when is populate_rootfs called?

Hi

I have read through
https://stackoverflow.com/questions/10540008/how-does-the-linux-kernel-determine-the-order-of-init-calls
and looked at the source code related to init, like `start_kernel`,
`kernel_init_mem`, `do_ido_initcall_levelnitcall_level` and
`offset_to_ptr`, but I am still totally confused at when is
`populate_rootfs` called.