Making RCU Safe for Deep Sub-Millisecond Response Realtime Applications

Overview

Linux has long been used for soft realtime applications. More recent work is preparing Linux for more aggressive realtime use, with scheduling latencies in the small number of hundreds of microseconds (that is right, microseconds, not milliseconds). The current Linux 2.6 RCU implementation both helps and hurts. It helps by removing locks, thus reducing latency in general, but hurts by causing large numbers of RCU callbacks to be invoked all at once at the end of the grace period. This paper describes modifications to RCU that greatly reduce its effect on scheduling latency, without significantly degrading performance for non-realtime Linux servers. Although these modifications appear to prevent RCU from interfering with realtime scheduling, other Linux kernel components are still problematic.