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.

Why not use a software solution ... start a timer (if you have a free timer available) at the beginning of the ISR, stop it at the end, and maximize its value with all previous results. So you have the maximum (longest duration) of many ISR calls.

Than at some time print it via UART (or another way) to check the value, or use a breakpoint.

If you want to use an oscilloscope: I only have a fairly simple oscilloscope myself, and what you want seems like quite advanced for an oscilloscope. However, some oscilloscopes have a feature to store all measured data to USB or to a PC, where you can write a program (or use some spreadsheet) to calculate the longest duration.