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

[Daniel Thompson <daniel.thompson@linaro.org>]

On Wed, May 02, 2018 at 09:44:55PM +0800, changbin.du@intel.com wrote:
> 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 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
>