[v2,4/4] kernel hacking: new config CC_OPTIMIZE_FOR_DEBUGGING to apply GCC -Og optimization
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Message ID 20181019124921.13780-5-changbin.du@gmail.com
State Superseded
Headers show
Series
  • kernel hacking: GCC optimization for better debug experience (-Og)
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Commit Message

Changbin Du Oct. 19, 2018, 12:49 p.m. UTC
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@gmail.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 Makefile                     |  5 +++++
 include/linux/compiler-gcc.h |  2 +-
 include/linux/compiler.h     |  2 +-
 init/Kconfig                 | 19 +++++++++++++++++++
 4 files changed, 26 insertions(+), 2 deletions(-)

Comments

Masahiro Yamada Oct. 28, 2018, 2:28 a.m. UTC | #1
On Fri, Oct 19, 2018 at 9:50 PM Du Changbin <changbin.du@gmail.com> 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@gmail.com>
> Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
>  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
>
Changbin Du Oct. 28, 2018, 12:51 p.m. UTC | #2
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@gmail.com> 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@gmail.com>
> > Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> > ---
> >  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
Masahiro Yamada Oct. 28, 2018, 1:09 p.m. UTC | #3
Hi Changbin,

On Sun, Oct 28, 2018 at 9:52 PM Changbin Du <changbin.du@gmail.com> wrote:

> >
> Thanks for pointing this out and kind suggestions. I have tested with your
> modification. Will update patch serias soon.


I have one more request.

The patch subject
"kernel hacking: new config CC_OPTIMIZE_FOR_DEBUGGING to apply GCC -Og
optimization"
is too long (83 characters)


I'd like the subject to be within 70 columns or so.

Thanks.
Changbin Du Oct. 28, 2018, 1:14 p.m. UTC | #4
On Sun, Oct 28, 2018 at 10:09:21PM +0900, Masahiro Yamada wrote:
> Hi Changbin,
> 
> On Sun, Oct 28, 2018 at 9:52 PM Changbin Du <changbin.du@gmail.com> wrote:
> 
> > >
> > Thanks for pointing this out and kind suggestions. I have tested with your
> > modification. Will update patch serias soon.
> 
> 
> I have one more request.
> 
> The patch subject
> "kernel hacking: new config CC_OPTIMIZE_FOR_DEBUGGING to apply GCC -Og
> optimization"
> is too long (83 characters)
> 
> 
> I'd like the subject to be within 70 columns or so.
> 
oh, I just sent new version. Could me update the title if there are more
suggestions then?

> Thanks.
> 
> 
> 
> 
> -- 
> Best Regards
> Masahiro Yamada
Masahiro Yamada Oct. 29, 2018, 1:01 p.m. UTC | #5
Hi.


On Sun, Oct 28, 2018 at 10:14 PM Changbin Du <changbin.du@gmail.com> wrote:
>
> On Sun, Oct 28, 2018 at 10:09:21PM +0900, Masahiro Yamada wrote:
> > Hi Changbin,
> >
> > On Sun, Oct 28, 2018 at 9:52 PM Changbin Du <changbin.du@gmail.com> wrote:
> >
> > > >
> > > Thanks for pointing this out and kind suggestions. I have tested with your
> > > modification. Will update patch serias soon.
> >
> >
> > I have one more request.
> >
> > The patch subject
> > "kernel hacking: new config CC_OPTIMIZE_FOR_DEBUGGING to apply GCC -Og
> > optimization"
> > is too long (83 characters)
> >
> >
> > I'd like the subject to be within 70 columns or so.
> >
> oh, I just sent new version. Could me update the title if there are more
> suggestions then?



How about this?

2/4: kernel hacking: add a config option to disable compiler auto-inlining
3/4: ARM: mm: fix build error in fix_to_virt with -Og optimization level
4/4: kernel hacking: support building kernel with -Og optimization level

(The CONFIG name is so long that the subject
would easily exceeds 80-columns with it.)


I applied this series to linux-kbuild,
and will send another pull request in this MW.


If you come up with better patch titles,
please let me know.
I will fix them up locally.
Changbin Du Oct. 29, 2018, 2:56 p.m. UTC | #6
On Mon, Oct 29, 2018 at 10:01:44PM +0900, Masahiro Yamada wrote:
> Hi.
> 
> 
> On Sun, Oct 28, 2018 at 10:14 PM Changbin Du <changbin.du@gmail.com> wrote:
> >
> > On Sun, Oct 28, 2018 at 10:09:21PM +0900, Masahiro Yamada wrote:
> > > Hi Changbin,
> > >
> > > On Sun, Oct 28, 2018 at 9:52 PM Changbin Du <changbin.du@gmail.com> wrote:
> > >
> > > > >
> > > > Thanks for pointing this out and kind suggestions. I have tested with your
> > > > modification. Will update patch serias soon.
> > >
> > >
> > > I have one more request.
> > >
> > > The patch subject
> > > "kernel hacking: new config CC_OPTIMIZE_FOR_DEBUGGING to apply GCC -Og
> > > optimization"
> > > is too long (83 characters)
> > >
> > >
> > > I'd like the subject to be within 70 columns or so.
> > >
> > oh, I just sent new version. Could me update the title if there are more
> > suggestions then?
> 
> 
> 
> How about this?
> 
> 2/4: kernel hacking: add a config option to disable compiler auto-inlining
> 3/4: ARM: mm: fix build error in fix_to_virt with -Og optimization level
> 4/4: kernel hacking: support building kernel with -Og optimization level
> 
> (The CONFIG name is so long that the subject
> would easily exceeds 80-columns with it.)
> 
> 
> I applied this series to linux-kbuild,
> and will send another pull request in this MW.
> 
> 
> If you come up with better patch titles,
> please let me know.
> I will fix them up locally.
>
I am just fine with these new titles. Thanks.

> 
> 
> -- 
> Best Regards
> Masahiro Yamada

Patch
diff mbox series

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)
+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