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