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Message-ID: <20250420010214.1963979-1-xur@google.com>
Date: Sat, 19 Apr 2025 18:02:14 -0700
From: xur@...gle.com
To: Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
Nicolas Schier <nicolas.schier@...ux.dev>,
Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>,
Justin Stitt <justinstitt@...gle.com>, Miguel Ojeda <ojeda@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>, Alice Ryhl <aliceryhl@...gle.com>,
Sami Tolvanen <samitolvanen@...gle.com>, "Mike Rapoport (Microsoft)" <rppt@...nel.org>,
Rafael Aquini <aquini@...hat.com>, Michael Ellerman <mpe@...erman.id.au>,
Stafford Horne <shorne@...il.com>, Christophe Leroy <christophe.leroy@...roup.eu>, Rong Xu <xur@...gle.com>,
Teresa Johnson <tejohnson@...gle.com>
Cc: linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
llvm@...ts.linux.dev
Subject: [PATCH] kbuild: distributed build support for Clang ThinLTO
From: Rong Xu <xur@...gle.com>
Add distributed ThinLTO build support for the Linux kernel.
This new mode offers several advantages: (1) Increased
flexibility in handling user-specified build options.
(2) Improved user-friendliness for developers. (3) Greater
convenience for integrating with objtool and livepatch.
Note that "distributed" in this context refers to a term
that differentiates in-process ThinLTO builds by invoking
backend compilation through the linker, not necessarily
building in distributed environments.
Distributed ThinLTO is enabled via the
`CONFIG_LTO_CLANG_THIN_DIST` Kconfig option. For example:
> make LLVM=1 defconfig
> scripts/config -e LTO_CLANG_THIN_DIST
> make LLVM=1 oldconfig
> make LLVM=1 vmlinux -j <..>
The implementation changes the top-level Makefile with a
macro for generating `vmlinux.o` for distributed ThinLTO
builds. It uses the existing Kbuild infrastructure to
perform two recursive passes through the subdirectories.
The first pass generates LLVM IR object files, similar to
in-process ThinLTO. Following the thin-link stage, a second
pass compiles these IR files into the final native object
files. The build rules and actions for this two-pass process
are primarily implemented in `scripts/Makefile.build`.
Currently, this patch focuses on building the main kernel
image (`vmlinux`) only. Support for building kernel modules
using this method is planned for a subsequent patch.
Tested on the following arch: x86, arm64, loongarch, and
riscv.
Some implementation details can be found here:
https://discourse.llvm.org/t/rfc-distributed-thinlto-build-for-kernel/85934
Signed-off-by: Rong Xu <xur@...gle.com>
---
MAINTAINERS | 5 +++
Makefile | 40 ++++++++++++++++++++---
arch/Kconfig | 12 +++++++
scripts/Makefile.build | 52 +++++++++++++++++++++++++++---
scripts/Makefile.lib | 7 +++-
scripts/Makefile.vmlinux_o | 16 +++++++---
scripts/Makefile.vmlinux_thinlink | 53 +++++++++++++++++++++++++++++++
scripts/head-object-list.txt | 1 +
8 files changed, 171 insertions(+), 15 deletions(-)
create mode 100644 scripts/Makefile.vmlinux_thinlink
diff --git a/MAINTAINERS b/MAINTAINERS
index c966e71ea60bb..40d3bc223e4b0 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -5782,6 +5782,11 @@ F: scripts/Makefile.clang
F: scripts/clang-tools/
K: \b(?i:clang|llvm)\b
+CLANG/LLVM THINLTO DISTRIBUTED BUILD
