cpuops: Use cmpxchg for xchg to avoid lock semantics

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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>

...

This commit is contained in:

Christoph Lameter 2010-12-14 10:28:47 -06:00 • committed by Tejun Heo

parent 7296e08aba

commit 8270137a0d

1 changed files with 15 additions and 6 deletions

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arch/x86/include/asm/percpu.h

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