arceos-fairsched

A standalone preemptive scheduling application running on ArceOS unikernel, with all dependencies sourced from crates.io. Demonstrates preemptive CFS (Completely Fair Scheduler) with timer-interrupt-driven task switching across multiple architectures.

What It Does

This application demonstrates preemptive scheduling and inter-task communication:

1. CFS scheduling: The sched-cfs feature enables the Completely Fair Scheduler, which assigns CPU time slices based on virtual runtime fairness.
2. Timer interrupts: The sched-cfs feature implies irq, enabling hardware timer interrupts that trigger preemption — tasks are switched automatically without explicit yield_now() calls.
3. Producer task (worker1): Pushes 257 messages (0..=256) into a shared queue without yielding. The CFS scheduler preempts it via timer interrupts.
4. Consumer task (worker2): Pops messages from the queue. Only yields when the queue is empty.
5. SpinNoIrq lock: The shared VecDeque is protected by an interrupt-safe spinlock to prevent deadlocks during preemption.

Cooperative vs Preemptive

	arceos-msgqueue (cooperative)	arceos-fairsched (preemptive)
Scheduler	FIFO	CFS
Producer yields?	Yes (yield_now())	No (preempted by timer)
Task switching	Voluntary	Automatic (timer IRQ)
Fairness	Manual	Automatic (vruntime)

Supported Architectures

Architecture	Rust Target	QEMU Machine	Platform
riscv64	riscv64gc-unknown-none-elf	qemu-system-riscv64 -machine virt	riscv64-qemu-virt
aarch64	aarch64-unknown-none-softfloat	qemu-system-aarch64 -machine virt	aarch64-qemu-virt
x86_64	x86_64-unknown-none	qemu-system-x86_64 -machine q35	x86-pc
loongarch64	loongarch64-unknown-none	qemu-system-loongarch64 -machine virt	loongarch64-qemu-virt

Prerequisites

• Rust nightly toolchain (edition 2024)

  rustup install nightly
  rustup default nightly

• Bare-metal targets (install the ones you need)

  rustup target add riscv64gc-unknown-none-elf
  rustup target add aarch64-unknown-none-softfloat
  rustup target add x86_64-unknown-none
  rustup target add loongarch64-unknown-none

• QEMU (install the emulators for your target architectures)

  # Ubuntu/Debian
  sudo apt install qemu-system-riscv64 qemu-system-aarch64 \
                   qemu-system-x86 qemu-system-loongarch64  # OR qemu-system-misc
  
  # macOS (Homebrew)
  brew install qemu

• rust-objcopy (from cargo-binutils, required for non-x86_64 targets)

  cargo install cargo-binutils
  rustup component add llvm-tools

Quick Start

# install cargo-clone sub-command
cargo install cargo-clone
# get source code of arceos-fairsched crate from crates.io
cargo clone arceos-fairsched
# into crate dir
cd arceos-fairsched
# Build and run on RISC-V 64 QEMU (default)
cargo xtask run

# Build and run on other architectures
cargo xtask run --arch aarch64
cargo xtask run --arch x86_64
cargo xtask run --arch loongarch64

# Build only (no QEMU)
cargo xtask build --arch riscv64
cargo xtask build --arch aarch64

Expected output (abbreviated, interleaving depends on timer preemption):

Multi-task(Preemptible) is starting ...
Wait for workers to exit ...
worker1 ... ThreadId(4)
worker1 [0]
worker1 [1]
...
worker1 [42]
worker2 ... ThreadId(5)
worker2 [0]
worker2 [1]
...
worker1 ok!
worker2 [256]
worker2 ok!
Multi-task(Preemptible) ok!

Note: The exact interleaving varies across runs and architectures because it depends on timer interrupt timing.

QEMU will automatically exit after printing the message.

