[$] Accurate timestamps for the ftrace ring buffer

The functiontracer (ftrace) subsystem has become an essential part of the kernel'sintrospection tooling. Like many kernel subsystems, ftrace uses a ring buffer toquickly communicate events to user space; those events include a timestamp toindicate when they occurred. Until recently, the design of the ring bufferhas led to the creation of inaccurate timestamps when events are generatedfrom interrupt handlers. That problem has now been solved; read on for anin-depth discussion of how this issue came about and the form of itssolution.