On Fri, Oct 19, 2018 at 9:50 PM Du Changbin <changbin.du@xxxxxxxxx> wrote: > > This will apply GCC '-Og' optimization level which is supported > since GCC 4.8. This optimization level offers a reasonable level > of optimization while maintaining fast compilation and a good > debugging experience. It is similar to '-O1' while perferring > to keep debug ability over runtime speed. > > If enabling this option breaks your kernel, you should either > disable this or find a fix (mostly in the arch code). Currently > this option has only been tested on x86_64 and arm platform. > > This option can satisfy people who was searching for a method > to disable compiler optimizations so to achieve better kernel > debugging experience with kgdb or qemu. > > The main problem of '-Og' is we must not use __attribute__((error(msg))). > The compiler will report error though the call to error function > still can be optimize out. So we must fallback to array tricky. > > Comparison of vmlinux size: a bit smaller. > > w/o CONFIG_CC_OPTIMIZE_FOR_DEBUGGING > $ size vmlinux > text data bss dec hex filename > 22665554 9709674 2920908 35296136 21a9388 vmlinux > > w/ CONFIG_CC_OPTIMIZE_FOR_DEBUGGING > $ size vmlinux > text data bss dec hex filename > 21499032 10102758 2920908 34522698 20ec64a vmlinux > > Comparison of system performance: a bit drop (~6%). > This benchmark of kernel compilation is suggested by Ingo Molnar. > https://lkml.org/lkml/2018/5/2/74 > > Preparation: Set cpufreq to 'performance'. > for ((cpu=0; cpu<120; cpu++)); do > G=/sys/devices/system/cpu/cpu$cpu/cpufreq/scaling_governor > [ -f $G ] && echo performance > $G > done > > w/o CONFIG_CC_OPTIMIZE_FOR_DEBUGGING > $ perf stat --repeat 5 --null --pre '\ > cp -a kernel ../kernel.copy.$(date +%s); \ > rm -rf *; \ > git checkout .; \ > echo 1 > /proc/sys/vm/drop_caches; \ > find ../kernel* -type f | xargs cat >/dev/null; \ > make -j kernel >/dev/null; \ > make clean >/dev/null 2>&1; \ > sync '\ > \ > make -j8 >/dev/null > > Performance counter stats for 'make -j8' (5 runs): > > 219.764246652 seconds time elapsed ( +- 0.78% ) > > w/ CONFIG_CC_OPTIMIZE_FOR_DEBUGGING > $ perf stat --repeat 5 --null --pre '\ > cp -a kernel ../kernel.copy.$(date +%s); \ > rm -rf *; \ > git checkout .; \ > echo 1 > /proc/sys/vm/drop_caches; \ > find ../kernel* -type f | xargs cat >/dev/null; \ > make -j kernel >/dev/null; \ > make clean >/dev/null 2>&1; \ > sync '\ > \ > make -j8 >/dev/null > > Performance counter stats for 'make -j8' (5 runs): > > 233.574187771 seconds time elapsed ( +- 0.19% ) > > Signed-off-by: Du Changbin <changbin.du@xxxxxxxxx> > Acked-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> > --- > Makefile | 5 +++++ > include/linux/compiler-gcc.h | 2 +- > include/linux/compiler.h | 2 +- > init/Kconfig | 19 +++++++++++++++++++ > 4 files changed, 26 insertions(+), 2 deletions(-) > > diff --git a/Makefile b/Makefile > index 757d6507cb5c..ea908cfe8594 100644 > --- a/Makefile > +++ b/Makefile > @@ -657,6 +657,10 @@ KBUILD_CFLAGS += $(call cc-disable-warning, format-truncation) > KBUILD_CFLAGS += $(call cc-disable-warning, format-overflow) > KBUILD_CFLAGS += $(call cc-disable-warning, int-in-bool-context) > > +ifdef CONFIG_CC_OPTIMIZE_FOR_DEBUGGING > +KBUILD_CFLAGS += $(call cc-option, -Og) This line is a problem. As you stated in your patch description, -Og is supported on GCC 4.8+ but the minimum GCC version supported for Linux kernel is GCC 4.6 If you use either GCC 4.6 or 4.7, $(call cc-option, -Og) is evaluated to empty. It means no optimization flag is given to the compiler, then the build fails. I'd like to suggest to change as follows: diff --git a/Makefile b/Makefile index 122ab74..9af3dc4 100644 --- a/Makefile +++ b/Makefile @@ -663,7 +663,7 @@ KBUILD_CFLAGS += $(call cc-disable-warning, format-overflow) KBUILD_CFLAGS += $(call cc-disable-warning, int-in-bool-context) ifdef