Scheduler And Wait Protocol

SlopOS uses a synchronous, preemptive scheduler. Blocking paths funnel through OSTD WaitQueue or typed EventBus queues; async behavior is built in userland on top of pollable file descriptors and SlopRing.

Scheduler State

The scheduler packs task status, block reason, CPU hint, and an epoch into one atomic word. This replaces older paired-atomic state where a wake producer could observe one field before the other.

The runnable invariant is:

A non-idle task with Ready status must also have scheduler placement: current CPU ownership, one ready queue, one remote wake inbox, a wake publisher reservation, or a migration handoff.

Ready without placement is a bug. The rescue sweep is diagnostic backstop, not normal scheduling policy.

WaitQueue Pattern

Blocking follows the Linux-style order:

consumer:
  check condition
  take WaitQueue lock
  check condition again
  enqueue current task
  commit Running -> Blocked under the same lock
  drop lock and yield

producer:
  publish durable state
  take WaitQueue lock
  remove waiters
  wake Blocked -> Ready
  drop lock

The under-lock recheck closes the window where the producer publishes and drains an empty queue between the consumer's first check and enqueue.

EventBus And Poll

File descriptors expose readiness through FileOps::poll. The poll/select path registers the calling task on the relevant queues before checking readiness, then blocks with an optional timeout. SlopRing harvest blocking reuses this same shape: the blocking ring_enter call registers the calling task, re-probes in-flight operations, and posts CQEs when resources are ready.

Timers

Sleep and scheduler ticks use the LAPIC timer. HPET provides the high precision clock and calibration reference. The scheduler can program LAPIC one-shot deadlines for idle wakeups and then restore periodic mode in the timer ISR.