+M: Rong Xu <xur@...gle.com>
+S: Supported
+F: scripts/Makefile.vmlinux_thinlink
+
CLK API
M: Russell King <linux@...linux.org.uk>
L: linux-clk@...r.kernel.org
diff --git a/Makefile b/Makefile
index e65f8735c7bf6..18da9a40dd2c8 100644
--- a/Makefile
+++ b/Makefile
@@ -298,7 +298,8 @@ no-dot-config-targets := $(clean-targets) \
outputmakefile rustavailable rustfmt rustfmtcheck
no-sync-config-targets := $(no-dot-config-targets) %install modules_sign kernelrelease \
image_name
-single-targets := %.a %.i %.ko %.lds %.ll %.lst %.mod %.o %.rsi %.s %/
+single-targets := %.a %.i %.ko %.lds %.ll %.lst %.mod %.o %.rsi %.s %.final_o \
+ %.final_a %.o.thinlto.bc %/
config-build :=
mixed-build :=
@@ -991,10 +992,10 @@ export CC_FLAGS_SCS
endif
ifdef CONFIG_LTO_CLANG
-ifdef CONFIG_LTO_CLANG_THIN
-CC_FLAGS_LTO := -flto=thin -fsplit-lto-unit
-else
+ifdef CONFIG_LTO_CLANG_FULL
CC_FLAGS_LTO := -flto
+else # for CONFIG_LTO_CLANG_THIN or CONFIG_LTO_CLANG_THIN_DIST
+CC_FLAGS_LTO := -flto=thin -fsplit-lto-unit
endif
CC_FLAGS_LTO += -fvisibility=hidden
@@ -1218,8 +1219,34 @@ vmlinux.a: $(KBUILD_VMLINUX_OBJS) scripts/head-object-list.txt FORCE
$(call if_changed,ar_vmlinux.a)
PHONY += vmlinux_o
+ifdef CONFIG_LTO_CLANG_THIN_DIST
+vmlinux.thinlink: vmlinux.a $(KBUILD_VMLINUX_LIBS) FORCE
+ $(Q)$(MAKE) -f $(srctree)/scripts/Makefile.vmlinux_thinlink
+targets += vmlinux.thinlink
+
+vmlinux_final_a := $(patsubst %.a,%.final_a,$(KBUILD_VMLINUX_OBJS))
+quiet_cmd_ar_vmlinux.final_a = AR $@
+ cmd_ar_vmlinux.final_a = \
+ rm -f $@; \
+ $(AR) cDPrST $@ $(vmlinux_final_a); \
+ $(AR) mPiT $$($(AR) t $@ | sed -n 1p) $@ $$($(AR) t $@ | grep -F -f $(srctree)/scripts/head-object-list.txt)
+
+define rule_gen_vmlinux_final_a
+ +$(Q)$(MAKE) $(build)=. need-builtin=1 thinlto_final_pass=1 need-modorder=1 built-in.final_a
+ $(call cmd_and_savecmd,ar_vmlinux.final_a)
+endef
+
+vmlinux.final_a: vmlinux.thinlink scripts/head-object-list.txt FORCE
+ $(call if_changed_rule,gen_vmlinux_final_a)
+
+targets += vmlinux.final_a
+
+vmlinux_o: vmlinux.final_a
+ $(Q)$(MAKE) thinlto_final_pass=1 -f $(srctree)/scripts/Makefile.vmlinux_o
+else
vmlinux_o: vmlinux.a $(KBUILD_VMLINUX_LIBS)
$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.vmlinux_o
+endif
vmlinux.o modules.builtin.modinfo modules.builtin: vmlinux_o
@:
@@ -1577,7 +1604,8 @@ CLEAN_FILES += vmlinux.symvers modules-only.symvers \
modules.builtin.ranges vmlinux.o.map vmlinux.unstripped \
compile_commands.json rust/test \
rust-project.json .vmlinux.objs .vmlinux.export.c \
- .builtin-dtbs-list .builtin-dtb.S
+ .builtin-dtbs-list .builtin-dtb.S \
+ .vmlinux_thinlto_bc_files vmlinux.thinlink
# Directories & files removed with 'make mrproper'
MRPROPER_FILES += include/config include/generated \
@@ -2028,6 +2056,8 @@ clean: $(clean-dirs)
-o -name '*.symtypes' -o -name 'modules.order' \
-o -name '*.c.[012]*.*' \
-o -name '*.ll' \
+ -o -name '*.final_a' -o -name '*.final_o' \
+ -o -name '*.o.thinlto.bc' \
-o -name '*.gcno' \
\) -type f -print \
-o -name '.tmp_*' -print \
diff --git a/arch/Kconfig b/arch/Kconfig
index b0adb665041f1..cbeeeb9b076d8 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -810,6 +810,18 @@ config LTO_CLANG_THIN
https://clang.llvm.org/docs/ThinLTO.html
If unsure, say Y.
