ReDAG-RT: Global Rate-Priority Scheduling for Real-Time Multi-DAG Execution in ROS 2

ROS 2 has become a dominant middleware for robotic systems, where perception, estimation, planning, and control pipelines are structured as directed acyclic graphs of callbacks executed under a shared executor. However, default ROS 2 executors use best-effort dispatch without cross-DAG priority enforcement, leading to callback contention, structural priority inversion, and deadline instability under concurrent workloads. These limitations restrict deployment in time-critical and safety-sensitive cyber-physical systems. This paper presents ReDAGRT, a user-space global scheduling framework for deterministic multi-DAG execution in unmodified ROS 2. The framework introduces a Rate-Priority driven global ready queue that orders callbacks by activation rate, enforces per-DAG concurrency bounds, and mitigates cross-graph priority inversion without modifying the ROS 2 API, executor interface, or underlying operating system scheduler. We formalize a multi-DAG task model for ROS 2 callback pipelines and analyze cross-DAG interference under Rate-Priority scheduling. Response-time recurrences and schedulability conditions are derived within classical Rate-Monotonic theory. Experiments in a ROS 2 Humble environment compare ReDAGRT against SingleThreadedExecutor and MultiThreadedExecutor using synthetic multi-DAG workloads. Results show up to 29.7 percent reduction in deadline miss rate, 42.9 percent reduction in 99th percentile response time, and 13.7 percent improvement over MultiThreadedExecutor under comparable utilization. Asymmetric per-DAG concurrency bounds further reduce interference by 40.8 percent. These results demonstrate that deterministic and analyzable multi-DAG scheduling can be achieved entirely in the ROS 2 user-space execution layer, providing a practical foundation for real-time robotic middleware in safety-critical systems.