From: Mikael Pettersson <mikpe@csd.uu.se>

The real problem is that SMP with nmi_watchdog=2 initialises the lapic NMI
watchdog but doesn't check it and therefore doesn't reduce nmi_hz.  This is
an SMP bug.

The patch changes smpboot.c to do a check_nmi_watchdog() at the appropriate
place, which fixes the high NMI frequency problem w/o changing anything
else.  I've verified that it solves the problem on my MP-capable UP box.