A method, computer program product, and data processing system for collecting metrics regarding completion stalls in an out-of-order superscalar processor with branch prediction is disclosed. A preferred embodiment of the present invention selectively samples particular instructions (or classes of instructions). Each selected instruction, as it passes through the processor datapath, is marked (tagged) for monitoring by a performance monitoring unit. The progress of marked instructions is monitored by the performance monitoring unit, and various stall counters are triggered by the progress of the marked instructions and the instruction groups they form a part of. The stall counters count cycles to give an indication of when certain delays associated with particular instructions occur and how serious the delays are.