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authorMasahiro Yamada <masahiroy@kernel.org>2023-06-12 00:50:57 +0900
committerMasahiro Yamada <masahiroy@kernel.org>2023-06-22 21:21:06 +0900
commit5e9e95cc9148b82074a5eae283e63bce3f1aacfe (patch)
tree72072798944c4a0ffcf2f294b8c264cbd7f60b05 /scripts/Makefile.modpost
parent700c48b439921b67715e25380e0f67e6e490d7b8 (diff)
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: - 5cf0fd591f2e ("Kbuild: disable TRIM_UNUSED_KSYMS option") - a555bdd0c58c ("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 f73edc8951b2 ("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/2o2rpn97-79nq-p7s2-nq5-8p83391473r@syhkavp.arg/ 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 <masahiroy@kernel.org>
Diffstat (limited to 'scripts/Makefile.modpost')
-rw-r--r--scripts/Makefile.modpost7
1 files changed, 7 insertions, 0 deletions
diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
index 074e27c0c140..39472e834b63 100644
--- 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