kbuild: implement CONFIG_TRIM_UNUSED_KSYMS without recursion

When CONFIG_TRIM_UNUSED_KSYMS is enabled, Kbuild recursively traverses the directory tree to determine which EXPORT_SYMBOL to trim. If an EXPORT_SYMBOL turns out to be unused by anyone, Kbuild begins the second traverse, where some source files are recompiled with their EXPORT_SYMBOL() tuned into a no-op. Linus stated negative opinions about this slowness in commits: - 5cf0fd5 ("Kbuild: disable TRIM_UNUSED_KSYMS option") - a555bdd ("Kbuild: enable TRIM_UNUSED_KSYMS again, with some guarding") We can do this better now. The final data structures of EXPORT_SYMBOL are generated by the modpost stage, so modpost can selectively emit KSYMTAB entries that are really used by modules. Commit f73edc8 ("kbuild: unify two modpost invocations") is another ground-work to do this in a one-pass algorithm. With the list of modules, modpost sets sym->used if it is used by a module. modpost emits KSYMTAB only for symbols with sym->used==true. BTW, Nicolas explained why the trimming was implemented with recursion: https://lore.kernel.org/all/[email protected]/ Actually, we never achieved that level of optimization where the chain reaction of trimming comes into play because: - CONFIG_LTO_CLANG cannot remove any unused symbols - CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is enabled only for vmlinux, but not modules If deeper trimming is required, we need to revisit this, but I guess that is unlikely to happen. Signed-off-by: Masahiro Yamada <[email protected]>
kawasaki · Jun 22, 2023 · 5e9e95c · 5e9e95c
1 parent 700c48b
commit 5e9e95c
Show file tree

Hide file tree

Showing 11 changed files with 78 additions and 264 deletions.
diff --git a/.gitignore b/.gitignore
@@ -51,7 +51,6 @@
 *.symversions
 *.tab.[ch]
 *.tar
-*.usyms
 *.xz
 *.zst
 Module.symvers
@@ -112,7 +111,6 @@ modules.order
 #
 /include/config/
 /include/generated/
-/include/ksym/
 /arch/*/include/generated/
 
 # stgit generated dirs

diff --git a/Makefile b/Makefile
@@ -1193,28 +1193,12 @@ endif
 export KBUILD_VMLINUX_LIBS
 export KBUILD_LDS          := arch/$(SRCARCH)/kernel/vmlinux.lds
 
-# Recurse until adjust_autoksyms.sh is satisfied
-PHONY += autoksyms_recursive
 ifdef CONFIG_TRIM_UNUSED_KSYMS
 # For the kernel to actually contain only the needed exported symbols,
 # we have to build modules as well to determine what those symbols are.
-# (this can be evaluated only once include/config/auto.conf has been included)
 KBUILD_MODULES := 1
-
-autoksyms_recursive: $(build-dir) modules.order
-	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/adjust_autoksyms.sh \
-	  "$(MAKE) -f $(srctree)/Makefile autoksyms_recursive"
 endif
 
-autoksyms_h := $(if $(CONFIG_TRIM_UNUSED_KSYMS), include/generated/autoksyms.h)
-
-quiet_cmd_autoksyms_h = GEN     $@
-      cmd_autoksyms_h = mkdir -p $(dir $@); \
-			$(CONFIG_SHELL) $(srctree)/scripts/gen_autoksyms.sh $@
-
-$(autoksyms_h):
-	$(call cmd,autoksyms_h)
-
 # '$(AR) mPi' needs 'T' to workaround the bug of llvm-ar <= 14
 quiet_cmd_ar_vmlinux.a = AR      $@
       cmd_ar_vmlinux.a = \
@@ -1223,7 +1207,7 @@ quiet_cmd_ar_vmlinux.a = AR      $@
 	$(AR) mPiT $$($(AR) t $@ | sed -n 1p) $@ $$($(AR) t $@ | grep -F -f $(srctree)/scripts/head-object-list.txt)
 
 targets += vmlinux.a
-vmlinux.a: $(KBUILD_VMLINUX_OBJS) scripts/head-object-list.txt autoksyms_recursive FORCE
+vmlinux.a: $(KBUILD_VMLINUX_OBJS) scripts/head-object-list.txt FORCE
 	$(call if_changed,ar_vmlinux.a)
 
