Interrupts and Tasks

Usually, timer interrupts are used for preemption
For example, both RM and EDF rely on preemption
In the simplest implementation, you fire a periodic timer
Every time the timer ticks, you make a scheduling choice
The NVIC even includes it's own "system tick" IRQ
Unfortunately, a periodic interrupt is a waste of CPU time
20us every ms is 2% of CPU power
With a slower clock, it can even be much worse
You can't raise your HZ if you interrupt at every tick
Linux deprecates the periodic interrupt since at at least 2011

And preemption brings in semaphores, spin locks, and more

Device interrupts are used to restart a stopped task
An interactive shell waiting for the serial port
An SPI data transfer waiting for a reply

But this creates the need for timeouts and recovery