From: Andi Kleen <ak@muc.de>

Set up SMT for the domain scheduler on x86-64.  This way the scheduling
works better on HyperThreading aware systems; in particular it will use
both physical CPUs before sharing two virtual CPUs on the same package. 
This improves performance considerably in some cases.

Based on the i386 code and a previous patch from Suresh B. Siddha.