As mentioned in several posts, the balance_irq code introduced in 2.5.9 is 
broken for any architecture that doesn't use flat logical addressing.  Also, 
even when it does work, there have been several reports of it degrading 
performance.  As the NUMA-Q architecture I work with doesn't use flat logical 
addressing, and it isn't worth the possible performance hit, I'm submitting 
this patch for inclusion into the mainline.

Cheers!

-Matt