> But If use the big.little platform, the task will be migrated to other cpu?
It depends how big.LITTLE is integrated in the OS - it's a hardware technology and can be used in multiple ways by the operating system.
In general, yes, your thread may get moved but you generally don't need to worry about it. You can view it as having similar behavior to CPU DVFS (dynamic voltage and frequency scaling). That has been around for years on embedded and mobile systems; even without big.LITTLE the OS may change the performance characteristics of the device under your feet. Trust the OS scheduler and power management framework to do the right thing ...
Hope that helps,
Pete