In many embedded real-time systems, the actual execution time of tasks is usually less than their worst-case execution time (WCET), which produces lots of dynamic slack time. Based on this fact, we proposed an energy-efficient real-time scheduling algorithm LRE-DVFS-ALL for sporadic tasks deployed on multi-cores, which is based on optimal online scheduling algorithm LRE-TL. The main idea of the algorithm is to reclaim dynamic slack time, during which DVFS techniques can be used to reduce the execution frequency of future tasks to eliminate the energy consumption. Meanwhile, our algorithm also dynamically scales the voltage and frequency at the initial time of each TL plane and the release time of a sporadic task in each TL plane to guarantee the optimal schedulability of sporadic tasks. Systematic mathematical analysis and extensive simulation results show that LRE-DVFS-ALL can always save more energy than the existing algorithms when the total workload of the system exceeds a threshold, and it can also guarantees the optimal schedulability at the same time.