Black-box instrumentation can support problem diagnosis in distributed systems without the need to modify the application code or to understand its semantics. We explore a novel, low-overhead black-box instrumentation source - CPU/hardware performance counters - for problem diagnosis. Based on our hypothesis that performance problems manifest as observable, anomalous changes in CPU performance counter-values collected across the nodes of a distributed system, we develop a diagnosis approach that is able to detect and localize performance problems injected into RUBiS, a three-tier e-commerce system.