linux-kernel - [PATCH] powerpc: Tweak copy selection parameter in __copy_tofrom_user

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Message-Id: <20170512035810.15070-1-andrew@aj.id.au>
Date:   Fri, 12 May 2017 13:28:10 +0930
From:   Andrew Jeffery <andrew@...id.au>
To:     linuxppc-dev@...ts.ozlabs.org
Cc:     Andrew Jeffery <andrew@...id.au>, benh@...nel.crashing.org,
        paulus@...ba.org, mpe@...erman.id.au, npiggin@...il.com,
        anton@...ba.org, linux-kernel@...r.kernel.org
Subject: [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 <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
 
-- 
2.9.3