How to set a flag while the function-call subsystem is executing?

I’m working with a multi‑rate system where task2 can be preempted by task1, and I need to measure the execution time of both tasks accurately.

Since task1 may interrupt task2, I want to subtract the time task1 executes during those interruptions from task2’s total execution time. My idea is to have task2 raise a flag whenever it is running, so task1 can detect the preemption and record how long it ran during that period.

How can I keep a signal high for the entire duration of task2’s execution? Both tasks are implemented inside function‑call subsystems.

Thanks!