[PATCH] arm64: Implement cpu_relax as yield

From: Peter Crosthwaite
Date: Mon Mar 02 2015 - 14:19:52 EST


ARM64 has the yield nop hint which has the intended semantics of
cpu_relax. Implement.

The immediate application is ARM CPU emulators. An emulator can take
advantage of the yield hint to de-prioritise an emulated CPU in favor
of other emulation tasks. QEMU A64 SMP emulation has yield awareness,
and sees a significant boot time performance increase with this change.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xxxxxxxxxx>
Acked-by: Will Deacon <will.deacon@xxxxxxx>
---
arch/arm64/include/asm/processor.h | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index f9be30e..ac2381d 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -126,7 +126,11 @@ extern void release_thread(struct task_struct *);

unsigned long get_wchan(struct task_struct *p);

-#define cpu_relax() barrier()
+static inline void cpu_relax(void)
+{
+ asm volatile("yield" ::: "memory");
+}
+
#define cpu_relax_lowlatency() cpu_relax()

/* Thread switching */
--
2.3.0.1.g27a12f1

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