How to call all cores in biglittle arm cpu

I run my model on rk3399’s cpu, it has 4 a53 core and 2 a72 core. when running the model ,I found only the big cores are occupied. My question is how to make the rest four little cores to run at the same time to speed the performace. (before autotvm, my model costs about 500ms, it is too slow)