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====== Threaded interrupt handler ======

In mainline Linux the interrupt service routine is processed in hard
interrupt context with hardware interrupts disabled.  Hardware
interrupts disabled implies that preemption and soft interrupts are
also disabled.  Interrupt handlers are processed in context of the
interrupt service routine with hard interrupts disabled as well.  The
kernel command line option `threadirqs` changes the the interrupt
handlers to run in a threaded context.  The scheduler policy of the
thread is set to SCHED_FIFO with a default priority of 50.

All interrupt handlers marked with the IRQF_NO_THREAD flag will not be
threaded and run with hard interrupts disabled.  This flag is used for
example by Inter-Processor Interrupts (IPIs).  Interrupt handlers set
up with the IRQF_TIMER or IRQF_PER_CPU flag are marked as
IRQF_NO_THREAD implicitly.

The PREEMPT_RT patch forces the `threadirqs` command line option.
There is a small difference between the behavior of threaded
interrupts in the mainline and PREEMPT_RT implementations, illustrated
in the following table:

<WRAP center 400px>
|                                ^ Mainline ^ PREEMPT_RT |
^hard interrupts disabled        |          |            |
^soft interrupts disabled        | ✓        | ✓          |
^preemption disabled             | ✓        |            |
</WRAP>

References:
  * [[https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/include/linux/interrupt.h|include/linux/interrupt.h]]
  * [[https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/kernel/irq/manage.c|kernel/irq/manage.c]]
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