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Subject[PATCH 0/5] kernel hacking: GCC optimization for 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 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 serias, firstly introduce a new config CONFIG_NO_AUTO_INLINE after two
fixes. Selecting this option will make compiler not auto-inline kernel
functions.This is useful when you are using ftrace to understand the control
flow of kernel code or tracing some static functions.

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.

w/o CONFIG_DEBUG_EXPERIENCE
$ time make -j4
real 6m43.619s
user 19m5.160s
sys 2m20.287s

w/ CONFIG_DEBUG_EXPERIENCE
$ time make -j4
real 6m55.054s
user 19m11.129s
sys 2m36.345s

Changbin Du (5):
x86/mm: surround level4_kernel_pgt with #ifdef
CONFIG_X86_5LEVEL...#endif
regulator: add dummy of_find_regulator_by_node
kernel hacking: new config NO_AUTO_INLINE to disable compiler
atuo-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 | 34 ++++++++++++++++++++++++++++++++++
8 files changed, 63 insertions(+), 12 deletions(-)

--
2.7.4

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