Sched_ext Schedulers and Tools

sched_ext is a Linux kernel feature which enables implementing kernel thread schedulers in BPF and dynamically loading them. This repository contains various scheduler implementations and support utilities.

sched_ext enables safe and rapid iterations of scheduler implementations, thus radically widening the scope of scheduling strategies that can be experimented with and deployed; even in massive and complex production environments.

• The scx_layered case study concretely demonstrates the power and benefits of sched_ext.
• For a high-level but thorough overview of the sched_ext (especially its motivation), please refer to the overview document.
• For a description of the schedulers shipped with this tree, please refer to the schedulers document.
• The following video is the scx_rustland scheduler which makes most scheduling decisions in userspace Rust code showing better FPS in terraria while kernel is being compiled. This doesn't mean that scx_rustland is a better scheduler but does demonstrate how safe and easy it is to implement a scheduler which is generally usable and can outperform the default scheduler in certain scenarios.

scx_rustland-terraria

While the kernel feature is not upstream yet, we believe sched_ext has a reasonable chance of landing upstream in the foreseeable future. Both Meta and Google are fully committed to sched_ext and Meta is in the process of mass production deployment. See (#kernel-feature-status) for more details.

In all example shell commands, $SCX refers to the root of this repository.

Getting Started

All that's necessary for running sched_ext schedulers is a kernel with sched_ext support and the scheduler binaries along with the libraries they depend on. Switching to a sched_ext scheduler is as simple as running a sched_ext binary:

root@test ~# cat /sys/kernel/sched_ext/state /sys/kernel/sched_ext/*/ops 2>/dev/null
disabled
root@test ~# scx_simple
local=1 global=0
local=74 global=15
local=78 global=32
local=82 global=42
local=86 global=54
^Zfish: Job 1, 'scx_simple' has stopped
root@test ~# cat /sys/kernel/sched_ext/state /sys/kernel/sched_ext/*/ops 2>/dev/null
enabled
simple
root@test ~# fg
Send job 1 (scx_simple) to foreground
local=635 global=179
local=696 global=192
^CEXIT: BPF scheduler unregistered

scx_simple is a very simple global vtime scheduler which can behave acceptably on CPUs with a simple topology (single socket and single L3 cache domain).

Above, we switch the whole system to use scx_simple by running the binary, suspend it with ctrl-z to confirm that it's loaded, and then switch back to the kernel default scheduler by terminating the process with ctrl-c. For scx_simple, suspending the scheduler process doesn't affect scheduling behavior because all that the userspace component does is print statistics. This doesn't hold for all schedulers.

In addition to terminating the program, there are two more ways to disable a sched_ext scheduler - sysrq-S and the watchdog timer. Ignoring kernel bugs, the worst damage a sched_ext scheduler can do to a system is starving some threads until the watchdog timer triggers.

As illustrated, once the kernel and binaries are in place, using sched_ext schedulers is straightforward and safe. While developing and building schedulers in this repository isn't complicated either, sched_ext makes use of many new BPF features, some of which require build tools which are newer than what many distros are currently shipping. This should become less of an issue in the future. For the time being, the following custom repositories are provided for select distros.

Ubuntu

Experimental sched_ext support for Ubuntu is provided by the following launchpad project:

https://launchpad.net/~arighi/+archive/ubuntu/sched-ext

Upgrading to 24.04 (NobleNumbat)

Currently, only the 24.04 release is supported. You can upgrade to 24.04 using the following command:

$ sudo do-release-upgrade -d

Installing the Kernel and Schedulers

$ sudo add-apt-repository -y --enable-source ppa:arighi/sched-ext
$ sudo apt install -y linux-generic-wip scx
$ sudo reboot

After the reboot, the scheduler binaries in /usr/sbin/scx_* should be usable. Note: they must be called with sudo like other BPF programs e.g. sudo scx_simple.

Setting up Dev Environment

$ apt source scx
$ sudo apt build-dep scx

Arch Linux

Adding the Repository

Install packages with a list of mirrors and GPG keys

sudo pacman -U 'https://mirror.cachyos.org/repo/x86_64/cachyos/cachyos-keyring-3-1-any.pkg.tar.zst' 'https://mirror.cachyos.org/repo/x86_64/cachyos/cachyos-mirrorlist-18-1-any.pkg.tar.zst'

Add the following custom repository section to /etc/pacman.conf.

# cachyos repos
[cachyos]
Include = /etc/pacman.d/cachyos-mirrorlist

You can also import the gpg key manually. This can be skipped if the signature checking is disabled when adding the repo.

$ sudo pacman-key --recv-keys F3B607488DB35A47 --keyserver keyserver.ubuntu.com
$ sudo pacman-key --lsign-key F3B607488DB35A47

If you haven't imported the GPG key, append the following line.

SigLevel = Never

Installing the Kernel and Schedulers

$ sudo pacman -Sy cachyos/linux-sched-ext cachyos/linux-sched-ext-headers cachyos/scx-scheds

Setting Up Dev Environment

In addition to the packages from the previous step, install the following.

