Re: [PATCH 10/13] kbuild: build modules in the same way with/without Clang LTO

Kees Cook
2021-08-19T06:59:06Z

On Thu, Aug 19, 2021 at 09:57:41AM +0900, Masahiro Yamada wrote:
> When Clang LTO is enabled, additional intermediate files *.lto.o are
> created because LLVM bitcode must be converted to ELF before modpost.
>
> For non-LTO builds:
>
> $(LD) $(LD)
> objects ---> <modname>.o -----> <modname>.ko
> |
> <modname>.mod.o ---/
>
> For Clang LTO builds:
>
> $(AR) $(LD) $(LD)
> objects ---> <modname>.o ---> <modname>.lto.o -----> <modname>.ko
> |
> <modname>.mod.o --/
>
> Since the Clang LTO introduction, ugly CONFIG_LTO_CLANG conditionals
> are sprinkled everywhere in the kbuild code.
>
> Another confusion for Clang LTO builds is, <modname>.o is an archive
> that contains LLVM bitcode files. The suffix should have been .a
> instead of .o
>
> To clean up the code, unify the build process of modules, as follows:
>
> $(AR) $(LD) $(LD)
> objects ---> <modname>.a ---> <modname>.prelink.o -----> <modname>.ko
> |
> <modname>.mod.o ------/
>
> Here, 'objects' are either ELF or LLVM bitcode. <modname>.a is an archive,
> <modname>.prelink.o is ELF.

I like this design, but I do see that it has a small but measurable
impact on build times:

allmodconfig build, GCC:

make -j72 allmodconfig
make -j72 -s clean && time make -j72

kbuild/for-next:
6m16.140s
6m19.742s
6m15.848s

+this-series:
6m22.742s
6m20.589s
6m19.911s

Thought with not so many modules, it's within the noise:

defconfig build, GCC:

make -j72 defconfig
make -j72 -s clean && time make -j72

kbuild/for-next:
0m41.579s
0m41.214s
0m41.370s

+series:
0m41.423s
0m41.434s
0m41.384s

However, I do see that even LTO builds are slightly slower now, so
perhaps the above numbers aren't due to the added $(AR) step:

allmodconfig + Clang ThinLTO:

make -j72 LLVM=1 LLVM_IAS=1 allmodconfig
./scripts/config -d GCOV_KERNEL -d KASAN -d LTO_NONE -e LTO_CLANG_THIN
make -j72 LLVM=1 LLVM_IAS=1 olddefconfig
make -j72 -s LLVM=1 LLVM_IAS=1 clean && time make -j72 LLVM=1 LLVM_IAS=1

kbuild/for-next:
9m53.927s
9m45.874s
9m47.722s

+series:
9m58.395s
9m53.201s
9m56.387s

I haven't been able to isolate where the changes in build times are
coming from (nor have I done link-phase-only timings -- I realize those
are really the most important).

I did notice some warnings from this patch, though, in the
$(modules-single) target:

scripts/Makefile.build:434: target 'drivers/scsi/libiscsi.a' given more than once in the same rule
scripts/Makefile.build:434: target 'drivers/atm/suni.a' given more than once in the same rule

(And I saw the new "FORCE prerequisite is missing" warnings, but those
are in kbuild/for-next and not new for this series.)

Anyway, this is a great clean-up; thank you very much for finding the
time for it! I'll keep poking at it tomorrow.

