tree 3490a31fcbea09ab5fffb6b2f4330dc92896f413
parent 7296e08abac0a22a2534a4f6e493c764f2c77583
author Christoph Lameter <cl@linux.com> 1292344127 -0600
committer Tejun Heo <tj@kernel.org> 1292684044 +0100

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>