sched refactor

[hexagonal-sun]

• sched: introduce Work as the unified scheduleable unit
  Refactor the scheduler so all scheduleable work is wrapped in Arc,
  replacing the previous per-CPU wait_q design where sleeping tasks were
  bound to a specific CPU. Wakers now hold direct Arc references and
  can re-enqueue tasks on any CPU upon wakeup.

  Key changes:

  • Add Work struct wrapping OwnedTask with an AtomicTaskState and
    scheduler metadata (SchedulerData), replacing the old SchedulableTask.
    Remove Task::state (Arc<SpinLock>). Work::state is now the
    single source of truth for task state.

  • Rewrite the run queue using BinaryHeap-based eligible/ineligible split
    (EEVDF) with a dedicated VClock, replacing the BTreeMap linear scan.
    Extract vclock into its own module.

  • Rewrite wakers to hold Arc directly instead of looking up tasks
    by TaskDescriptor from TASK_LIST.

  • Replace lock-based sleep transitions in uspc_ret with atomic CAS
    (try_sleep_current) that correctly detects concurrent Woken state.

  • Simplify least-tasked-CPU metric to use only run-queue weight, since
    sleeping tasks are no longer bound to any CPU.

  • Add current_work() accessor.

• sched: runqueue: derfer task drops and drop Finished tasks
  Fix two issues:

  1. When a task is dropped from the runqueue, it will trigger the
     destructors for the task to run. This may well call wakers to wake up
     parent processes, other ends of pipes, etc. If we do that while
     SCHED_STATE is still borrowed, this causes a double-borrow panic. Fix
     this by deferring all drops until after we have unlocked SCHED_STATE.

  2. Tasks inside the runqueue which are yet to be scheduled which become
     finished will be returned by find_next_task and the state will be set
     to TaskState::Running overwriting the fact that this task had
     Finished. We'd then queue this task forever. Filter finished tasks in
     find_next_task and add them to the defered drop list.

• sched: runqueue: fix tick and runqueue insertion bug
  Ensure that tick() is called on the current task, and allow the task
  account decide whether we should try to switch to another task.

  Also, ensure accounting is updated for freshly inserted tasks into the
  runqueue.

[arihant2math]

LGTM

[arihant2math]

The reason I included the number of waiting tasks was to prevent a single CPU from taking on all tasks because they were bursty or I/O bound. Ideally we'd rather schedule onto a CPU with less waiting tasks, all else equal. Maybe we should consider total weight (run queue + waiting queue).

Not something that needs to change in the PR.

[arihant2math]

Also probably worth removing this atomic. I think linux does proper searching. However, for this we'd need a concept of "shared scheduler state".

[arihant2math]

nice diagram :)