Patchwork [1/2] sparc64: Fix cpu strand yielding.

For atomic backoff, we just loop over an exponentially backed off
counter.  This is extremely ineffective as it doesn't actually yield
the cpu strand so that other competing strands can use the cpu core.

In cpus previous to SPARC-T4 we have to do this in a slightly hackish
way, by doing an operation with no side effects that also happens to
mark the strand as unavailable.

The mechanism we choose for this is three reads of the %ccr
(condition-code) register into %g0 (the zero register).

SPARC-T4 has an explicit "pause" instruction, and we'll make use of
that in a subsequent commit.

Yield strands also in cpu_relax().  We really should have done this a
very long time ago.

Signed-off-by: David S. Miller <davem@davemloft.net>
---
 arch/sparc/include/asm/backoff.h      | 5 ++++-
 arch/sparc/include/asm/processor_64.h | 5 ++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

Hi Davem.

Nitpick only.

On Sun, Oct 28, 2012 at 02:13:33AM -0400, David Miller wrote:
> 
> For atomic backoff, we just loop over an exponentially backed off
> counter.  This is extremely ineffective as it doesn't actually yield
> the cpu strand so that other competing strands can use the cpu core.
> 
> In cpus previous to SPARC-T4 we have to do this in a slightly hackish
> way, by doing an operation with no side effects that also happens to
> mark the strand as unavailable.
> 
> The mechanism we choose for this is three reads of the %ccr
> (condition-code) register into %g0 (the zero register).
> 
> SPARC-T4 has an explicit "pause" instruction, and we'll make use of
> that in a subsequent commit.
> 
> Yield strands also in cpu_relax().  We really should have done this a
> very long time ago.
> 
> Signed-off-by: David S. Miller <davem@davemloft.net>
> ---
>  arch/sparc/include/asm/backoff.h      | 5 ++++-
>  arch/sparc/include/asm/processor_64.h | 5 ++++-
>  2 files changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/sparc/include/asm/backoff.h b/arch/sparc/include/asm/backoff.h
> index db3af0d..64b077b 100644
> --- a/arch/sparc/include/asm/backoff.h
> +++ b/arch/sparc/include/asm/backoff.h
> @@ -13,7 +13,10 @@
>  

A small comment would be nice:

/* For atomic backoff, loop over an exponentially backed off
 * counter.
 */

>  #define BACKOFF_SPIN(reg, tmp, label)	\
>  	mov	reg, tmp; \
> -88:	brnz,pt	tmp, 88b; \
> +88:	rd	%ccr, %g0; \
> +	rd	%ccr, %g0; \
> +	rd	%ccr, %g0; \
> +	brnz,pt	tmp, 88b; \
>  	 sub	tmp, 1, tmp; \
>  	set	BACKOFF_LIMIT, tmp; \
>  	cmp	reg, tmp; \
> diff --git a/arch/sparc/include/asm/processor_64.h b/arch/sparc/include/asm/processor_64.h
> index 4e5a483..9865634 100644
> --- a/arch/sparc/include/asm/processor_64.h
> +++ b/arch/sparc/include/asm/processor_64.h
> @@ -196,7 +196,10 @@ extern unsigned long get_wchan(struct task_struct *task);
>  #define KSTK_EIP(tsk)  (task_pt_regs(tsk)->tpc)
>  #define KSTK_ESP(tsk)  (task_pt_regs(tsk)->u_regs[UREG_FP])
>  
> -#define cpu_relax()	barrier()
> +#define cpu_relax()	asm volatile("rd	%%ccr, %%g0\n\t" \
> +				     "rd	%%ccr, %%g0\n\t" \
> +				     "rd	%%ccr, %%g0" \
> +				     ::: "memory")

You could have cared to let the formatting match next patch so
we saw less changes in the follow-up patch.

	Sam
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