[PATCH] powerpc: Tweak copy selection parameter in __copy_tofrom_user_power7()

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.

Signed-off-by: Andrew Jeffery <redacted>
---
 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
 

-- 
2.9.3

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 <redacted>

Anton

On Fri, 2017-05-12 at 03:58:10 UTC, Andrew Jeffery wrote:

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.

Signed-off-by: Andrew Jeffery <redacted>
Acked-by: Anton Blanchard <redacted>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/a3f952df3c6d205fe3b1e0e4848d3c

cheers