Re: [PATCH v2 4/4] kernel hacking: new config CC_OPTIMIZE_FOR_DEBUGGING to apply GCC -Og optimization
From: Masahiro Yamada
Date: Sat Oct 27 2018 - 22:29:25 EST
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