From: changbin.du@intel.com
To: yamada.masahiro@socionext.com, michal.lkml@markovi.net,
tglx@linutronix.de, mingo@redhat.com, akpm@linux-foundation.org
Cc: rostedt@goodmis.org, rdunlap@infradead.org, x86@kernel.org,
lgirdwood@gmail.com, broonie@kernel.org, arnd@arndb.de,
linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-arch@vger.kernel.org, Changbin Du <changbin.du@intel.com>
Subject: [PATCH v3 4/5] kernel hacking: new config CC_OPTIMIZE_FOR_DEBUGGING to apply GCC -Og optimization
Date: Sun, 6 May 2018 08:20:15 +0800 [thread overview]
Message-ID: <1525566016-30172-5-git-send-email-changbin.du@intel.com> (raw)
In-Reply-To: <1525566016-30172-1-git-send-email-changbin.du@intel.com>
From: Changbin Du <changbin.du@intel.com>
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: Changbin Du <changbin.du@intel.com>
---
v3:
o Rename DEBUG_EXPERIENCE to CC_OPTIMIZE_FOR_DEBUGGING
o Move new configuration item to "General setup->Compiler optimization level"
v2:
o Improve performance benchmark as suggested by Ingo.
o Grammar updates in description. (Randy Dunlap)
---
Makefile | 4 ++++
include/linux/compiler-gcc.h | 2 +-
include/linux/compiler.h | 2 +-
init/Kconfig | 19 +++++++++++++++++++
4 files changed, 25 insertions(+), 2 deletions(-)
diff --git a/Makefile b/Makefile
index eb694f6..ae1dea7 100644
--- a/Makefile
+++ b/Makefile
@@ -639,6 +639,9 @@ 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)
+else
ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
KBUILD_CFLAGS += $(call cc-option,-Oz,-Os)
KBUILD_CFLAGS += $(call cc-disable-warning,maybe-uninitialized,)
@@ -649,6 +652,7 @@ 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 b4bf73f..586ed11 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -192,7 +192,7 @@
#define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
-#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)))
#endif /* __CHECKER__ */
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index ab4711c..e97caf4 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -301,7 +301,7 @@ unsigned long read_word_at_a_time(const void *addr)
* 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 f013afc..aa52535 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1036,6 +1036,25 @@ 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 SYSCTL
--
2.7.4
next prev parent reply other threads:[~2018-05-06 0:31 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-06 0:20 [PATCH v3 0/5] kernel hacking: GCC optimization for better debug experience (-Og) changbin.du
2018-05-06 0:20 ` [PATCH v3 1/5] x86/mm: surround level4_kernel_pgt with #ifdef CONFIG_X86_5LEVEL...#endif changbin.du
2018-05-06 0:20 ` [PATCH v3 2/5] regulator: add dummy function of_find_regulator_by_node changbin.du
2018-05-09 8:21 ` Mark Brown
2018-05-09 8:13 ` Du, Changbin
2018-05-09 8:39 ` Mark Brown
2018-05-06 0:20 ` [PATCH v3 3/5] kernel hacking: new config NO_AUTO_INLINE to disable compiler auto-inline optimizations changbin.du
2018-05-06 5:50 ` kbuild test robot
2018-05-06 6:15 ` kbuild test robot
2018-05-06 0:20 ` changbin.du [this message]
2018-05-06 0:20 ` [PATCH v3 5/5] asm-generic: fix build error in fix_to_virt with CONFIG_CC_OPTIMIZE_FOR_DEBUGGING changbin.du
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