About the PMU
The Cortex®‑A77 core includes performance monitors that enable you to gather various statistics on the operation of the core and its memory system during runtime. These provide useful information about the behavior of the core that you can use when debugging or profiling code.
The PMU provides six counters. Each counter can count any of the events available in the core. The absolute counts recorded might vary because of pipeline effects. This has negligible effect except in cases where the counters are enabled for a very short time.