From ef6edc9746dc2bfdacf44eefd5f881179971c478 Mon Sep 17 00:00:00 2001 From: Martin Schwidefsky Date: Sat, 30 Sep 2006 23:27:43 -0700 Subject: [PATCH] Directed yield: cpu_relax variants for spinlocks and rw-locks On systems running with virtual cpus there is optimization potential in regard to spinlocks and rw-locks. If the virtual cpu that has taken a lock is known to a cpu that wants to acquire the same lock it is beneficial to yield the timeslice of the virtual cpu in favour of the cpu that has the lock (directed yield). With CONFIG_PREEMPT="n" this can be implemented by the architecture without common code changes. Powerpc already does this. With CONFIG_PREEMPT="y" the lock loops are coded with _raw_spin_trylock, _raw_read_trylock and _raw_write_trylock in kernel/spinlock.c. If the lock could not be taken cpu_relax is called. A directed yield is not possible because cpu_relax doesn't know anything about the lock. To be able to yield the lock in favour of the current lock holder variants of cpu_relax for spinlocks and rw-locks are needed. The new _raw_spin_relax, _raw_read_relax and _raw_write_relax primitives differ from cpu_relax insofar that they have an argument: a pointer to the lock structure. Signed-off-by: Martin Schwidefsky Cc: Ingo Molnar Cc: Paul Mackerras Cc: Haavard Skinnemoen Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/asm-arm/spinlock.h | 4 ++++ 1 file changed, 4 insertions(+) (limited to 'include/asm-arm/spinlock.h') diff --git a/include/asm-arm/spinlock.h b/include/asm-arm/spinlock.h index 01b7c26a3038..861092fbaa53 100644 --- a/include/asm-arm/spinlock.h +++ b/include/asm-arm/spinlock.h @@ -218,4 +218,8 @@ static inline int __raw_read_trylock(raw_rwlock_t *rw) /* read_can_lock - would read_trylock() succeed? */ #define __raw_read_can_lock(x) ((x)->lock < 0x80000000) +#define _raw_spin_relax(lock) cpu_relax() +#define _raw_read_relax(lock) cpu_relax() +#define _raw_write_relax(lock) cpu_relax() + #endif /* __ASM_SPINLOCK_H */ -- cgit v1.2.2