[PATCH] arm64: Implement cpu_relax as yield

[PATCH] arm64: Implement cpu_relax as yield

[Peter Crosthwaite]

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@xilinx.com>
Acked-by: Will Deacon <will.deacon@arm.com>
---
 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

[PATCH] arm64: Implement cpu_relax as yield

[Peter Crosthwaite]

Ping!

Anything I need to do or send to to get a merge on this?

Regards,
Peter

On Tue, Mar 3, 2015 at 12:49 AM, Peter Crosthwaite
<peter.crosthwaite@xilinx.com> wrote:
> 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@xilinx.com>
> Acked-by: Will Deacon <will.deacon@arm.com>
> ---
>  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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[PATCH] arm64: Implement cpu_relax as yield

[Will Deacon]

On Wed, Mar 25, 2015 at 09:58:26AM +0000, Peter Crosthwaite wrote:
> Ping!
> 
> Anything I need to do or send to to get a merge on this?

It's queued for 4.1 and should already be in linux-next.

Cheers,

Will