Why disabling interrupts disables kernel preemption and how spin lock disables preemption

Question

I am reading Linux Kernel Development recently, and I have a few questions related to disabling preemption.

1. In the "Interrupt Control" section of chapter 7, it says:

   Moreover, disabling interrupts also disables kernel preemption.

   I also read from the book that kernel preemption can occur in the follow cases:

   When an interrupt handler exits, before returning to kernel-space.
   When kernel code becomes preemptible again.
   If a task in the kernel explicitly calls schedule()
   If a task in ther kernel blocks (which results in a call to schedule())

   But I can't relate disabling interrupts with these cases.

2. As far as I know, a spinlock would disable preemption with the preempt_disable() function.

   The post What exactly are "spin-locks"? says:

   On a single core machine a spinlock is simply a "disable interrupts" or "raise IRQL" which prevents thread scheduling completely.

   Does preempt_disable() disable preemption by disabling interrupts?

Answer 1

I am not a scheduler guru, but I would like to explain how I see it. Here are several things.

1. preempt_disable() doesn't disable IRQ. It just increases a thread_info->preempt_count variable.
2. Disabling interrupts also disables preemption because scheduler isn't working after that - but only on a single-CPU machine. On the SMP it isn't enough because when you close the interrupts on one CPU the other / others still does / do something asynchronously.
3. The Big Lock (means - closing all interrupts on all CPUs) is slowing the system down dramatically - so it is why it not anymore in use. This is also the reason why preempt_disable() doesn't close the IRQ.

You can see what is preempt_disable(). Try this: 1. Get a spinlock. 2. Call schedule()

In the dmesg you will see something like "BUG: scheduling while atomic". This happens when scheduler detects that your process in atomic (not preemptive) context but it schedules itself.

Good luck.

Answer 2

In a test kernel module I wrote to monitor/profile a task, I've tried disabling interrupts by:

1 - Using local_irq_save()

2 - Using spin_lock_irqsave()

3 - Manually disable_irq() to all IRQs in /proc/interrupts

In all 3 cases I could still use the hrtimer to measure time even though IRQs were disabled (and a task I was monitoring got preempted as well).

I find this veeeeerrrryyyy strange... I personally was anticipating what Sebastian Mountaniol pointed out -> No interrupts - no clock. No clock - no timers...

Linux kernel 2.6.32 on a single core, single CPU... Can anyone have a better explanation ?