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Date:   Fri, 19 May 2017 03:08:23 +1000
From:   Anton Blanchard <anton@...ba.org>
To:     Andrew Jeffery <andrew@...id.au>
Cc:     linuxppc-dev@...ts.ozlabs.org, benh@...nel.crashing.org,
        paulus@...ba.org, mpe@...erman.id.au, npiggin@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc: Tweak copy selection parameter in
 __copy_tofrom_user_power7()

Hi Andrew,

> Experiments with the netperf benchmark indicated that the size
> selecting VMX-based copies in __copy_tofrom_user_power7() was
> suboptimal on POWER8. Measurements showed that parity was in the
> neighbourhood of 3328 bytes, rather than greater than 4096. The
> change gives a 1.5-2.0% improvement in performance for 4096-byte
> buffers, reducing the relative time spent in
> __copy_tofrom_user_power7() from approximately 7% to approximately 5%
> in the TCP_RR benchmark.

Nice work! All our context switch optimisations we've made over
the last year has likely moved the break even point for this.

Acked-by: Anton Blanchard <anton@...ba.org>

Anton

> Signed-off-by: Andrew Jeffery <andrew@...id.au>
> ---
>  arch/powerpc/lib/copyuser_power7.S | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/lib/copyuser_power7.S
> b/arch/powerpc/lib/copyuser_power7.S index a24b4039352c..706b7cc19846
> 100644 --- a/arch/powerpc/lib/copyuser_power7.S
> +++ b/arch/powerpc/lib/copyuser_power7.S
> @@ -82,14 +82,14 @@
>  _GLOBAL(__copy_tofrom_user_power7)
>  #ifdef CONFIG_ALTIVEC
>  	cmpldi	r5,16
> -	cmpldi	cr1,r5,4096
> +	cmpldi	cr1,r5,3328
>  
>  	std	r3,-STACKFRAMESIZE+STK_REG(R31)(r1)
>  	std	r4,-STACKFRAMESIZE+STK_REG(R30)(r1)
>  	std	r5,-STACKFRAMESIZE+STK_REG(R29)(r1)
>  
>  	blt	.Lshort_copy
> -	bgt	cr1,.Lvmx_copy
> +	bge	cr1,.Lvmx_copy
>  #else
>  	cmpldi	r5,16
>  

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