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Date: Wed, 2 May 2018 10:17:07 -0400
From: Steven Rostedt <rostedt@...dmis.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,
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 4/5] kernel hacking: new config DEBUG_EXPERIENCE to
apply GCC -Og optimization
On Wed, 2 May 2018 21:44:59 +0800
changbin.du@...el.com wrote:
> From: Changbin Du <changbin.du@...el.com>
>
> This will apply GCC '-Og' optimization level which is supported
> since GCC 4.8. This optimization level 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.
>
> If enabling this option breaks your kernel, you should either
> disable this or find a fix (mostly in the arch code). Currently
> this option has only be tested on x86_64 platform.
If this becomes an issue, you probably need to add an arch config that
it depends on like CONFIG_HAVE_DEBUG_EXPERIENCE (or another name, as I
mention below).
>
> This option can satisfy people who was searching for a method
> to disable compiler optimizations so to achieve better kernel
> debugging experience with kgdb or qemu.
>
> The main problem of '-Og' is we must not use __attribute__((error(msg))).
> The compiler will report error though the call to error function
> still can be optimize out. So we must fallback to array tricky.
>
> Comparison of vmlinux size: a bit smaller.
>
> w/o CONFIG_DEBUG_EXPERIENCE
I hate the config name.
I probably can't come up with better ones but let's try:
CONFIG_DEBUG_OPTIMIZE ?
CONFIG_OPTIMIZE_DEBUG ?
But "EXPERIENCE" sounds like I'm on some DEBUG LSD.
> $ 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% )
>
> Signed-off-by: Changbin Du <changbin.du@...el.com>
>
> ---
> v2:
> o Improve performance benchmark as suggested by Ingo.
> o Grammar updates in description. (Randy Dunlap)
> ---
> Makefile | 4 ++++
> include/linux/compiler-gcc.h | 2 +-
> include/linux/compiler.h | 2 +-
> lib/Kconfig.debug | 21 +++++++++++++++++++++
> 4 files changed, 27 insertions(+), 2 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index eb694f6..6a10469 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -639,6 +639,9 @@ KBUILD_CFLAGS += $(call cc-disable-warning, format-truncation)
> KBUILD_CFLAGS += $(call cc-disable-warning, format-overflow)
> KBUILD_CFLAGS += $(call cc-disable-warning, int-in-bool-context)
>
> +ifdef CONFIG_DEBUG_EXPERIENCE
> +KBUILD_CFLAGS += $(call cc-option, -Og)
> +else
> ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
> KBUILD_CFLAGS += $(call cc-option,-Oz,-Os)
> KBUILD_CFLAGS += $(call cc-disable-warning,maybe-uninitialized,)
> @@ -649,6 +652,7 @@ else
> KBUILD_CFLAGS += -O2
> endif
> endif
> +endif
>
> KBUILD_CFLAGS += $(call cc-ifversion, -lt, 0409, \
> $(call cc-disable-warning,maybe-uninitialized,))
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index b4bf73f..b8b3832 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -192,7 +192,7 @@
>
> #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
>
> -#ifndef __CHECKER__
> +#if !defined(__CHECKER__) && !defined(CONFIG_DEBUG_EXPERIENCE)
> # define __compiletime_warning(message) __attribute__((warning(message)))
> # define __compiletime_error(message) __attribute__((error(message)))
> #endif /* __CHECKER__ */
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index ab4711c..952cc7f 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -301,7 +301,7 @@ unsigned long read_word_at_a_time(const void *addr)
> * sparse see a constant array size without breaking compiletime_assert on old
> * versions of GCC (e.g. 4.2.4), so hide the array from sparse altogether.
> */
> -# ifndef __CHECKER__
> +# if !defined(__CHECKER__) && !defined(CONFIG_DEBUG_EXPERIENCE)
> # define __compiletime_error_fallback(condition) \
> do { ((void)sizeof(char[1 - 2 * condition])); } while (0)
> # endif
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index ab55801..e264199 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -216,6 +216,27 @@ config NO_AUTO_INLINE
>
> If unsure, select N.
>
> +config DEBUG_EXPERIENCE
> + bool "Optimize for better debugging experience (-Og)"
> + default n
You don't need to add "default n" because that's the default if it
isn't specified.
-- Steve
> + select NO_AUTO_INLINE
> + depends on !CC_OPTIMIZE_FOR_SIZE
> + help
> + This will apply GCC '-Og' optimization level which is supported
> + since GCC 4.8. This optimization level offers a reasonable level
> + of optimization while maintaining fast compilation and a good
> + debugging experience. It is similar to '-O1' while preferring to
> + keep debug ability over runtime speed. The overall performance
> + will drop a bit (~6%).
> +
> + Use only if you want to debug the kernel, especially if you want
> + to have better kernel debugging experience with gdb facilities
> + like kgdb or qemu. If enabling this option breaks your kernel,
> + you should either disable this or find a fix (mostly in the arch
> + code). Currently this option has only be tested on x86_64 platform.
> +
> + If unsure, select N.
> +
> config ENABLE_WARN_DEPRECATED
> bool "Enable __deprecated logic"
> default y
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