Not all interrupts are triggers for the real-time tasks but all interrupts steal cycles from the real-time task. Threaded interrupt handlers allow a priority to be associated with the interrupt and for it to be scheduled at an appropriate time as shown in the following diagram:

If the interrupt handler code is run as a kernel thread there is no reason why it cannot be preempted by a user space thread of higher priority, and so the interrupt handler does not contribute towards scheduling latency of the user space thread. Threaded interrupt handlers have been a feature of mainline Linux since 2.6.30. You can request that an individual interrupt handler is threaded by registering it with request_threaded_irq() in place of the normal request_irq(). You can make threaded IRQs the default by configuring the kernel with CONFIG_IRQ_FORCED_THREADING=y which makes all handlers into threads unless they have explicitly prevented this by setting the IRQF_NO_THREAD flag. When you apply the PREEMPT_RT patches, interrupts are, by default, configured as threads in this way. Here is an example of what you might see:

# ps -Leo pid,tid,class,rtprio,stat,comm,wchan | grep FF
PID     TID     CLS     RTPRIO  STAT    COMMAND          WCHAN
3       3       FF      1       S      ksoftirqd/0      smpboot_th
7       7       FF      99      S      posixcputmr/0    posix_cpu_
19      19      FF      50      S      irq/28-edma      irq_thread
20      20      FF      50      S      irq/30-edma_err  irq_thread
42      42      FF      50      S      irq/91-rtc0      irq_thread
43      43      FF      50      S      irq/92-rtc0      irq_thread
44      44      FF      50      S      irq/80-mmc0      irq_thread
45      45      FF      50      S      irq/150-mmc0     irq_thread
47      47      FF      50      S      irq/44-mmc1      irq_thread
52      52      FF      50      S      irq/86-44e0b000  irq_thread
59      59      FF      50      S      irq/52-tilcdc    irq_thread
65      65      FF      50      S      irq/56-4a100000  irq_thread
66      66      FF      50      S      irq/57-4a100000  irq_thread
67      67      FF      50      S      irq/58-4a100000  irq_thread
68      68      FF      50      S      irq/59-4a100000  irq_thread
76      76      FF      50      S      irq/88-OMAP UAR  irq_thread

In this case, a BeagleBone running linux-yocto-rt, only the gp_timer interrupt was not threaded. It is normal that the timer interrupt handler be run in-line.

Here is a suggested order of descending thread priorities:

You can change the priorities using the chrt command as part of the boot script, using a command like this:

# chrt -f -p 90 `pgrep irq/28-edma`

The pgrep command is part of the procps package.