linux scheduler

Question

In my kernel configuration CONFIG_PREEMPT is not set. Since schedule() is not allowed in interrupt handler how does round robin type of scheduling is implemented in linux kernel. i.e. Who calls the scheduler so often. In entry_32.S it calls preempt_schedule_irq only if CONFIG_PREEMPT is set.

Answer 1

What happens is the timer on the CPU is set to interrupt the kernel every so often. But we can't just call schedule from interrupt context right? So what the kernel does is a neat trick. It changes the currently executing task while executing the handler and then returns. What this effectively does is switch out the context from underneath the handler so the handler completes but at the same time the next context to run is now the next task that will execute. Read up on do_context_switch (IIRC I think that's what it's called) and you will see that it switches it's stack and context from underneath the current execution and resumes the same function in another context.

And CONFIG_PREEMPT only applies to kernel code preemption in kernel context. Userspace tasks will always preempt. All this means is that any kernel code that starts to execute runs to completion (unless you call schedule() yourself or block waiting for I/O, etc....). Normally the kernel can preempt as long as it does not hold any locks except in certain cases where acquiring a lock can put the thread to sleep.