On Wed, 2 May 2018 21:44:59 +0800 [email protected] wrote: > From: Changbin Du <[email protected]> > > 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.
If this becomes an issue, you probably need to add an arch config that it depends on like CONFIG_HAVE_DEBUG_EXPERIENCE (or another name, as I mention below). > > 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 I hate the config name. I probably can't come up with better ones but let's try: CONFIG_DEBUG_OPTIMIZE ? CONFIG_OPTIMIZE_DEBUG ? But "EXPERIENCE" sounds like I'm on some DEBUG LSD. > $ 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 <[email protected]> > > --- > 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 You don't need to add "default n" because that's the default if it isn't specified. -- Steve > + 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
