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Subject[PATCH v5 0/4] kernel hacking: GCC optimization for better debug experience (-Og)
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From: Changbin Du <changbin.du@intel.com>

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 perferring to
keep 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 CC_OPTIMIZE_FOR_DEBUGGING which apply '-Og'
optimization level for whole kernel, with a simple fix in fix_to_virt().
Currently I have only tested this option on x86 and ARM platform. Other
platforms should also work but probably need some compiling fixes as what
having done in this series. I leave that to who want to try this debug
option.

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% )

v5:
o Exchange the position of last two patches to avoid compiling error.
v4:
o Remove aready merged one "regulator: add dummy function of_find_regulator_by_node".

Changbin Du (4):
x86/mm: surround level4_kernel_pgt with #ifdef
CONFIG_X86_5LEVEL...#endif
kernel hacking: new config NO_AUTO_INLINE to disable compiler
auto-inline optimizations
ARM: mm: fix build error in fix_to_virt with
CONFIG_CC_OPTIMIZE_FOR_DEBUGGING
kernel hacking: new config CC_OPTIMIZE_FOR_DEBUGGING to apply GCC -Og
optimization

Makefile | 10 ++++++++++
arch/arm/mm/mmu.c | 2 +-
arch/x86/include/asm/pgtable_64.h | 2 ++
arch/x86/kernel/head64.c | 13 ++++++-------
include/linux/compiler-gcc.h | 2 +-
include/linux/compiler.h | 2 +-
init/Kconfig | 19 +++++++++++++++++++
lib/Kconfig.debug | 17 +++++++++++++++++
8 files changed, 57 insertions(+), 10 deletions(-)

--
2.7.4

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