I am reading Linux Kernel Development recently, and I have a few questions related to disabling preemption.
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.
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?
I am not a scheduler guru, but I would like to explain how I see it.
Here are several things.
- preempt_disable() doesn't disable IRQ. It just increases a
thread_info->preempt_count variable.
- 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.
- 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.
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 ?
