On Wed, May 02, 2018 at 09:44:55PM +0800, changbin.du@xxxxxxxxx wrote: > From: Changbin Du <changbin.du@xxxxxxxxx> > > Hi all, > I know some kernel developers was searching for a method to dissable GCC > optimizations, probably they want to apply GCC '-O0' option. But since Linux > kernel replys on GCC optimization to remove some dead code, so '-O0' just > breaks the build. They do need this because they want to debug kernel with > qemu, simics, kgtp or kgdb. > > Thanks for the GCC '-Og' optimization level introduced in GCC 4.8, which > 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. With '-Og', we can build a kernel with > better debug ability and little performance drop after some simple change. > > In this series, firstly introduce a new config CONFIG_NO_AUTO_INLINE after two > fixes for this new option. With this option, only functions explicitly marked > with "inline" will be inlined. This will allow the function tracer to trace > more functions because it only traces functions that the compiler has not > inlined. > > Then introduce new config CONFIG_DEBUG_EXPERIENCE which apply '-Og' > optimization level for whole kernel, with a simple fix in fix_to_virt(). > Currently this option is only tested on a QEMU gust and it works fine. > > > 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 In my mind was the opposite question. When running on the same kernel does a kernel whose config contains CONFIG_DEBUG_EXPERIENCE build faster than one without (due to the disabled optimization passes). To be honest this is more curiosity than a review comment though... if you have the figures please share, if not then don't sweat it on my account! Daniel. > > 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% ) > > Changbin Du (5): > x86/mm: surround level4_kernel_pgt with #ifdef > CONFIG_X86_5LEVEL...#endif > regulator: add dummy function of_find_regulator_by_node > kernel hacking: new config NO_AUTO_INLINE to disable compiler > auto-inline optimizations > kernel hacking: new config DEBUG_EXPERIENCE to apply GCC -Og > optimization > asm-generic: fix build error in fix_to_virt with > CONFIG_DEBUG_EXPERIENCE > > Makefile | 10 ++++++++++ > arch/x86/include/asm/pgtable_64.h | 2 ++ > arch/x86/kernel/head64.c | 13 ++++++------- > drivers/regulator/internal.h | 9 +++++++-- > include/asm-generic/fixmap.h | 3 ++- > include/linux/compiler-gcc.h | 2 +- > include/linux/compiler.h | 2 +- > lib/Kconfig.debug | 39 +++++++++++++++++++++++++++++++++++++++ > 8 files changed, 68 insertions(+), 12 deletions(-) > > -- > 2.7.4 >
