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost
 

-- 
2.56.1

Re: [PATCH 3/3] kbuild: move scripts/mod target out of single-build

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Low] kbuild: unordered modpost_prepare and stack_protector_prepare
  mutates CFLAGS
--

commit 08ffc13380249741520c2ea434688d701b5d0e9b
Author: Vincent Mailhol <mailhol@kernel.org>

kbuild: move scripts/mod target out of single-build

This commit moves the scripts/mod build out of prepare0 into a new
modpost_prepare target depended on by modules_prepare and modpost.

> diff --git a/Makefile b/Makefile
> index 2c1289a44c560..93236f310f080 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1405,7 +1405,7 @@ scripts: scripts_basic scripts_dtc
>  # archprepare is used in arch Makefiles and when processed asm symlink,
>  # version.h and scripts_basic is processed / created.
>  
> -PHONY += prepare archprepare
> +PHONY += prepare archprepare modpost_prepare
>  
>  archprepare: outputmakefile archheaders archscripts scripts include/config/kernel.release \
>  	asm-generic $(version_h) include/generated/utsrelease.h \
> @@ -1414,6 +1414,8 @@ archprepare: outputmakefile archheaders archscripts scripts include/config/kerne
>  
>  prepare0: archprepare
>  	$(Q)$(MAKE) $(build)=. prepare $(if $(single-build),,prepare_checks)
> +
> +modpost_prepare: archprepare
>  	$(Q)$(MAKE) $(build)=scripts/mod

[Severity: Low]
Should modpost_prepare be ordered relative to stack_protector_prepare (or
depend on prepare)?

On arm, arm64, powerpc, and riscv, prepare depends on
stack_protector_prepare, which depends on prepare0 and mutates the
exported KBUILD_CFLAGS variable at recipe execution time:

arch/arm64/Makefile:
prepare: stack_protector_prepare
stack_protector_prepare: prepare0
	$(eval KBUILD_CFLAGS += -mstack-protector-guard=sysreg		  \
				-mstack-protector-guard-reg=sp_el0	  \
				-mstack-protector-guard-offset=$(shell	  \
			awk '{if ($$2 == "TSK_STACK_CANARY") print $$3;}' \
				$(objtree)/include/generated/asm-offsets.h))

Previously, scripts/mod was built inside prepare0, so it always ran before
stack_protector_prepare modified KBUILD_CFLAGS.  With modpost_prepare
only depending on archprepare, a parallel build runs modpost_prepare
concurrently with prepare0 before stack_protector_prepare updates
KBUILD_CFLAGS, whereas a serial build or make modules_prepare runs
prepare first and passes the updated KBUILD_CFLAGS to scripts/mod.

Because scripts/mod/empty.o is compiled with KBUILD_CFLAGS, switching
between parallel and serial or single-module builds changes its recorded
command line, rebuilding empty.o, regenerating elfconfig.h (which uses
if_changed), and forcing a full rebuild of modpost.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261111-reduce-single-build-dependencies-v1-0-7196ca986a85@kernel.org?part=3