In kernel 4.16 and above init_task is initialized directly

[l-0-l]

Kernel entry point -> First steps in the start_kernel

You can see the definition of the init_task and it initialized by the INIT_TASK macro. This macro is from include/linux/init_task.h and it just fills the init_task with the values for the first process.

In earlier kernels init_task used to be initialized with the INIT_TASK macro, which is now absent. From 4.16 and on, there's a direct initialization in the init_task.c file.