With Simulink Real-Time and Multi-Rate Subsystems, is there a way to set a "block execution finished" flag?

I have a multi-rate simulink real-time model with data dependency. I'm trying to figure out a good way to notifiy blocks when data is ready because there are times when the computation required for a subsystem is not as demanding and data will be ready sooner. And then there are times when data may take too long and I want the next subsystem to know it wasn't ready, (CPU overloads are disabled). So is there a good way to set some sort of "subsystem execution done" flag that would happen at the end of a subsystem execution and then reset on the next model cycle?