Fork migration
This operates when the fork system call is used to create a new software thread. At this point, clearly no historical load information is available. The system defaults to a big core for new threads on the assumption that a 'light' thread will quickly migrate down to a LITTLE core as a result of Wake migration.
Fork migration benefits demanding tasks without being expensive. Threads that are low intensity and persistent, such as Android system services, will only be moved to big cores at creation time, quickly moving to more suitable LITTLE cores thereafter. Threads that are clearly demanding throughout, will not be penalized by being made to launch on LITTLE cores first. Threads that are episodic but tend to require performance on the whole will benefit from being launched on the big cluster and will continue to run there as required.
app启动时sched_setaffinity 无任何优化，因为arm架构fork进程时都会在大核心上执行。
