Run-time reconfigurable logic is a very attractive alterative in the design of SoC. However, configuration overhead can largely decrease the system performance. In this work, we present a novel configuration locking technique to reduce the effect of the overhead. The idea is to at run-time lock a number of the most frequently used tasks on the configuration memory so that they cannot be evicted by other tasks. With real applications in validation, the results show that using proper amount of resources to lock tasks can significantly outperform simply using more resources. In addition, an algorithm has been developed for estimating the lock ratio. Experimental results show that the estimates are close to optimal results and the measured computer runtime is less than 4 us in a commercial embedded processor.