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

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On Fri, Oct 19, 2018 at 9:50 PM Du Changbin <changbin.du@xxxxxxxxx> wrote:
>
> 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 perferring
> to keep 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 been tested on x86_64 and arm platform.
>
> 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_CC_OPTIMIZE_FOR_DEBUGGING
>     $ size vmlinux
>        text    data     bss     dec     hex filename
>     22665554   9709674  2920908 35296136        21a9388 vmlinux
>
>     w/ CONFIG_CC_OPTIMIZE_FOR_DEBUGGING
>     $ 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_CC_OPTIMIZE_FOR_DEBUGGING
>     $ 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_CC_OPTIMIZE_FOR_DEBUGGING
>     $ 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: Du Changbin <changbin.du@xxxxxxxxx>
> Acked-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> ---
>  Makefile                     |  5 +++++
>  include/linux/compiler-gcc.h |  2 +-
>  include/linux/compiler.h     |  2 +-
>  init/Kconfig                 | 19 +++++++++++++++++++
>  4 files changed, 26 insertions(+), 2 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 757d6507cb5c..ea908cfe8594 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -657,6 +657,10 @@ 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_CC_OPTIMIZE_FOR_DEBUGGING
> +KBUILD_CFLAGS  += $(call cc-option, -Og)


This line is a problem.

As you stated in your patch description, -Og is supported on GCC 4.8+
but the minimum GCC version supported for Linux kernel is GCC 4.6


If you use either GCC 4.6 or 4.7,
$(call cc-option, -Og) is evaluated to empty.

It means no optimization flag is given to the compiler, then
the build fails.

I'd like to suggest to change as follows:




diff --git a/Makefile b/Makefile
index 122ab74..9af3dc4 100644
--- a/Makefile
+++ b/Makefile
@@ -663,7 +663,7 @@ KBUILD_CFLAGS       += $(call cc-disable-warning,
format-overflow)
 KBUILD_CFLAGS  += $(call cc-disable-warning, int-in-bool-context)

 ifdef CONFIG_CC_OPTIMIZE_FOR_DEBUGGING
-KBUILD_CFLAGS  += $(call cc-option, -Og)
+KBUILD_CFLAGS  += -Og
 KBUILD_CFLAGS  += $(call cc-disable-warning,maybe-uninitialized,)
 else
 ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
diff --git a/init/Kconfig b/init/Kconfig
index b01c7c6..a0424dc 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1083,6 +1083,7 @@ config CC_OPTIMIZE_FOR_SIZE

 config CC_OPTIMIZE_FOR_DEBUGGING
        bool "Optimize for better debugging experience (-Og)"
+       depends on $(cc-option,-Og)
        select NO_AUTO_INLINE
        help
          This will apply GCC '-Og' optimization level which is supported





If -Og is unsupported by the compiler, this is hidden from the Kconfig menu.



Also, in order to fix a warning from 'make tiny.config'
could you add the following change?



diff --git a/kernel/configs/tiny.config b/kernel/configs/tiny.config
index 7fa0c4a..599ea86 100644
--- a/kernel/configs/tiny.config
+++ b/kernel/configs/tiny.config
@@ -1,5 +1,6 @@
 # CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE is not set
 CONFIG_CC_OPTIMIZE_FOR_SIZE=y
+# CONFIG_CC_OPTIMIZE_FOR_DEBUGGING is not set
 # CONFIG_KERNEL_GZIP is not set
 # CONFIG_KERNEL_BZIP2 is not set
 # CONFIG_KERNEL_LZMA is not set







> +KBUILD_CFLAGS  += $(call cc-disable-warning,maybe-uninitialized,)
> +else
>  ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
>  KBUILD_CFLAGS  += $(call cc-option,-Oz,-Os)
>  KBUILD_CFLAGS  += $(call cc-disable-warning,maybe-uninitialized,)
> @@ -667,6 +671,7 @@ 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 4d36b27214fd..2a76f7c64b54 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -85,7 +85,7 @@
>
>  #define __compiletime_object_size(obj) __builtin_object_size(obj, 0)
>
> -#ifndef __CHECKER__
> +#if !defined(__CHECKER__) && !defined(CONFIG_CC_OPTIMIZE_FOR_DEBUGGING)
>  #define __compiletime_warning(message) __attribute__((warning(message)))
>  #define __compiletime_error(message) __attribute__((error(message)))
>
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 681d866efb1e..9385c62e9f00 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -320,7 +320,7 @@ static inline void *offset_to_ptr(const int *off)
>   * 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_CC_OPTIMIZE_FOR_DEBUGGING)
>  #  define __compiletime_error_fallback(condition) \
>         do { ((void)sizeof(char[1 - 2 * condition])); } while (0)
>  # endif
> diff --git a/init/Kconfig b/init/Kconfig
> index 1e234e2f1cba..b01c7c654570 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1081,6 +1081,25 @@ config CC_OPTIMIZE_FOR_SIZE
>
>           If unsure, say N.
>
> +config CC_OPTIMIZE_FOR_DEBUGGING
> +       bool "Optimize for better debugging experience (-Og)"
> +       select NO_AUTO_INLINE
> +       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).
> +
> +         If unsure, select N.
> +
>  endchoice
>
>  config HAVE_LD_DEAD_CODE_DATA_ELIMINATION
> --
> 2.17.1
>


-- 
Best Regards
Masahiro Yamada



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