Linux kernel stable tree https://git.rulkc.org/pub/scm/linux/kernel/git/stable/linux.git/atom?h=master 2023-06-22T12:21:06+00:00 2023-06-22T12:21:06+00:00 Masahiro Yamada masahiroy@kernel.org 2023-06-11T15:50:57+00:00 urn:sha1:5e9e95cc9148b82074a5eae283e63bce3f1aacfe 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> 2021-09-02T23:17:21+00:00 Masahiro Yamada masahiroy@kernel.org 2021-08-30T08:20:33+00:00 urn:sha1:bc7cd2dd1f8e5889cc68b69984033ac5bef6ba61 Commit 0e0345b77ac4 ("kbuild: redo fake deps at include/config/*.h") simplified the Kconfig/fixdep interaction a lot. For CONFIG_FOO_BAR_BAZ, Kconfig now touches include/config/FOO_BAR_BAZ instead of the previous include/config/foo/bar/baz.h . This commit simplifies the TRIM_UNUSED_KSYMS feature in a similar way: - delete .h suffix - delete tolower() - put everything in 1 directory For EXPORT_SYMBOL(FOO_BAR_BAZ), scripts/adjust_autoksyms.sh now touches include/ksym/FOO_BAR_BAZ instead of include/ksym/foo/bar/baz.h . This is more precise, avoiding possibly unnecessary rebuilds. EXPORT_SYMBOL(FOO_BAR_BAZ) EXPORT_SYMBOL(_FOO_BAR_BAZ) EXPORT_SYMBOL(__FOO_BAR_BAZ) were previously mapped to the same header, include/ksym/foo/bar/baz.h but now are handled separately. Signed-off-by: Masahiro Yamada <masahiroy@kernel.org> 2021-09-02T23:17:20+00:00 Masahiro Yamada masahiroy@kernel.org 2021-08-19T00:01:14+00:00 urn:sha1:52d83df682c82055961531853c066f4f16e234ea When CONFIG_TRIM_UNUSED_KSYMS is enabled, I see some warnings like this: nm: arch/x86/entry/vdso/vdso32/note.o: no symbols $NM (both GNU nm and llvm-nm) warns when no symbol is found in the object. Suppress the stderr. Fangrui Song mentioned binutils>=2.37 `nm -q` can be used to suppress "no symbols" [1], and llvm-nm>=13.0.0 supports -q as well. We cannot use it for now, but note it as a TODO. [1]: https://sourceware.org/bugzilla/show_bug.cgi?id=27408 Fixes: bbda5ec671d3 ("kbuild: simplify dependency generation for CONFIG_TRIM_UNUSED_KSYMS") Signed-off-by: Masahiro Yamada <masahiroy@kernel.org> Reviewed-by: Nathan Chancellor <nathan@kernel.org> 2018-12-01T14:13:14+00:00 Masahiro Yamada yamada.masahiro@socionext.com 2018-11-30T01:05:26+00:00 urn:sha1:bbda5ec671d3fe62faefa1cab7270aa586042a4b My main motivation of this commit is to clean up scripts/Kbuild.include and scripts/Makefile.build. Currently, CONFIG_TRIM_UNUSED_KSYMS works with a tricky gimmick; possibly exported symbols are detected by letting $(CPP) replace EXPORT_SYMBOL* with a special string '=== __KSYM_*===', which is post-processed by sed, and passed to fixdep. The extra preprocessing is costly, and hacking cmd_and_fixdep is ugly. I came up with a new way to find exported symbols; insert a dummy symbol __ksym_marker_* to each potentially exported symbol. Those dummy symbols are picked up by $(NM), post-processed by sed, then appended to .*.cmd files. I collected the post-process part to a new shell script scripts/gen_ksymdeps.sh for readability. The dummy symbols are put into the .discard.* section so that the linker script rips them off the final vmlinux or modules. A nice side-effect is building with CONFIG_TRIM_UNUSED_KSYMS will be much faster. Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com> Reviewed-by: Nicolas Pitre <nico@linaro.org>