From: Christoph Lameter <cl@linux.com>
Date: Tue, 14 Dec 2010 16:28:47 +0000 (-0600)
Subject: cpuops: Use cmpxchg for xchg to avoid lock semantics
X-Git-Tag: v2.6.38-rc1~455^2~7^2
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=8270137a0d50507a5b40f880db636527045b8466;p=pandora-kernel.git

cpuops: Use cmpxchg for xchg to avoid lock semantics

Use cmpxchg instead of xchg to realize this_cpu_xchg.

xchg will cause LOCK overhead since LOCK is always implied but cmpxchg
will not.

Baselines:

xchg()		= 18 cycles (no segment prefix, LOCK semantics)
__this_cpu_xchg = 1 cycle

(simulated using this_cpu_read/write, two prefixes. Looks like the
cpu can use loop optimization to get rid of most of the overhead)

Cycles before:

this_cpu_xchg	 = 37 cycles (segment prefix and LOCK (implied by xchg))

After:

this_cpu_xchg	= 11 cycle (using cmpxchg without lock semantics)

Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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