Re: [PATCH 2/3] arm64: vdso32: lazily invoke COMPAT_CC
From: Nick Desaulniers <ndesaulniers@google.com>
Date: 2021-10-14 20:59:29
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On Tue, Oct 12, 2021 at 8:03 PM Masahiro Yamada [off-list ref] wrote:
On Wed, Oct 13, 2021 at 8:46 AM Nick Desaulniers [off-list ref] wrote:quoted
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 <redacted> 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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel