Len Brown of Intel OTC wrote yesterday to the Linux kernel mailing list about a 50 Watt idle power regression bisected to Linux-3.10. On a system with 40 Intel Xeon CPU cores, Len Brown discovered the system was running through 50 more Watts of power on all kernel version since Linux 3.10 -- up through and including the latest Linux 3.13-rc3 kernel.
The issue was tracked down to an x86 change for using a generic idle loop. The change made the system go from idling in its package C6 state nearly the entire time to never having a chance to idle in this low-power C6 state. A commit to the Linux kernel from September attempted to improve the generic idle loop code, but it failed to properly do so and the power regression is still outstanding.
A proper fix for the solution is still being investigated but it looks like setting the "idle=mwait" kernel command-line parameter may help the situation.