See this presentation about Google's scheduler patches, which claims the major overhead is logic in the scheduler (pg16): http://www.linuxplumbersconf.org/2013/ocw/system/presentatio...
I'd love to hear otherwise or in more detail from someone at Google. It's frustrating to have to do archaeology just to guess what the state of the art is. Isn't sharing your findings part of "organizing the world's information"?
It's the bit on the end that sounds a lot less noble.