That lkml answer is not quite complete, as it only describes exec done from an already existing user process via the execve() syscall. In Linux, context switches happen often implicitly. There is an explicit timer that runs CONFIG_HZ times per second for that, but often the scheduler runs implicitly in e.g. mutex_lock(), and several other places too (grep for cond_resched(), or might_sleep()). Process creation just sets up the process and puts it in the scheduler's task list, where it is started "sometime later", AFAIK. Returning from syscalls is one of those implicit places, like the lkml answer says. After having handled an interrupt is another I guess.
If you put printk()s all over the place where kernel_init() is called and run_init_process() too, you'll see that it just returns. Track schedule() to see that init actually gets set up:
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2849,6 +2851,10 @@ asmlinkage __visible void __sched schedule(void)
struct task_struct *tsk = current;
sched_submit_work(tsk);
+ printk("schedule %d:%s @ %lx\n", tsk->pid, tsk->comm, instruction_pointer(current_pt_regs()));
__schedule();
}
EXPORT_SYMBOL(schedule);
Result: continuous prints between init (its ip doesn't increment though :-/) and ksoftirqd. (not sure if ksoftirqd actually has something to do or not... and its ip appears zero since it doesn't have an user process - regs of a process get saved in the end of its stack; for kernel threads, that memory has other uses)
(the log contains some of my other debug prints too, but that's irrelevant)
I recommend a good book, as these things are not consistently described elsewhere unless you really dig through the whole code - Robert Love's Linux Kernel Development is a recent one. (edit: by consistent, I mean that you have to swim around the internets to find small bits of information at a time. That's probably necessary for some ARM specific stuff anyway, though.)
I'll discuss this at work - we've got many old Linux experts that are probably interested. Will keep you updated.