Project Structure

app-fairsched/
├── .cargo/
│   └── config.toml       # cargo xtask alias & AX_CONFIG_PATH
├── xtask/
│   └── src/
│       └── main.rs       # build/run tool (QEMU launch, no pflash)
├── configs/
│   ├── riscv64.toml      # Platform config
│   ├── aarch64.toml
│   ├── x86_64.toml
│   └── loongarch64.toml
├── src/
│   └── main.rs           # Preemptive producer-consumer with CFS
├── build.rs              # Linker script path setup (auto-detects arch)
├── Cargo.toml            # Dependencies (axstd with sched-cfs feature)
└── README.md

Key Components

Component	Role
axstd	ArceOS standard library (replaces Rust's std in no_std environment)
axtask	Task scheduler with CFS algorithm, run queues, and preemption support
axsync	Synchronization primitives, provides SpinNoIrq for the message queue
sched-cfs feature	Enables CFS (Completely Fair Scheduler) with timer-interrupt-driven preemption
multitask feature	Enables multi-task support with thread::spawn / thread::join
paging feature	Enables page table management for MMIO region mapping

ArceOS Tutorial Crates

This crate is part of a series of tutorial crates for learning OS development with ArceOS. The crates are organized by functionality and complexity progression:

#	Crate Name	Description
1	arceos-helloworld	Minimal ArceOS unikernel application that prints Hello World, demonstrating the basic boot flow
2	arceos-collections	Dynamic memory allocation on a unikernel, demonstrating the use of String, Vec, and other collection types
3	arceos-readpflash	MMIO device access via page table remapping, reading data from QEMU's PFlash device
4	arceos-childtask	Multi-tasking basics: spawning a child task (thread) that accesses a PFlash MMIO device
5	arceos-msgqueue	Cooperative multi-task scheduling with a producer-consumer message queue, demonstrating inter-task communication
6	arceos-fairsched (this crate)	Preemptive CFS scheduling with timer-interrupt-driven task switching, demonstrating automatic task preemption
7	arceos-readblk	VirtIO block device driver discovery and disk I/O, demonstrating device probing and block read operations
8	arceos-loadapp	FAT filesystem initialization and file I/O, demonstrating the full I/O stack from VirtIO block device to filesystem
9	arceos-userprivilege	User-privilege mode switching: loading a user-space program, switching to unprivileged mode, and handling syscalls
10	arceos-lazymapping	Lazy page mapping (demand paging): user-space program triggers page faults, and the kernel maps physical pages on demand
11	arceos-runlinuxapp	Loading and running real Linux ELF applications (musl libc) on ArceOS, with ELF parsing and Linux syscall handling
12	arceos-guestmode	Minimal hypervisor: creating a guest address space, entering guest mode, and handling a single VM exit (shutdown)
13	arceos-guestaspace	Hypervisor address space management: loop-based VM exit handling with nested page fault (NPF) on-demand mapping
14	arceos-guestvdev	Hypervisor virtual device support: timer virtualization, console I/O forwarding, and NPF passthrough; guest runs preemptive multi-tasking
15	arceos-guestmonolithickernel	Full hypervisor + guest monolithic kernel: the guest kernel supports user-space process management, syscall handling, and preemptive scheduling

Progression Logic:

• #1–#8 (Unikernel Stage): Starting from the simplest output, these crates progressively introduce memory allocation, device access (MMIO / VirtIO), multi-task scheduling (both cooperative and preemptive), and filesystem support, building up the core capabilities of a unikernel.
• #9–#11 (Monolithic Kernel Stage): Building on the unikernel foundation, these crates add user/kernel privilege separation, page fault handling, and ELF loading, progressively evolving toward a monolithic kernel.
• #12–#15 (Hypervisor Stage): Starting from minimal VM lifecycle management, these crates progressively add address space management, virtual devices, timer injection, and ultimately run a full monolithic kernel inside a virtual machine.

License

GPL-3.0-or-later OR Apache-2.0 OR MulanPSL-2.0