-Kees

>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> ---
>
> scripts/Makefile.build | 103 ++++++++++++++++++--------------------
> scripts/Makefile.lib | 11 ++--
> scripts/Makefile.modfinal | 4 +-
> scripts/Makefile.modpost | 7 +--
> scripts/mod/modpost.c | 6 +--
> scripts/mod/sumversion.c | 6 +--
> 6 files changed, 61 insertions(+), 76 deletions(-)
>
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 37d6f6da34d6..957addea830b 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -88,9 +88,7 @@ endif
>
> targets-for-modules := $(patsubst %.o, %.mod, $(filter %.o, $(obj-m)))
>
> -ifdef CONFIG_LTO_CLANG
> -targets-for-modules += $(patsubst %.o, %.lto.o, $(filter %.o, $(obj-m)))
> -endif
> +targets-for-modules += $(patsubst %.o, %.prelink.o, $(filter %.o, $(obj-m)))
>
> ifdef need-modorder
> targets-for-modules += $(obj)/modules.order
> @@ -282,33 +280,12 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) $(objtool_dep) FORCE
> $(call if_changed_rule,cc_o_c)
> $(call cmd,force_checksrc)
>
> -ifdef CONFIG_LTO_CLANG
> -# Module .o files may contain LLVM bitcode, compile them into native code
> -# before ELF processing
> -quiet_cmd_cc_lto_link_modules = LTO [M] $@
> -cmd_cc_lto_link_modules = \
> - $(LD) $(ld_flags) -r -o $@ \
> - $(shell [ -s $(@:.lto.o=.o.symversions) ] && \
> - echo -T $(@:.lto.o=.o.symversions)) \
> - --whole-archive $(filter-out FORCE,$^)
> -
> -ifdef CONFIG_STACK_VALIDATION
> -# objtool was skipped for LLVM bitcode, run it now that we have compiled
> -# modules into native code
> -cmd_cc_lto_link_modules += ; \
> - $(objtree)/tools/objtool/objtool $(objtool_args) --module $@
> -endif
> -
> -$(obj)/%.lto.o: $(obj)/%.o FORCE
> - $(call if_changed,cc_lto_link_modules)
> -endif
> -
> cmd_mod = { \
> echo $(if $($*-objs)$($*-y)$($*-m), $(addprefix $(obj)/, $($*-objs) $($*-y) $($*-m)), $(@:.mod=.o)); \
> $(undefined_syms) echo; \
> } > $@
>
> -$(obj)/%.mod: $(obj)/%$(mod-prelink-ext).o FORCE
> +$(obj)/%.mod: $(obj)/%.prelink.o FORCE
> $(call if_changed,mod)
>
> quiet_cmd_cc_lst_c = MKLST $@
> @@ -412,17 +389,6 @@ $(obj)/%.asn1.c $(obj)/%.asn1.h: $(src)/%.asn1 $(objtree)/scripts/asn1_compiler
> $(subdir-builtin): $(obj)/%/built-in.a: $(obj)/% ;
> $(subdir-modorder): $(obj)/%/modules.order: $(obj)/% ;
>
> -# combine symversions for later processing
> -ifeq ($(CONFIG_LTO_CLANG) $(CONFIG_MODVERSIONS),y y)
> - cmd_update_lto_symversions = \
> - rm -f $@.symversions \
> - $(foreach n, $(filter-out FORCE,$^), \
> - $(if $(shell test -s $(n).symversions && echo y), \
> - ; cat $(n).symversions >> $@.symversions))
> -else
> - cmd_update_lto_symversions = echo >/dev/null
> -endif
> -
> #
> # Rule to compile a set of .o files into one .a file (without symbol table)
> #
> @@ -442,10 +408,10 @@ $(obj)/built-in.a: $(real-obj-y) FORCE
> # modules.order unless contained modules are updated.
>
> cmd_modules_order = { $(foreach m, $(real-prereqs), \
> - $(if $(filter %/modules.order, $m), cat $m, echo $(patsubst %.o,%.ko,$m));) :; } \
> + $(if $(filter %/modules.order, $m), cat $m, echo $(patsubst %.a,%.ko,$m));) :; } \
> | $(AWK) '!x[$$0]++' - > $@
>
> -$(obj)/modules.order: $(obj-m) FORCE
> +$(obj)/modules.order: $(modules) FORCE
> $(call if_changed,modules_order)
>
> #
> @@ -454,26 +420,55 @@ $(obj)/modules.order: $(obj-m) FORCE