+
+config LTO_CLANG_THIN_DIST
+ bool "Clang ThinLTO in distributed mode (EXPERIMENTAL)"
+ depends on HAS_LTO_CLANG && ARCH_SUPPORTS_LTO_CLANG_THIN
+ select LTO_CLANG
+ help
+ This option enables Clang's ThinLTO in distributed build mode.
+ In this mode, the linker performs the thin-link, generating
+ ThinLTO index files. Subsequently, the build system explicitly
+ invokes ThinLTO backend compilation using these index files
+ and pre-linked IR objects. The resulting final object files
+ are with the .final_o suffix.
endchoice
config ARCH_SUPPORTS_AUTOFDO_CLANG
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 13dcd86e74ca8..87259b0e2bfc8 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -50,18 +50,23 @@ endif
# ===========================================================================
+builtin_suffix := $(if $(filter %.final_a, $(MAKECMDGOALS)),final_a,a)
+ifeq ($(thinlto_final_pass),1)
+builtin_suffix := final_a
+endif
+
# subdir-builtin and subdir-modorder may contain duplications. Use $(sort ...)
-subdir-builtin := $(sort $(filter %/built-in.a, $(real-obj-y)))
+subdir-builtin := $(sort $(filter %/built-in.$(builtin_suffix), $(real-obj-y)))
subdir-modorder := $(sort $(filter %/modules.order, $(obj-m)))
targets-for-builtin := $(extra-y)
ifneq ($(strip $(lib-y) $(lib-m) $(lib-)),)
-targets-for-builtin += $(obj)/lib.a
+targets-for-builtin += $(obj)/lib.$(builtin_suffix)
endif
ifdef need-builtin
-targets-for-builtin += $(obj)/built-in.a
+targets-for-builtin += $(obj)/built-in.$(builtin_suffix)
endif
targets-for-modules := $(foreach x, o mod, \
@@ -337,6 +342,10 @@ $(obj)/%.o: $(obj)/%.S FORCE
targets += $(filter-out $(subdir-builtin), $(real-obj-y))
targets += $(filter-out $(subdir-modorder), $(real-obj-m))
targets += $(lib-y) $(always-y)
+ifeq ($(builtin_suffix), final_a)
+final_o_targets = $(patsubst,%.o,%.final_o,$(targets))
+targets += $(final_targets)
+endif
# Linker scripts preprocessor (.lds.S -> .lds)
# ---------------------------------------------------------------------------
@@ -347,6 +356,24 @@ quiet_cmd_cpp_lds_S = LDS $@
$(obj)/%.lds: $(src)/%.lds.S FORCE
$(call if_changed_dep,cpp_lds_S)
+ifdef CONFIG_LTO_CLANG_THIN_DIST
+# Generate .final_o (obj) from .o (bitcode) file
+# ---------------------------------------------------------------------------
+quiet_cmd_cc_o_bc = CC $(quiet_modtag) $@
+
+cmd_cc_o_bc = $(if $(filter bitcode, $(shell file -b $<)),$(CC) \
+ $(filter-out -Wp% $(LINUXINCLUDE) %.h.gch %.h -D% \
+ -flto=thin, $(c_flags)) \
+ -Wno-unused-command-line-argument \
+ -x ir -fthinlto-index=$<.thinlto.bc -c -o $@ \
+ $(if $(findstring ../,$<), \
+ $$(realpath --relative-to=$(srcroot) $<), $<), \
+ cp $< $@)
+
+$(obj)/%.final_o: $(obj)/%.o FORCE
+ $(call if_changed,cc_o_bc)
+endif
+
# ASN.1 grammar
# ---------------------------------------------------------------------------
quiet_cmd_asn1_compiler = ASN.1 $(basename $@).[ch]
@@ -360,7 +387,7 @@ $(obj)/%.asn1.c $(obj)/%.asn1.h: $(src)/%.asn1 $(objtree)/scripts/asn1_compiler
# ---------------------------------------------------------------------------
# To build objects in subdirs, we need to descend into the directories
-$(subdir-builtin): $(obj)/%/built-in.a: $(obj)/% ;
+$(subdir-builtin): $(obj)/%/built-in.$(builtin_suffix): $(obj)/% ;
$(subdir-modorder): $(obj)/%/modules.order: $(obj)/% ;
#
@@ -377,6 +404,12 @@ quiet_cmd_ar_builtin = AR $@
$(obj)/built-in.a: $(real-obj-y) FORCE
$(call if_changed,ar_builtin)
+ifdef CONFIG_LTO_CLANG_THIN_DIST
+# Rule to compile a set of .final_o files into one .final_a file.
