[PATCH v2 0/5] kernel hacking: GCC optimization for debug experience (-Og)

[changbin . du]

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

    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