MFC r239194:

MFC r239194:
Allow idle threads to steal second threads from other cores on systems with
8 or more cores to improve utilization.  None of my tests on 2xXeon (2x6x2)
system shown any slowdown from mentioned "excess thrashing".  Same time in
pbzip2 test with number of threads more then number of CPUs I see up to 10%
speedup with SMT disabled and up 5% with SMT enabled.  Thinking about
trashing I was trying to limit that stealing within same last level cache,
but got only worse results.  Present code any way prefers to steal threads
from topologically closer cores.

git-svn-id: svn://svn.freebsd.org/base/stable/9@241250 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f