Kernel thread scheduling

[Jeff Haran]

-----Original Message-----
From: kernelnewbies-bounces at kernelnewbies.org [mailto:kernelnewbies-bounces at kernelnewbies.org] On Behalf Of Vincenzo Scotti
Sent: Friday, March 20, 2015 4:20 PM
To: kernelnewbies at kernelnewbies.org
Subject: Kernel thread scheduling

Hello,
I am actually studying kernel threads, and I have some doubts about them.
Let's take for example this snippet of code

static int thread_function(void *data)
{
	while (!kthread_should_stop()) {
		schedule();
	}

	pr_err("Stopped");
	return 0;
}

This way it works just fine, and waits until I call kthread_stop on it.
But if I comment out that schedule() call, it just hangs my system when I load it (it is part of a module). I see that the loop-schedule-wakeup pattern is used among all the others kernel threads. But I don't get why I need to call the scheduler explicitly.
I know that the kernel is fully preemptible, and in my interpretation I thought that it could stop every running thread, even in kernel space, using a timer-based interrupt handler, to give cpu to other threads. Doesn't this pattern resemble a voluntary preemption model?

Where am I wrong?

Are you sure your kernel is configured with kernel preemption on? It is a configurable option. Grep for PREEMPT in your .config file.

Jeff