> $(obj)/lib.a: $(lib-y) FORCE
> $(call if_changed,ar)
>
> -# NOTE:
> -# Do not replace $(filter %.o,^) with $(real-prereqs). When a single object
> -# module is turned into a multi object module, $^ will contain header file
> -# dependencies recorded in the .*.cmd file.
> +#
> +# Rule to prelink modules
> +#
> +
> +ifeq ($(CONFIG_LTO_CLANG) $(CONFIG_MODVERSIONS),y y)
> +
> +cmd_merge_symver = \
> + rm -f $@; \
> + touch $@; \
> + for o in $$($(AR) t $<); do \
> + if [ -s $${o}.symversions ]; then \
> + cat $${o}.symversions >> $@; \
> + fi; \
> + done
> +
> +$(obj)/%.prelink.symversions: $(obj)/%.a FORCE
> + $(call if_changed,merge_symver)
> +
> +$(obj)/%.prelink.o: ld_flags += --script=$(filter %.symversions,$^)
> +module-symver = $(obj)/%.prelink.symversions
> +
> +endif
> +
> +quiet_cmd_ld_o_a = LD [M] $@
> + cmd_ld_o_a = $(LD) $(ld_flags) -r -o $@ --whole-archive $<
> +
> +$(obj)/%.prelink.o: $(obj)/%.a $(module-symver) FORCE
> + $(call if_changed,ld_o_a)
> +
> ifdef CONFIG_LTO_CLANG
> -quiet_cmd_link_multi-m = AR [M] $@
> -cmd_link_multi-m = \
> - $(cmd_update_lto_symversions); \
> - rm -f $@; \
> - $(AR) cDPrsT $@ $(filter %.o,$^)
> -else
> -quiet_cmd_link_multi-m = LD [M] $@
> - cmd_link_multi-m = $(LD) $(ld_flags) -r -o $@ $(filter %.o,$^)
> +ifdef CONFIG_STACK_VALIDATION
> +# objtool was skipped for LLVM bitcode, run it now that we have compiled
> +# modules into native code
> +cmd_ld_o_a += ; $(objtool) $(objtool_args) --module $@
> endif
> +endif
> +
> +quiet_cmd_ar_module = AR [M] $@
> + cmd_ar_module = rm -f $@; $(AR) cDPrST $@ $(real-prereqs)
> +
> +$(modules-single): %.a: %.o FORCE
> + $(call if_changed,ar_module)
> +
> +$(modules-multi): FORCE
> + $(call if_changed,ar_module)
> +$(call multi_depend, $(modules-multi), .a, -objs -y -m)
>
> -$(multi-obj-m): FORCE
> - $(call if_changed,link_multi-m)
> -$(call multi_depend, $(multi-obj-m), .o, -objs -y -m)
> +targets += $(modules-single) $(modules-multi)
>
> -targets += $(multi-obj-m)
> targets := $(filter-out $(PHONY), $(targets))
>
> # Add intermediate targets:
> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> index 34c4c11c4bc1..f604d2d01cad 100644
> --- a/scripts/Makefile.lib
> +++ b/scripts/Makefile.lib
> @@ -106,6 +106,10 @@ multi-dtb-y := $(addprefix $(obj)/, $(multi-dtb-y))
> real-dtb-y := $(addprefix $(obj)/, $(real-dtb-y))
> subdir-ym := $(addprefix $(obj)/,$(subdir-ym))
>
> +modules := $(patsubst %.o, %.a, $(obj-m))
> +modules-multi := $(patsubst %.o, %.a, $(multi-obj-m))
> +modules-single := $(filter-out $(modules-multi), $(filter %.a, $(modules)))
> +
> # Finds the multi-part object the current object will be linked into.
> # If the object belongs to two or more multi-part objects, list them all.
> modname-multi = $(sort $(foreach m,$(multi-obj-ym),\
> @@ -225,13 +229,6 @@ dtc_cpp_flags = -Wp,-MMD,$(depfile).pre.tmp -nostdinc \
> $(addprefix -I,$(DTC_INCLUDE)) \
> -undef -D__DTS__
>
> -ifeq ($(CONFIG_LTO_CLANG),y)
> -# With CONFIG_LTO_CLANG, .o files in modules might be LLVM bitcode, so we
> -# need to run LTO to compile them into native code (.lto.o) before further
> -# processing.
> -mod-prelink-ext := .lto
> -endif
> -
> # Useful for describing the dependency of composite objects
> # Usage:
