Re: [PATCH] powerpc: Optimise the 64bit optimised __clear_user

On Jun 4, 2012, at 8:12 AM, Olof Johansson wrote:

Hi,
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On Mon, Jun 4, 2012 at 12:58 AM, Anton Blanchard [off-list ref] =

wrote:

quoted

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I blame Mikey for this. He elevated my slightly dubious testcase:
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# dd if=3D/dev/zero of=3D/dev/null bs=3D1M count=3D10000
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to benchmark status. And naturally we need to be number 1 at creating
zeros. So lets improve __clear_user some more.
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As Paul suggests we can use dcbz for large lengths. This patch gets
the destination 128 byte aligned then uses dcbz on whole cachelines.
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Before:
10485760000 bytes (10 GB) copied, 0.414744 s, 25.3 GB/s
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After:
10485760000 bytes (10 GB) copied, 0.268597 s, 39.0 GB/s
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39 GB/s, a new record.
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Signed-off-by: Anton Blanchard <redacted>
---
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Index: linux-build/arch/powerpc/lib/string_64.S
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=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
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quoted

--- linux-build.orig/arch/powerpc/lib/string_64.S       2012-06-04 =

16:18:56.351604302 +1000

quoted

+++ linux-build/arch/powerpc/lib/string_64.S    2012-06-04 =

16:47:10.538500871 +1000

quoted

@@ -78,7 +78,7 @@ _GLOBAL(__clear_user)

[..]
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quoted

+15:
+err2;  dcbz    r0,r3
+       addi    r3,r3,128
+       addi    r4,r4,-128
+       bdnz    15b

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This breaks architecture spec (and at least one implementation); cache
lines are not guaranteed to be 128 bytes.

I'm guessing it breaks more than one (FSL 64-bit is 64byte cache lines).

- k=