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

--
Thanks,
Changbin Du

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