I'm being confused a bit.
How do you do "context switch"?
In userspace you can't.
In kernelspace you can, but to withhold the scheduler from scheduling kthreads would require to patch it, no? But you asked where the scheduler is in the kernel tree, so I assume you haven't.

So what do you context-switch?

Secondly: If a kthread starts to execute it cannot be scheduled-out. It will execute until finished or give up voluntary by calling schedule(). I don't know much about preemptive kernels (well, all are nowadays). I am sure that are very certain points of where a kernel preemption can occur -- after an interrupt for example. If you don't have an interrupt I can't imagine how the kthread could be unscheduled (by who??).

So what I think in a n-core pc:
pthread0...pthreadN sleep
kthread0...kthreadn-1 become runnable (as many as the cores)
kthreadx finishes
either a pthreadx or a kthreadx become runnable
Meanwhile, more kthreadx will finish.
Chances are as kthreadx finish, some pthreadx will became runnable.
So with time, all pthreads will become runnable and all kthreads will terminate.

But I am not 100% sure if everything works as I assume. I remember when I was programming a driver for the parallel port, upon entry to the interrupt service routine (triggered by a hardware interrupt) I did a CLI so no one could interrupt me. I did my job the fastest I could, then executed an STI and a schedule() to give up cpu. Preemptible kernel of not, there was no difference.
I don't seem to have the code to check it however, it's so many years that have passed.

Gotta go to bed() now because I'm cast to (void *) as I am sleepless for 2 days. Please be more specific (technically) and/or give or describe some code you have run.