From: Anton Blanchard <anton@samba.org>
Date: Thu, 2 Oct 2014 05:44:21 +0000 (+1000)
Subject: powerpc: Speed up clear_page by unrolling it
X-Git-Tag: fixes-against-v3.18-rc2~117^2~35
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=e35735b9a5d8d38d9ffe2f1f0cdcbb0d45c42eff;p=pandora-kernel.git

powerpc: Speed up clear_page by unrolling it

Unroll clear_page 8 times. A simple microbenchmark which
allocates and frees a zeroed page:

for (i = 0; i < iterations; i++) {
	unsigned long p = __get_free_page(GFP_KERNEL | __GFP_ZERO);
	free_page(p);
}

improves 20% on POWER8.

This assumes cacheline sizes won't grow beyond 512 bytes or
page sizes wont drop below 1kB, which is unlikely, but we could
add a runtime check during early init if it makes people nervous.

Michael found that some versions of gcc produce quite bad code
(all multiplies), so we give gcc a hand by using shifts and adds.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
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