McKenney: Stupid RCU Tricks: Is RCU Watching?

Paul McKenney writesabout why read-copy-update coverage is not universal in the kernel, thehazards that can result from that, and what is being done to improve thesituation.

Unfortunately, an eternally watchful RCU is impractical in theLinux kernel due to energy-efficiency considerations. The problemis that if RCU watches an idle CPU, RCU needs that CPU to executeinstructions. And making an idle CPU unnecessarily executeinstructions (for a rather broad definition of the wordunnecessary") will terminally annoy a great many people in thebattery-powered embedded world. And for good reason: Making RCUavoid watching idle CPUs can provide 30-40% increases in batterylifetime.