Hi,
For mesuring interrupt latency, I think you need a oscilloscope or a logic state analyser. By entering the interrupt service routine (ISR), you need to activite an available port on your hardware (like a led port or so on) and desactivate it just before returning from the ISR. You can do that by writing the appropriate code.
By connecting one input of the oscilloscope (or logic state analyser) to the INTR pin of the microprocessor and the second one to the port you activate/desactivate, you can measure the latency time and the duration of the ISR.
I have done this with some succes in the past.