From: Changbin Du <changbin.du@xxxxxxxxx> 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 perfer keeping 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 be tested on x86_64 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_DEBUG_EXPERIENCE $ size vmlinux text data bss dec hex filename 22665554 9709674 2920908 35296136 21a9388 vmlinux w/ CONFIG_DEBUG_EXPERIENCE $ 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_DEBUG_EXPERIENCE $ 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_DEBUG_EXPERIENCE $ 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: Changbin Du <changbin.du@xxxxxxxxx> --- v2: o Improve performance benchmark as suggested by Ingo. o Grammar updates in description. (Randy Dunlap) --- Makefile | 4 ++++ include/linux/compiler-gcc.h | 2 +- include/linux/compiler.h | 2 +- lib/Kconfig.debug | 21 +++++++++++++++++++++ 4 files changed, 27 insertions(+), 2 deletions(-) diff --git a/Makefile b/Makefile index eb694f6..6a10469 100644 --- a/Makefile +++ b/Makefile @@ -639,6 +639,9 @@ 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_DEBUG_EXPERIENCE +KBUILD_CFLAGS += $(call cc-option, -Og) +else ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE KBUILD_CFLAGS += $(call cc-option,-Oz,-Os) KBUILD_CFLAGS += $(call cc-disable-warning,maybe-uninitialized,) @@ -649,6 +652,7 @@ 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 b4bf73f..b8b3832 100644 --- a/include/linux/compiler-gcc.h +++ b/include/linux/compiler-gcc.h @@ -192,7 +192,7 @@ #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__) -#ifndef __CHECKER__ +#if !defined(__CHECKER__) && !defined(CONFIG_DEBUG_EXPERIENCE) # define __compiletime_warning(message) __attribute__((warning(message))) # define __compiletime_error(message) __attribute__((error(message))) #endif /* __CHECKER__ */ diff --git a/include/linux/compiler.h b/include/linux/compiler.h index ab4711c..952cc7f 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -301,7 +301,7 @@ unsigned long read_word_at_a_time(const void *addr) * 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_DEBUG_EXPERIENCE) # define __compiletime_error_fallback(condition) \ do { ((void)sizeof(char[1 - 2 * condition])); } while (0) # endif diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index ab55801..e264199 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -216,6 +216,27 @@ config NO_AUTO_INLINE If unsure, select N. +config DEBUG_EXPERIENCE + bool "Optimize for better debugging experience (-Og)" + default n + select NO_AUTO_INLINE + depends on !CC_OPTIMIZE_FOR_SIZE + 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). Currently this option has only be tested on x86_64 platform. + + If unsure, select N. + config ENABLE_WARN_DEPRECATED bool "Enable __deprecated logic" default y -- 2.7.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html