While investigating a performance issue on RT 3.14, I noticed that __HAVE_ARCH_CMPXCHG is not defined for ARM. This causes -rt locks to always use the slow path and impacts performance. ARMv6 and above have a cmpxchg implementation already that will work just fine. In 2012, Yong Zhang submitted a patch to fix this[1], but it mysteriously disappeared afterwards. I did notice that the s-o-b line has a different email address than the windriver one. Yong, do you mind re-submitting the patch with a fixed s-o-b? Thanks! Nathan Sullivan [1]: http://lists.infradead.org/pipermail/linux-arm-kernel/2012-May/102122.html (duplicated below) -- 8< -- From: Yong Zhang <yong.zhang at windriver.com> Both pi_stress and sigwaittest in rt-test show performance gain with __HAVE_ARCH_CMPXCHG. Testing result on coretile_express_a9x4: pi_stress -p 99 --duration=300 (on linux-3.4-rc5; bigger is better) vanilla: Total inversion performed: 5493381 patched: Total inversion performed: 5621746 sigwaittest -p 99 -l 100000 (on linux-3.4-rc5-rt6; less is better) 3.4-rc5-rt6: Min 24, Cur 27, Avg 30, Max 98 patched: Min 19, Cur 21, Avg 23, Max 96 Signed-off-by: Yong Zhang <yong.zhang0 at gmail.com> Cc: Russell King <rmk+kernel at arm.linux.org.uk> Cc: Nicolas Pitre <nico at linaro.org> Cc: Will Deacon <will.deacon at arm.com> Cc: Catalin Marinas <catalin.marinas at arm.com> Cc: Thomas Gleixner <tglx at linutronix.de> Cc: linux-arm-kernel at lists.infradead.org --- arch/arm/include/asm/cmpxchg.h | 2 ++ 1 files changed, 2 insertions(+), 0 deletions(-) diff --git a/arch/arm/include/asm/cmpxchg.h b/arch/arm/include/asm/cmpxchg.h index 7eb18c1..a91b44e 100644 --- a/arch/arm/include/asm/cmpxchg.h +++ b/arch/arm/include/asm/cmpxchg.h @@ -127,6 +127,8 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr, int size #else /* min ARCH >= ARMv6 */ +#define __HAVE_ARCH_CMPXCHG 1 + extern void __bad_cmpxchg(volatile void *ptr, int size); /* -- 1.7.5.4 -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
