CS 452/652 Winter 2024 - Lecture 9

Events, Clock, Idle

Feb 6, 2024 prev next

AwaitEvent

• synchronization: user tasks ↔ device
• AwaitEvent() argument and return code → design decision
  • eventIds are defined by the kernel, known to the application
    • example: clock tick
  • return code - event-specific interpretation
    • example: subtype, count, data
  • micro-kernel: put as much functionality as possible in user-level tasks
• AwaitEvent() blocks calling task
• matchmaking task ↔ event
  • 0, 1, N tasks waiting? store in queue (cf. SRR)?
  • N tasks wait for event? complexity? unblock one or all?
  • 0 tasks wait for interrupt? buffer in kernel?
    • multiple same interrupts without waiting task indicate performance problem
    • use task prioritity to make sure that hardware interrupts are handled promptly
• intention of AwaitEvent() is to expose device interrupts to the application
  • program periodic interrupt in kernel
  • can be decoupled: 1ms timer interrupt vs. 10ms clock tick

Timer Interrupt Handling

• save running task state
• read GICC_IAR to learn interrupt type
• if system timer interrupt
  • update Cn compare register - overflow?
  • clear interrupt via CS status register
  • confirm interrupt to GICC_EOIR
  • unblock task(s) waiting for event
• loop: read GICC_IAR again?
• schedule next task
• resume task
• note: system timer interrupts cannot be disabled at device

Clock Server

• API:
  Time(int tid) - current time (in ticks)
  Delay(tid, ticks) - delay, relative to current time
  DelayUntil(tid, ticks) - delay, to absolute time
• these are Send() wrappers; tid is tid of a Clock server
• Time measured in "ticks"

Server Implementaion

• AwaitEvent() might block; server pattern?
  • if server calls AwaitEvent(), it cannot Receive() new requests (e.g., Time())
• delegate event blocking to special task: Notifier pattern

Clock Notifier

    for (;;) {
        AwaitEvent();
        Send(); // send tick to clock server
    }

Clock Server

    for (;;) {
        Receive();
        process(); // manage tasks
        Reply(); // if applicable
    }

• needs to manage list of suspended tasks
• DelayUntil = Time + Delay, but at server
• convencience for real-time loop

Idle/Halt

• what to do when processor has no other work?

  • SETI@home? crypto-mining? → 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 instruction wfi (or wfe?)

• how to measure idle time?

• idle time shoud be high: >> 90%

K3 overview

• Note: system with 1ms clock tick is fine as well