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

Answer 1

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 ?