CONFIG_CC_OPTIMIZE_FOR_DEBUGGING -KBUILD_CFLAGS += $(call cc-option, -Og) +KBUILD_CFLAGS += -Og KBUILD_CFLAGS += $(call cc-disable-warning,maybe-uninitialized,) else ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE diff --git a/init/Kconfig b/init/Kconfig index b01c7c6..a0424dc 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1083,6 +1083,7 @@ config CC_OPTIMIZE_FOR_SIZE config CC_OPTIMIZE_FOR_DEBUGGING bool "Optimize for better debugging experience (-Og)" + depends on $(cc-option,-Og) select NO_AUTO_INLINE help This will apply GCC '-Og' optimization level which is supported If -Og is unsupported by the compiler, this is hidden from the Kconfig menu. Also, in order to fix a warning from 'make tiny.config' could you add the following change? diff --git a/kernel/configs/tiny.config b/kernel/configs/tiny.config index 7fa0c4a..599ea86 100644 --- a/kernel/configs/tiny.config +++ b/kernel/configs/tiny.config @@ -1,5 +1,6 @@ # CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE is not set CONFIG_CC_OPTIMIZE_FOR_SIZE=y +# CONFIG_CC_OPTIMIZE_FOR_DEBUGGING is not set # CONFIG_KERNEL_GZIP is not set # CONFIG_KERNEL_BZIP2 is not set # CONFIG_KERNEL_LZMA is not set > +KBUILD_CFLAGS += $(call cc-disable-warning,maybe-uninitialized,) > +else > ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE > KBUILD_CFLAGS += $(call cc-option,-Oz,-Os) > KBUILD_CFLAGS += $(call cc-disable-warning,maybe-uninitialized,) > @@ -667,6 +671,7 @@ else > KBUILD_CFLAGS += -O2 > endif > endif > +endif > > KBUILD_CFLAGS += $(call cc-ifversion, -lt, 0409, \ > $(call cc-disable-warning,maybe-uninitialized,)) > diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h > index 4d36b27214fd..2a76f7c64b54 100644 > --- a/include/linux/compiler-gcc.h > +++ b/include/linux/compiler-gcc.h > @@ -85,7 +85,7 @@ > > #define __compiletime_object_size(obj) __builtin_object_size(obj, 0) > > -#ifndef __CHECKER__ > +#if !defined(__CHECKER__) && !defined(CONFIG_CC_OPTIMIZE_FOR_DEBUGGING) > #define __compiletime_warning(message) __attribute__((warning(message))) > #define __compiletime_error(message) __attribute__((error(message))) > > diff --git a/include/linux/compiler.h b/include/linux/compiler.h > index 681d866efb1e..9385c62e9f00 100644 > --- a/include/linux/compiler.h > +++ b/include/linux/compiler.h > @@ -320,7 +320,7 @@ static inline void *offset_to_ptr(const int *off) > * sparse see a constant array size without breaking compiletime_assert on old > * versions of GCC (e.g. 4.2.4), so hide the array from sparse altogether. > */ > -# ifndef __CHECKER__ > +# if !defined(__CHECKER__) && !defined(CONFIG_CC_OPTIMIZE_FOR_DEBUGGING) > # define __compiletime_error_fallback(condition) \ > do { ((void)sizeof(char[1 - 2 * condition])); } while (0) > # endif > diff --git a/init/Kconfig b/init/Kconfig > index 1e234e2f1cba..b01c7c654570 100644 > --- a/init/Kconfig > +++ b/init/Kconfig > @@ -1081,6 +1081,25 @@ config CC_OPTIMIZE_FOR_SIZE > > If unsure, say N. > > +config CC_OPTIMIZE_FOR_DEBUGGING > + bool "Optimize for better debugging experience (-Og)" > + select NO_AUTO_INLINE > + help > + This will apply GCC '-Og' optimization level which is supported > + since GCC 4.8. This optimization level offers a reasonable level > + of optimization while maintaining fast compilation and a good > + debugging experience. It is similar to '-O1' while preferring to > + keep debug ability over runtime speed. The overall performance > + will drop a bit (~6%). > + > + Use only if you want to debug the kernel, especially if you want > + to have better kernel debugging experience with gdb facilities > + like kgdb or qemu. If enabling this option breaks your kernel, > + you should either disable this or find a fix (mostly in the arch > + code). > + > + If unsure, select N. > + > endchoice > > config HAVE_LD_DEAD_CODE_DATA_ELIMINATION > -- > 2.17.1 > -- Best Regards Masahiro Yamada