 PHONY += vmlinux_o
@@ -1279,7 +1263,7 @@ scripts: scripts_basic scripts_dtc
 PHONY += prepare archprepare
 
 archprepare: outputmakefile archheaders archscripts scripts include/config/kernel.release \
-	asm-generic $(version_h) $(autoksyms_h) include/generated/utsrelease.h \
+	asm-generic $(version_h) include/generated/utsrelease.h \
 	include/generated/compile.h include/generated/autoconf.h remove-stale-files
 
 prepare0: archprepare
@@ -2039,7 +2023,7 @@ clean: $(clean-dirs)
 		-o -name '*.dtb.S' -o -name '*.dtbo.S' \
 		-o -name '*.dt.yaml' \
 		-o -name '*.dwo' -o -name '*.lst' \
-		-o -name '*.su' -o -name '*.mod' -o -name '*.usyms' \
+		-o -name '*.su' -o -name '*.mod' \
 		-o -name '.*.d' -o -name '.*.tmp' -o -name '*.mod.c' \
 		-o -name '*.lex.c' -o -name '*.tab.[ch]' \
 		-o -name '*.asn1.[ch]' \

diff --git a/include/linux/export.h b/include/linux/export.h
@@ -42,32 +42,14 @@ extern struct module __this_module;
 	.long sym
 #endif
 
-#define ____EXPORT_SYMBOL(sym, license, ns)		\
+#define ___EXPORT_SYMBOL(sym, license, ns)		\
 	.section ".export_symbol","a"		ASM_NL	\
 	__export_symbol_##sym:			ASM_NL	\
 		.asciz license			ASM_NL	\
 		.asciz ns			ASM_NL	\
 		__EXPORT_SYMBOL_REF(sym)	ASM_NL	\
 	.previous
 
-#ifdef __GENKSYMS__
-
-#define ___EXPORT_SYMBOL(sym, sec, ns)	__GENKSYMS_EXPORT_SYMBOL(sym)
-
-#elif defined(__ASSEMBLY__)
-
-#define ___EXPORT_SYMBOL(sym, license, ns) \
-	____EXPORT_SYMBOL(sym, license, ns)
-
-#else
-
-#define ___EXPORT_SYMBOL(sym, license, ns)			\
-	extern typeof(sym) sym;					\
-	__ADDRESSABLE(sym)					\
-	asm(__stringify(____EXPORT_SYMBOL(sym, license, ns)))
-
-#endif
-
 #if !defined(CONFIG_MODULES) || defined(__DISABLE_EXPORTS)
 
 /*
@@ -77,50 +59,21 @@ extern struct module __this_module;
  */
 #define __EXPORT_SYMBOL(sym, sec, ns)
 
-#elif defined(CONFIG_TRIM_UNUSED_KSYMS)
+#elif defined(__GENKSYMS__)
 
-#include <generated/autoksyms.h>
+#define __EXPORT_SYMBOL(sym, sec, ns)	__GENKSYMS_EXPORT_SYMBOL(sym)
 
-/*
- * For fine grained build dependencies, we want to tell the build system
- * about each possible exported symbol even if they're not actually exported.
- * We use a symbol pattern __ksym_marker_<symbol> that the build system filters
- * from the $(NM) output (see scripts/gen_ksymdeps.sh). These symbols are
- * discarded in the final link stage.
- */
-
-#ifdef __ASSEMBLY__
-
-#define __ksym_marker(sym)					\
-	.section ".discard.ksym","a" ;				\
-__ksym_marker_##sym: ;						\
-	.previous
-
-#else
-
-#define __ksym_marker(sym)	\
-	static int __ksym_marker_##sym[0] __section(".discard.ksym") __used
-
-#endif
+#elif defined(__ASSEMBLY__)
 
-#define __EXPORT_SYMBOL(sym, sec, ns)					\
-	__ksym_marker(sym);						\
-	__cond_export_sym(sym, sec, ns, __is_defined(__KSYM_##sym))
-#define __cond_export_sym(sym, sec, ns, conf)				\
-	___cond_export_sym(sym, sec, ns, conf)
-#define ___cond_export_sym(sym, sec, ns, enabled)			\
-	__cond_export_sym_##enabled(sym, sec, ns)
-#define __cond_export_sym_1(sym, sec, ns) ___EXPORT_SYMBOL(sym, sec, ns)
-
-#ifdef __GENKSYMS__
-#define __cond_export_sym_0(sym, sec, ns) __GENKSYMS_EXPORT_SYMBOL(sym)
-#else
-#define __cond_export_sym_0(sym, sec, ns) /* nothing */
-#endif
+#define __EXPORT_SYMBOL(sym, license, ns) \
+	___EXPORT_SYMBOL(sym, license, ns)
 
 #else
 
-#define __EXPORT_SYMBOL(sym, sec, ns)	___EXPORT_SYMBOL(sym, sec, ns)
+#define __EXPORT_SYMBOL(sym, license, ns)			\
+	extern typeof(sym) sym;					\
+	__ADDRESSABLE(sym)					\
+	asm(__stringify(___EXPORT_SYMBOL(sym, license, ns)))
 
 #endif /* CONFIG_MODULES */
 

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
@@ -82,7 +82,7 @@ ifdef need-builtin
 targets-for-builtin += $(obj)/built-in.a
 endif
 
-targets-for-modules := $(foreach x, o mod $(if $(CONFIG_TRIM_UNUSED_KSYMS), usyms), \
+targets-for-modules := $(foreach x, o mod, \
 				$(patsubst %.o, %.$x, $(filter %.o, $(obj-m))))
 
 ifdef need-modorder
@@ -217,18 +217,12 @@ is-standard-object = $(if $(filter-out y%, $(OBJECT_FILES_NON_STANDARD_$(basetar
 
 $(obj)/%.o: objtool-enabled = $(if $(is-standard-object),$(if $(delay-objtool),$(is-single-obj-m),y))
 
-ifdef CONFIG_TRIM_UNUSED_KSYMS
-cmd_gen_ksymdeps = \
-	$(CONFIG_SHELL) $(srctree)/scripts/gen_ksymdeps.sh $@ >> $(dot-target).cmd
-endif
-
 ifneq ($(findstring 1, $(KBUILD_EXTRA_WARN)),)
 cmd_warn_shared_object = $(if $(word 2, $(modname-multi)),$(warning $(kbuild-file): $*.o is added to multiple modules: $(modname-multi)))
 endif
 
 define rule_cc_o_c
 	$(call cmd_and_fixdep,cc_o_c)
-	$(call cmd,gen_ksymdeps)
 	$(call cmd,checksrc)
 	$(call cmd,checkdoc)
 	$(call cmd,gen_objtooldep)
@@ -239,7 +233,6 @@ endef
 
 define rule_as_o_S
 	$(call cmd_and_fixdep,as_o_S)
-	$(call cmd,gen_ksymdeps)
 	$(call cmd,gen_objtooldep)
 	$(call cmd,gen_symversions_S)
 	$(call cmd,warn_shared_object)
@@ -258,12 +251,6 @@ cmd_mod = printf '%s\n' $(call real-search, $*.o, .o, -objs -y -m) | \
 $(obj)/%.mod: FORCE
 	$(call if_changed,mod)
 
-# List module undefined symbols
-cmd_undefined_syms = $(NM) $< | sed -n 's/^  *U //p' > $@
-
-$(obj)/%.usyms: $(obj)/%.o FORCE
-	$(call if_changed,undefined_syms)
-
 quiet_cmd_cc_lst_c = MKLST   $@
       cmd_cc_lst_c = $(CC) $(c_flags) -g -c -o $*.o $< && \
 		     $(CONFIG_SHELL) $(srctree)/scripts/makelst $*.o \

diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
@@ -91,6 +91,13 @@ targets += .vmlinux.objs
 .vmlinux.objs: vmlinux.a $(KBUILD_VMLINUX_LIBS) FORCE
 	$(call if_changed,vmlinux_objs)
 
+ifdef CONFIG_TRIM_UNUSED_KSYMS
+ksym-wl := $(CONFIG_UNUSED_KSYMS_WHITELIST)
+ksym-wl := $(if $(filter-out /%, $(ksym-wl)),$(srctree)/)$(ksym-wl)
+modpost-args += -t $(addprefix -u , $(ksym-wl))
+modpost-deps += $(ksym-wl)
+endif
+
 ifeq ($(wildcard vmlinux.o),)
 missing-input := vmlinux.o
 output-symdump := modules-only.symvers

diff --git a/scripts/adjust_autoksyms.sh b/scripts/adjust_autoksyms.sh
diff --git a/scripts/basic/fixdep.c b/scripts/basic/fixdep.c
@@ -246,8 +246,7 @@ static void *read_file(const char *filename)
 /* Ignore certain dependencies */
 static int is_ignored_file(const char *s, int len)
 {
-	return str_ends_with(s, len, "include/generated/autoconf.h") ||
-	       str_ends_with(s, len, "include/generated/autoksyms.h");
+	return str_ends_with(s, len, "include/generated/autoconf.h");
 }
 
 /* Do not parse these files */

diff --git a/scripts/gen_autoksyms.sh b/scripts/gen_autoksyms.sh