+$(obj)/built-in.final_a: $(patsubst %.o,%.final_o,$(real-obj-y)) FORCE
+ $(call if_changed,ar_builtin)
+endif
+
# This is a list of build artifacts from the current Makefile and its
# sub-directories. The timestamp should be updated when any of the member files.
@@ -394,6 +427,14 @@ $(obj)/modules.order: $(obj-m) FORCE
$(obj)/lib.a: $(lib-y) FORCE
$(call if_changed,ar)
+ifdef CONFIG_LTO_CLANG_THIN_DIST
+quiet_cmd_ar_final = AR $@
+ cmd_ar_final = rm -f $@; $(AR) cDPrsT $@ $(patsubst %.o,%.final_o,$(real-prereqs))
+
+$(obj)/lib.final_a: $(patsubst %.o,%.final_o,$(lib-y)) FORCE
+ $(call if_changed,ar_final)
+endif
+
quiet_cmd_ld_multi_m = LD [M] $@
cmd_ld_multi_m = $(LD) $(ld_flags) -r -o $@ @$< $(cmd_objtool)
@@ -459,7 +500,8 @@ $(single-subdir-goals): $(single-subdirs)
PHONY += $(subdir-ym)
$(subdir-ym):
$(Q)$(MAKE) $(build)=$@ \
- need-builtin=$(if $(filter $@...ilt-in.a, $(subdir-builtin)),1) \
+ need-builtin=$(if $(filter $@...ilt-in.$(builtin_suffix), $(subdir-builtin)),1) \
+ thinlto_final_pass=$(if $(filter final_a, $(builtin_suffix)),1) \
need-modorder=$(if $(filter $@...dules.order, $(subdir-modorder)),1) \
$(filter $@/%, $(single-subdir-goals))
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 2fe73cda0bddb..ce341ed8d1df3 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -34,8 +34,13 @@ else
obj-m := $(filter-out %/, $(obj-m))
endif
+builtin_suffix := $(if $(filter %.final_a, $(MAKECMDGOALS)),final_a,a)
+ifeq ($(thinlto_final_pass),1)
+ builtin_suffix := final_a
+endif
+
ifdef need-builtin
-obj-y := $(patsubst %/, %/built-in.a, $(obj-y))
+obj-y := $(patsubst %/, %/built-in.$(builtin_suffix), $(obj-y))
else
obj-y := $(filter-out %/, $(obj-y))
endif
diff --git a/scripts/Makefile.vmlinux_o b/scripts/Makefile.vmlinux_o
index 938c7457717ea..4f04eb2c11412 100644
--- a/scripts/Makefile.vmlinux_o
+++ b/scripts/Makefile.vmlinux_o
@@ -9,6 +9,14 @@ include $(srctree)/scripts/Kbuild.include
# for objtool
include $(srctree)/scripts/Makefile.lib
+ifeq ($(thinlto_final_pass),1)
+vmlinux_a := vmlinux.final_a
+vmlinux_libs := $(patsubst %.a,%.final_a,$(KBUILD_VMLINUX_LIBS))
+else
+vmlinux_a := vmlinux.a
+vmlinux_libs := $(KBUILD_VMLINUX_LIBS)
+endif
+
# Generate a linker script to ensure correct ordering of initcalls for Clang LTO
# ---------------------------------------------------------------------------
@@ -18,7 +26,7 @@ quiet_cmd_gen_initcalls_lds = GEN $@
$(PERL) $(real-prereqs) > $@
.tmp_initcalls.lds: $(srctree)/scripts/generate_initcall_order.pl \
- vmlinux.a $(KBUILD_VMLINUX_LIBS) FORCE
+ $(vmlinux_a) $(vmlinux_libs) FORCE
$(call if_changed,gen_initcalls_lds)
targets := .tmp_initcalls.lds
@@ -59,8 +67,8 @@ quiet_cmd_ld_vmlinux.o = LD $@
$(LD) ${KBUILD_LDFLAGS} -r -o $@ \
$(vmlinux-o-ld-args-y) \
$(addprefix -T , $(initcalls-lds)) \
- --whole-archive vmlinux.a --no-whole-archive \
