Fixed-priority preemptive scheduler

docs/scheduling.md

@@ -0,0 +1,92 @@
+# Scheduling
+
+The scheduler turns danos from a linear "boot then halt" kernel into a **running
+multitasking system**. It's **fixed-priority preemptive**: the highest-priority
+ready task always runs, and tasks at the same priority take turns. That model is
+chosen for [real-time](vision.md) — it's predictable (you can reason about which
+task runs when) and its decisions are O(1), unlike a fair-share scheduler.
+
+The scheduler proper (`src/sched.zig`) is generic; the context switch and new-task
+stack setup are architecture-specific (`src/arch/x86_64/`, see [arch](arch.md)).
+
+## Tasks
+
+A **task** is a kernel thread: ring-0 code with its own 16 KiB stack (allocated
+from the [heap](heap.md)). A task struct holds its saved stack pointer, priority,
+state, and a ready-queue link. The currently-running kernel context (kmain)
+registers itself as task 0, so there's always something to switch *from*.
+
+## The context switch
+
+Switching tasks means swapping stacks. `switch_context(old, new)` (in `isr.s`)
+saves the **callee-saved** registers on the current stack, stores the stack pointer
+into the old task, loads the new task's stack pointer, restores *its* callee-saved
+registers, and `ret`s — landing wherever the new task was last suspended. Only
+callee-saved registers are handled explicitly: to the compiler this looks like a
+normal function call, so it already preserves the caller-saved ones itself (this is
+the [SysV](sysv.md) convention doing the work).
+
+A **freshly spawned** task has never run, so there's nothing to restore. Its stack
+is faked to look as if it had just called `switch_context`: `init_task_stack` lays
+down a return address pointing at `task_trampoline` and zeroed callee-saved slots
+(smuggling the entry function in via the `r15` slot). When first switched to, the
+`ret` lands in the trampoline, which enables interrupts and calls the entry.
+
+## Two ways to switch, one flag discipline
+
+`schedule()` — pick the best task and switch — runs from two places:
+
+- **`yield()`** — a task voluntarily gives up the CPU.
+- **`tick()`** — the 1000 Hz [timer](device-interrupts.md) preempts the running
+  task. This is what lets a task that never yields still share the CPU.
+
+The subtlety in mixing them is the **interrupt flag (IF)**. The rule: `switch_context`
+is always entered with interrupts *disabled* — naturally so inside the timer ISR,
+and explicitly (`cli`) in `yield`. Then every task ends up with interrupts enabled
+again through whichever path resumes it:
+
+- a task suspended in `yield` re-enables them (`sti`) right after `schedule` returns;
+- a task suspended mid-ISR resumes through the interrupt return (`iretq`), which
+  restores the `RFLAGS` it had when it was preempted (IF set);
+- a brand-new task enables them in the trampoline.
+
+One related detail: the timer interrupt is **acknowledged (EOI) before** its handler
+runs, so a handler that switches tasks and doesn't return promptly can't stall the
+LAPIC from delivering the next tick.
+
+## Priority selection, in O(1)
+
+Ready tasks live in a **FIFO queue per priority level** (8 levels), plus a
+**bitmap** with one bit per non-empty level. Picking the next task is: find the
+highest set bit (one instruction), take the front of that level's queue. No list
+walking, no scanning — the decision cost is constant regardless of how many tasks
+exist, which is what a real-time scheduler needs.
+
+- **Highest priority wins.** A ready high-priority task always runs before a
+  lower-priority one.
+- **Round-robin within a level.** When a task is descheduled it goes to the *back*
+  of its level's queue, so equal-priority tasks share the CPU fairly.
+
+## Verifying it
+
+Two tests (see [testing.md](testing.md)) prove the two guarantees:
+
+- **`sched`** spawns three tasks that busy-loop *without ever yielding*. They all
+  make progress — which can only happen if the timer is **preempting** between them
+  and the context switch is correct (nothing yields voluntarily).
+- **`priority`** (with preemption off, for determinism) spawns tasks at three
+  priorities; they run and exit **highest-priority first** — `[6, 4, 2]`.
+
+## What's next (not done here)
+
+- **Blocking and sleep.** Right now a task can only yield or exit; it can't wait for
+  a condition or a duration. `sleep(ms)` (on the calibrated clock) and blocking
+  come next, and are what a real-time task really needs.
+- **Priority inheritance.** Once tasks block on shared resources (locks, IPC),
+  danos will need it to bound priority inversion — a [real-time](vision.md)
+  requirement.
+- **Task exit / a reaper.** `exit` currently leaks the task's stack; nothing frees
+  finished tasks' memory yet.
+- **Per-address-space tasks.** Today all tasks share the kernel address space. User
+  processes will each get their own, switching page tables (CR3) on the context
+  switch.