How to set an oscilloscope for measurement of interrupt service routine execution time
On TDS2014, the DISPLAY button menu includes persistence, which can be set to infinite. It should be able to capture the pulse-width of the GPIO signal. If the ISR duration was constant, you should see a single pulse-fall, and a constant pulse-width assuming that trigger was set to start on the GPIO's rising edge.
Do ensure that your target ISR is not pre-empted by a different higher-priority ISR.