Messages in this thread | | | From | Nick Desaulniers <> | Date | Thu, 14 Oct 2021 13:59:12 -0700 | Subject | Re: [PATCH 2/3] arm64: vdso32: lazily invoke COMPAT_CC |
| |
On Tue, Oct 12, 2021 at 8:03 PM Masahiro Yamada <masahiroy@kernel.org> wrote: > > On Wed, Oct 13, 2021 at 8:46 AM Nick Desaulniers > <ndesaulniers@google.com> wrote: > > > > When running the following command without arm-linux-gnueabi-gcc in > > one's $PATH, the following warning is observed: > > > > $ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make -j72 LLVM=1 mrproper > > make[1]: arm-linux-gnueabi-gcc: No such file or directory > > > > This is because KCONFIG is not run for mrproper, so CONFIG_CC_IS_CLANG > > is not set, and we end up eagerly evaluating various variables that try > > to invoke CC_COMPAT. > > > > This is a similar problem to what was observed in > > commit 3ec8a5b33dea ("kbuild: do not export LDFLAGS_vmlinux") > > > > Cc: Masahiro Yamada <masahiroy@kernel.org> > > Reported-by: Lucas Henneman <henneman@google.com> > > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> > > > There are two ways to fix it: > > [1]: sink the error message to /dev/null > (as in commit dc960bfeedb01cf832c5632ed1f3daed4416b142) > [2]: use a recursively-expanded variable as you did. > > > "Simple variable (:=) vs recursive variable (=)" is a trade-off. > > Please be careful about the cost when you try the [2] approach. > > > > Simple variables are immediately expanded while parsing Makefile. > There are 7 call-sites for cc32-option, hence > the compiler is invoked 7 times for building vdso32, > 0 times for cleaning. > (Since 57fd251c789647552d32d2fc51bedd4f90d70f9f, > try-run is no-op for 'make clean'). > > > > > Recursive variables are expanded every time they are used. > > IIUC, if_changed expands the command line 3 times. > There are 2 objects (note.o and vgettimeofday.o) > There are 7 call-sites for cc32-option. > > So, the compiler is invoked 42 (3 * 2 * 7) times > for building vdso32.
With this patch applied: $ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make LLVM=1 -j72 clean defconfig $ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make LLVM=1 -j72 arch/arm64/kernel/vdso32/ V=1 | tr -s ' ' | cut -d ' ' -f 2 | grep clang | wc -l 55 $ find arch/arm64/kernel/vdso32/ -name \*.o | xargs rm $ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make LLVM=1 -j72 arch/arm64/kernel/vdso32/ V=1 | tr -s ' ' | cut -d ' ' -f 2 | grep clang | wc -l 2
Prior to this series: $ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make LLVM=1 -j72 clean defconfig $ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make LLVM=1 -j72 arch/arm64/kernel/vdso32/ V=1 | tr -s ' ' | cut -d ' ' -f 2 | grep clang | wc -l 55 $ find arch/arm64/kernel/vdso32/ -name \*.o | xargs rm $ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make LLVM=1 -j72 arch/arm64/kernel/vdso32/ V=1 | tr -s ' ' | cut -d ' ' -f 2 | grep clang | wc -l 2
With patch 3 applied, we can drop CROSS_COMPILE_COMPAT, and we now get: $ ARCH=arm64 make LLVM=1 -j72 clean defconfig ARCH=arm64 make LLVM=1 -j72 arch/arm64/kernel/vdso32/ V=1 | tr -s ' ' | cut -d ' ' -f 2 | grep clang | wc -l 44 $ find arch/arm64/kernel/vdso32/ -name \*.o | xargs rm $ ARCH=arm64 make LLVM=1 -j72 arch/arm64/kernel/vdso32/ V=1 | tr -s ' ' | cut -d ' ' -f 2 | grep clang | wc -l 2
Please confirm; perhaps my pipeline missed some invocations? Or was there a different target you were referring to?
-- Thanks, ~Nick Desaulniers
| |