Re: [PATCH v2 4/5] kernel hacking: new config DEBUG_EXPERIENCE to apply GCC -Og optimization

[Steven Rostedt]

On Wed,  2 May 2018 21:44:59 +0800
changbin.du@xxxxxxxxx wrote:

> From: Changbin Du <changbin.du@xxxxxxxxx>
> 
> 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@xxxxxxxxx>
> 
> ---
> 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