Messages in this thread | | | From | Masahiro Yamada <> | Date | Thu, 11 Mar 2021 05:49:42 +0900 | Subject | Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION |
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On Mon, Mar 1, 2021 at 10:11 AM Nicholas Piggin <npiggin@gmail.com> wrote: > > Excerpts from Arnd Bergmann's message of February 27, 2021 7:49 pm: > > On Fri, Feb 26, 2021 at 10:13 PM 'Fangrui Song' via Clang Built Linux > > <clang-built-linux@googlegroups.com> wrote: > >> > >> For folks who are interested in --gc-sections on metadata sections, > >> I want to bring you awareness of the implication of __start_/__stop_ symbols and C identifier name sections. > >> You can see https://github.com/ClangBuiltLinux/linux/issues/1307 for a summary. > >> (Its linked blog article has some examples.) > >> > >> In the kernel linker scripts, most C identifier name sections begin with double-underscore __. > >> Some are surrounded by `KEEP(...)`, some are not. > >> > >> * A `KEEP` keyword has GC root semantics and makes ld --gc-sections ineffectful. > >> * Without `KEEP`, __start_/__stop_ references from a live input section > >> can unnecessarily retain all the associated C identifier name input > >> sections. The new ld.lld option `-z start-stop-gc` can defeat this rule. > >> > >> As an example, a __start___jump_table reference from a live section > >> causes all `__jump_table` input section to be retained, even if you > >> change `KEEP(__jump_table)` to `(__jump_table)`. > >> (If you change the symbol name from `__start_${section}` to something > >> else (e.g. `__start${section}`), the rule will not apply.) > > > > I suspect the __start_* symbols are cargo-culted by many developers > > copying stuff around between kernel linker scripts, that's certainly how I > > approach making changes to it normally without a deeper understanding > > of how the linker actually works or what the different bits of syntax mean > > there. > > > > I see the original vmlinux.lds linker script showed up in linux-2.1.23, and > > it contained > > > > + . = ALIGN(16); /* Exception table */ > > + __start___ex_table = .; > > + __ex_table : { *(__ex_table) } > > + __stop___ex_table = .; > > + > > + __start___ksymtab = .; /* Kernel symbol table */ > > + __ksymtab : { *(__ksymtab) } > > + __stop___ksymtab = .; > > > > originally for arch/sparc, and shortly afterwards for i386. The magic > > __ex_table section was first used in linux-2.1.7 without a linker > > script. It's probably a good idea to try cleaning these up by using > > non-magic start/stop symbols for all sections, and relying on KEEP() > > instead where needed. > > > >> There are a lot of KEEP usage. Perhaps some can be dropped to facilitate > >> ld --gc-sections. > > > > I see a lot of these were added by Nick Piggin (added to Cc) in this commit: > > > > commit 266ff2a8f51f02b429a987d87634697eb0d01d6a > > Author: Nicholas Piggin <npiggin@gmail.com> > > Date: Wed May 9 22:59:58 2018 +1000 > > > > kbuild: Fix asm-generic/vmlinux.lds.h for LD_DEAD_CODE_DATA_ELIMINATION > > > > KEEP more tables, and add the function/data section wildcard to more > > section selections. > > > > This is a little ad-hoc at the moment, but kernel code should be moved > > to consistently use .text..x (note: double dots) for explicit sections > > and all references to it in the linker script can be made with > > TEXT_MAIN, and similarly for other sections. > > > > For now, let's see if major architectures move to enabling this option > > then we can do some refactoring passes. Otherwise if it remains unused > > or superseded by LTO, this may not be required. > > > > Signed-off-by: Nicholas Piggin <npiggin@gmail.com> > > Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com> > > > > which apparently was intentionally cautious. > > > > Unlike what Nick expected in his submission, I now think the annotations > > will be needed for LTO just like they are for --gc-sections. > > Yeah I wasn't sure exactly what LTO looks like or how it would work. > I thought perhaps LTO might be able to find dead code with circular / > back references, we could put references from the code back to these > tables or something so they would be kept without KEEP. I don't know, I > was handwaving! > > I managed to get powerpc (and IIRC x86?) working with gc sections with > those KEEP annotations, but effectiveness of course is far worse than > what Nicolas was able to achieve with all his techniques and tricks. > > But yes unless there is some other mechanism to handle these tables, > then KEEP probably has to stay. I suggest this wants a very explicit and > systematic way to handle it (maybe with some toolchain support) rather > than trying to just remove things case by case and see what breaks. > > I don't know if Nicolas is still been working on his shrinking patches > recenty but he probably knows more than anyone about this stuff. > > Thanks, > Nick >
I tested LD_DEAD_CODE_DATA_ELIMINATION for the latest kernel.
I added an unused function, this_func_is_unused(), then built the ppc kernel with LD_DEAD_CODE_DATA_ELIMINATION.
It remained in vmlinux.
masahiro@oscar:~/ref/linux$ echo 'void this_func_is_unused(void) {}' >> kernel/cpu.c masahiro@oscar:~/ref/linux$ export CROSS_COMPILE=/home/masahiro/tools/powerpc-10.1.0/bin/powerpc-linux- masahiro@oscar:~/ref/linux$ make ARCH=powerpc defconfig masahiro@oscar:~/ref/linux$ ./scripts/config -e EXPERT masahiro@oscar:~/ref/linux$ ./scripts/config -e LD_DEAD_CODE_DATA_ELIMINATION masahiro@oscar:~/ref/linux$ ~/tools/powerpc-10.1.0/bin/powerpc-linux-nm -n vmlinux | grep this_func c000000000170560 T .this_func_is_unused c000000001d8d560 D this_func_is_unused masahiro@oscar:~/ref/linux$ grep DEAD_CODE_ .config CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y
If I remember correctly, LD_DEAD_CODE_DATA_ELIMINATION dropped unused functions when I tried it last time.
I also tried arm64 with a HAVE_LD_DEAD_CODE_DATA_ELIMINATION hack. The result was the same.
Am I missing something?
-- Best Regards Masahiro Yamada
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