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Message-ID: <20180502145631.bzfqpw4izlrqhda6@holly.lan>
Date: Wed, 2 May 2018 15:56:31 +0100
From: Daniel Thompson <daniel.thompson@...aro.org>
To: changbin.du@...el.com
Cc: yamada.masahiro@...ionext.com, michal.lkml@...kovi.net,
tglx@...utronix.de, mingo@...hat.com, akpm@...ux-foundation.org,
rostedt@...dmis.org, rdunlap@...radead.org, x86@...nel.org,
lgirdwood@...il.com, broonie@...nel.org, arnd@...db.de,
linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-arch@...r.kernel.org
Subject: Re: [PATCH v2 0/5] kernel hacking: GCC optimization for debug
experience (-Og)
On Wed, May 02, 2018 at 09:44:55PM +0800, changbin.du@...el.com wrote:
> From: Changbin Du <changbin.du@...el.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
>
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