The preemptive EDF (Earliest Deadline First) scheduler requires a mechanism for tasks to voluntarily yield CPU time while blocked on delays. In RISC-V M-mode, the ecall instruction provides a clean way to trigger a synchronous trap that invokes the scheduler without relying on timer interrupts.

The dispatcher function now accepts a from_timer parameter to distinguish between timer-driven preemption and ecall-driven yields. Timer interrupts increment the system tick counter and process time slices, while ecall-based yields skip tick advancement to prevent time drift.

When handling ecall from M-mode, the trap handler advances mepc past the 4-byte ecall instruction to prevent re-execution upon return. Critically, the ISR stack frame must also be updated because the ISR epilogue restores mepc from the saved frame rather than the CSR directly. Without this fix, mret would jump back to the ecall instruction causing an infinite trap loop.

The logger subsystem gains a flush mechanism with a direct_mode flag that bypasses the async queue. This ensures multi-line output like statistics reports prints in order, as printf normally enqueues to the ring buffer which the logger task drains asynchronously.

The rtsched test application validates the EDF implementation by running periodic RT tasks with different periods and deadlines, measuring execution counts, deadline misses, response times, and jitter to verify correct scheduler behavior.

Close #26

Summary by cubic

Introduce a preemptive EDF scheduler with ecall-based context switching on RISC‑V and a flushable logger for ordered output. Includes a test app that validates deadlines, response times, jitter, fairness, and non‑RT starvation.

• New Features

  • EDF RT scheduler: picks earliest absolute deadline; deadlines reset on period expiry; non‑RT uses round‑robin.
  • RISC‑V ecall yield: trap advances mepc and ISR frame, passes/returns SP to do_trap, and calls dispatcher(0); timer ISR calls dispatcher(1) and maintains mtimecmp.
  • Preemptive plumbing: dispatcher(int from_timer); TCB.sp; hal_build_initial_frame(), hal_switch_stack(); hal_timer_irq_enable()/disable(); NOSCHED toggles MTIE and is gated by scheduler_started; direct UART in traps; default task stack is 8 KiB.
  • Logger: mo_logger_flush(), mo_logger_async_resume(), mo_logger_direct_mode(); printf/puts honor direct mode to keep multi‑line reports ordered.
  • rtsched app: 3 periodic RT tasks + background + idle; aggregates executions/misses/response/jitter and prints fairness/starvation summary.

• Migration

  • Update dispatcher() calls to dispatcher(int from_timer) (timer: 1; ecall/yield: 0).
  • Wrap multi‑line reports with mo_logger_flush() … mo_logger_async_resume() for ordered output.
  • For non‑RISC‑V ports, implement hal_switch_stack(), hal_build_initial_frame(), and hal_timer_irq_enable()/disable().
  • Note: NOSCHED macros no longer retarget mtimecmp; they only toggle the timer interrupt bit and respect scheduler_started. Default task stack is now 8 KiB.

^{Written for commit 424616f. Summary will update automatically on new commits.}