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

[Changbin Du]

On Sun, Oct 28, 2018 at 11:28:24AM +0900, Masahiro Yamada wrote:
> 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:
> 
>
Thanks for pointing this out and kind suggestions. I have tested with your
modification. Will update patch serias soon.

> 
> 
> 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

-- 
Thanks,
Changbin Du