CS 452/652 Fall 2022 - Lecture 10

Interrupt Handling

1. user task executing
   [interrupt]
2. handler saves task state and switches to kernel
3. kernel makes task ready
4. kernel determines interrupt number
5. kernel processes interrupt
6. kernel switches to next ready task

Interrupt Processing

• multiple interrupts pending: loop over IAR or one-shot processing
• cause for interrupt (at device):
  • event, such as timer underflow → edge trigger
    • device will not trigger interrupt again unless previous one confirmed
  • status, such as buffer empty → level trigger
    • after completion of interrupt handler, interrupt would trigger again!
      → force status change or disable/mask interrupt
• example: confirm timer interrupt by writing to EOIR.

AwaitEvent

• matchmaking: task ↔ event
• task(s) waiting for interrupt to occur: store one or queue? → design decision
• interrupt without waiting task: buffer one or several? or failure?
  • excessive buffering is not helpful in any producer/consumer scenario
  • multiple same interrupts without waiting task indicate performance problem!
  • use task prioritity to make sure that hardware interrupts are handled promptly
• AwaitEvent() argument and return code → design decision

Timer Interrupt

• timer compare register (CN reg vs. interrupt #)
  • rearm timer - new value (cf. real-time loop)
  • overflow? no problem...
• confirm interrupt before/during/after processing

Clock Server

• main loop of receiving and processing requests
• server never blocks ⇒ cannot call AwaitEvent()
• task blocking delegated to clock notifier
• server & notifier → general design pattern

Server Processing

• TIME: reply to client with current clock value
• DELAY: park client task (using tick & tid) in timer list
  • two variations: absolute vs. relative tick
• TICK: advance clock value and check parked tasks in timer list
  • reply to clients whose waiting times have completed
• timer list should probably be a sorted data structure

Clock Notifier

• main loop (at high priority)

  loop:
    AwaitEvent()
    Send() tick to clock server
  

Idle/Halt

• what to do when processor has not other work?
  • SETI? cryptomining? → often not appropriate!
  • maintenance, e.g., garbage collection → not relevant for CS 452 kernel
  • diagnosis, such as computation of idle fraction
  • save energy → low-power halt! (cf. "race-to-idle/sleep")
• implement as dedicated idle task at lowest priority

  for (;;) {
    while (maintenance()) yield();
    park();
  }
  

• ARMv8 instructions wfi (or wfe)