- --start-group $(KBUILD_VMLINUX_LIBS) --end-group \
+ --whole-archive $(vmlinux_a) --no-whole-archive \
+ --start-group $(vmlinux_libs) --end-group \
$(cmd_objtool)
define rule_ld_vmlinux.o
@@ -68,7 +76,7 @@ define rule_ld_vmlinux.o
$(call cmd,gen_objtooldep)
endef
-vmlinux.o: $(initcalls-lds) vmlinux.a $(KBUILD_VMLINUX_LIBS) FORCE
+vmlinux.o: $(initcalls-lds) $(vmlinux_a) $(vmlinux_libs) FORCE
$(call if_changed_rule,ld_vmlinux.o)
targets += vmlinux.o
diff --git a/scripts/Makefile.vmlinux_thinlink b/scripts/Makefile.vmlinux_thinlink
new file mode 100644
index 0000000000000..13e4026c7d45b
--- /dev/null
+++ b/scripts/Makefile.vmlinux_thinlink
@@ -0,0 +1,53 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+PHONY := __default
+__default: vmlinux.thinlink
+
+include include/config/auto.conf
+include $(srctree)/scripts/Kbuild.include
+
+
+# Generate a linker script to ensure correct ordering of initcalls for Clang LTO
+# ---------------------------------------------------------------------------
+
+quiet_cmd_gen_initcalls_lds = GEN $@
+ cmd_gen_initcalls_lds = \
+ $(PYTHON3) $(srctree)/scripts/jobserver-exec \
+ $(PERL) $(real-prereqs) > $@
+
+.tmp_initcalls_thinlink.lds: $(srctree)/scripts/generate_initcall_order.pl \
+ vmlinux.a FORCE
+ $(call if_changed,gen_initcalls_lds)
+
+targets := .tmp_initcalls_thinlink.lds
+
+initcalls-lds := .tmp_initcalls_thinlink.lds
+
+quiet_cmd_ld_vmlinux.thinlink = LD $@
+ cmd_ld_vmlinux.thinlink = \
+ $(AR) t vmlinux.a > .vmlinux_thinlto_bc_files; \
+ $(LD) ${KBUILD_LDFLAGS} -r $(addprefix -T , $(initcalls-lds)) \
+ --thinlto-index-only @.vmlinux_thinlto_bc_files; \
+ touch vmlinux.thinlink
+
+vmlinux.thinlink: vmlinux.a $(initcalls-lds) FORCE
+ $(call if_changed,ld_vmlinux.thinlink)
+
+targets += vmlinux.thinlink
+
+# Add FORCE to the prerequisites of a target to force it to be always rebuilt.
+# ---------------------------------------------------------------------------
+
+PHONY += FORCE
+FORCE:
+
+# Read all saved command lines and dependencies for the $(targets) we
+# may be building above, using $(if_changed{,_dep}). As an
+# optimization, we don't need to read them if the target does not
+# exist, we will rebuild anyway in that case.
+
+existing-targets := $(wildcard $(sort $(targets)))
+
+-include $(foreach f,$(existing-targets),$(dir $(f)).$(notdir $(f)).cmd)
+
+.PHONY: $(PHONY)
diff --git a/scripts/head-object-list.txt b/scripts/head-object-list.txt
index 7274dfc65af60..b300222e99a6b 100644
--- a/scripts/head-object-list.txt
+++ b/scripts/head-object-list.txt
@@ -18,6 +18,7 @@ arch/arm/kernel/head.o
arch/csky/kernel/head.o
arch/hexagon/kernel/head.o
arch/loongarch/kernel/head.o
+arch/loongarch/kernel/head.final_o
arch/m68k/68000/head.o
arch/m68k/coldfire/head.o
arch/m68k/kernel/head.o
--
2.49.0.805.g082f7c87e0-goog
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