> # $(call multi_depend, multi_used_targets, suffix_to_remove, suffix_to_add)
> diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
> index ff805777431c..1b6401f53662 100644
> --- a/scripts/Makefile.modfinal
> +++ b/scripts/Makefile.modfinal
> @@ -9,7 +9,7 @@ __modfinal:
> include include/config/auto.conf
> include $(srctree)/scripts/Kbuild.include
>
> -# for c_flags and mod-prelink-ext
> +# for c_flags
> include $(srctree)/scripts/Makefile.lib
>
> # find all modules listed in modules.order
> @@ -55,7 +55,7 @@ if_changed_except = $(if $(call newer_prereqs_except,$(2))$(cmd-check), \
>
>
> # Re-generate module BTFs if either module's .ko or vmlinux changed
> -$(modules): %.ko: %$(mod-prelink-ext).o %.mod.o scripts/module.lds $(if $(KBUILD_BUILTIN),vmlinux) FORCE
> +$(modules): %.ko: %.prelink.o %.mod.o scripts/module.lds $(if $(KBUILD_BUILTIN),vmlinux) FORCE
> +$(call if_changed_except,ld_ko_o,vmlinux)
> ifdef CONFIG_DEBUG_INFO_BTF_MODULES
> +$(if $(newer-prereqs),$(call cmd,btf_ko))
> diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
> index eef56d629799..11883b31c615 100644
> --- a/scripts/Makefile.modpost
> +++ b/scripts/Makefile.modpost
> @@ -41,9 +41,6 @@ __modpost:
> include include/config/auto.conf
> include $(srctree)/scripts/Kbuild.include
>
> -# for mod-prelink-ext
> -include $(srctree)/scripts/Makefile.lib
> -
> MODPOST = scripts/mod/modpost \
> $(if $(CONFIG_MODVERSIONS),-m) \
> $(if $(CONFIG_MODULE_SRCVERSION_ALL),-a) \
> @@ -128,9 +125,9 @@ endif
> # Read out modules.order to pass in modpost.
> # Otherwise, allmodconfig would fail with "Argument list too long".
> quiet_cmd_modpost = MODPOST $@
> - cmd_modpost = sed 's/\.ko$$/$(mod-prelink-ext)\.o/' $< | $(MODPOST) -T -
> + cmd_modpost = sed 's/ko$$/prelink.o/' $< | $(MODPOST) -T -
>
> -$(output-symdump): $(MODORDER) $(input-symdump) $(modules:.ko=$(mod-prelink-ext).o) FORCE
> +$(output-symdump): $(MODORDER) $(input-symdump) $(modules:ko=prelink.o) FORCE
> $(call if_changed,modpost)
>
> targets += $(output-symdump)
> diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> index 270a7df898e2..8c63c52af88d 100644
> --- a/scripts/mod/modpost.c
> +++ b/scripts/mod/modpost.c
> @@ -1991,9 +1991,9 @@ static void read_symbols(const char *modname)
> /* strip trailing .o */
> tmp = NOFAIL(strdup(modname));
> tmp[strlen(tmp) - 2] = '\0';
> - /* strip trailing .lto */
> - if (strends(tmp, ".lto"))
> - tmp[strlen(tmp) - 4] = '\0';
> + /* strip trailing .prelink */
> + if (strends(tmp, ".prelink"))
> + tmp[strlen(tmp) - 8] = '\0';
> mod = new_module(tmp);
> free(tmp);
> }
> diff --git a/scripts/mod/sumversion.c b/scripts/mod/sumversion.c
> index 760e6baa7eda..8ea0f7b23c63 100644
> --- a/scripts/mod/sumversion.c
> +++ b/scripts/mod/sumversion.c
> @@ -391,14 +391,10 @@ void get_src_version(const char *modname, char sum[], unsigned sumlen)
> struct md4_ctx md;
> char *fname;
> char filelist[PATH_MAX + 1];
> - int postfix_len = 1;
> -
> - if (strends(modname, ".lto.o"))
> - postfix_len = 5;
>
> /* objects for a module are listed in the first line of *.mod file. */
> snprintf(filelist, sizeof(filelist), "%.*smod",
> - (int)strlen(modname) - postfix_len, modname);
> + (int)(strlen(modname) - strlen("prelink.o")), modname);
>
> buf = read_text_file(filelist);
>
> --
> 2.30.2
>

--
Kees Cook