code: private void checkForStall(long currentTime) {
      long delta = currentTime - _lastMonitoringStep;
      if (delta < _shortestObservedDelta) {
        _shortestObservedDelta = delta;
      }
      long stall = Math.max(0, delta - _shortestObservedDelta);
      if (stall > _minStallNano) {
        _stalls.put(_lastMonitoringStep, stall);
        if (_stalls.size() > _stallsHistorySize) {
          _stalls.pollFirstEntry();
        }
      }
    }
docs: Check how much we missed scheduled wake up and if it is larger than _minStallNano
  then consider it a stall and remember it.