$ sudo pacman -Sy cachyos/clang-github-bin meson cargo bpf pahole

clang-github-bin is necessary because the recommended clang version is 17 while Arch is still shipping 16. It's built from the AUR package of the same name. This is a repackage of the official LLVM release in .deb format.

Repository Structure

scx
|-- scheds               : Sched_ext scheduler implementations
|   |-- include          : Shared BPF and user C include files including vmlinux.h
|   |-- c                : Example schedulers - userspace code written C
|   \-- rust             : Example schedulers - userspace code written Rust
\-- rust                 : Rust support code
    \-- scx_utils        : Common utility library for rust schedulers

Build & Install

meson is the main build system but each Rust sub-project is its own self-contained cargo project and can be built and published separately. The followings are the dependencies and version requirements.

Note: Many distros only have earlier versions of meson, in that case just clone the meson repo and call meson.py e.g. /path/to/meson/repo/meson.py compile -C build. Alternatively, use pip e.g. pip install meson or pip install meson --break-system-packages (if needed).

• meson: >=1.2, build scripts under meson-scripts/ use bash and standard utilities including awk.
• clang: >=16 required, >=17 recommended
• libbpf: >=1.2.2 required, >=1.3 recommended (RESIZE_ARRAY support is new in 1.3). It's preferred to link statically against the source from the libbpf git repo, which is cloned during setup.
• Rust toolchain: >=1.72
• libelf, libz, libzstd if linking against staic libbpf.a
• bpftool (usually available in linux-tools-common)

Setting Up and Building

meson always uses a separate build directory. Running the following commands in the root of the tree builds and installs all schedulers under ~/bin.

Static linking against libbpf (preferred)

$ cd $SCX
$ meson setup build --prefix ~
$ meson compile -C build
$ meson install -C build

Note: meson setup will also clone the libbpf repo and meson compile will build it.

Dynamic linking against libbpf

$ cd $SCX
$ meson setup build --prefix ~ -D libbpf_a=disabled
$ meson compile -C build
$ meson install -C build

Note that meson compile step is not strictly necessary as install implies compile. The above also will build debug binaries with optimizations turned off, which is useful for development but they aren't optimized and big. For actual use you want to build release binaries. meson uses -D argument to specify build options. The configuration options can be specified at setup time but can also be changed afterwards and meson will do the right thing. To switch to release builds, run the following in the build directory and then compile and install again.

$ meson configure -Dbuildtype=release

Running meson configure without any argument shows all current build options. For more information on meson arguments and built-in options, please refer to meson --help and its documentation.

Building Specific Schedulers and Binary Locations

If you just want to build a subset of schedulers, you can specify the scheduler names as arguments to meson compile. For example, if we just want to build the simple example scheduler scheds/c/scx_simple and the Rust userspace scheduler scheds/rust/scx_rusty:

$ cd $SCX
$ meson setup build -Dbuildtype=release
$ meson compile -C build scx_simple scx_rusty

You can also specify -v if you want to see the commands being used:

$ meson compile -C build -v scx_pair

For C userspace schedulers such as the ones under scheds/c, the built binaries are located in the same directory under the build root. For example, here, the scx_simple binary can be found at $SCX/build/scheds/c/scx_simple.

For Rust userspace schedulers such as the ones under scheds/rust, the same directory under the build root is used as the cargo build target directory. Thus, here, the scx_rusty binary can be found at $SCX/build/scheds/rust/scx_rusty/release/scx_rusty.

SCX specific build options

While the default options should work in most cases, it may be desirable to override some of the toolchains and dependencies - e.g. to directly use libbpf built from the kernel source tree. The following meson build options can be used in such cases.

• bpf_clang: clang to use when compiling .bpf.c
• bpftool: bpftool to use when generating .bpf.skel.h
• libbpf_a: Static libbpf.a to use. Set this to "disabled" to link libbpf dynamically
• libbpf_h: libbpf header directories, only meaningful with libbpf_a option
• cargo: cargo to use when building rust sub-projects
• 'cargo_home': 'CARGO_HOME env to use when invoking cargo'
• offline: 'Compilation step should not access the internet'
• enable_rust: 'Enable the build of rust sub-projects'

For example, let's say you want to use bpftool and libbpf shipped in the kernel tree located at $KERNEL. We need to build bpftool in the kernel tree first, set up SCX build with the related options and then build & install.

$ cd $KERNEL
$ make -C tools/bpf/bpftool
$ cd $SCX
$ BPFTOOL=$KERNEL/tools/bpf/bpftool
$ meson setup build -Dbuildtype=release -Dprefix=~/bin \
    -Dbpftool=$BPFTOOL/bpftool \
    -Dlibbpf_a=$BPFTOOL/libbpf/libbpf.a \
    -Dlibbpf_h=$BPFTOOL/libbpf/include
$ meson install -C build

Note that we use libbpf which was produced as a part of bpftool build process rather than buliding libbpf directly. This is necessary because libbpf header files need to be installed for them to be in the expected relative locations.

Offline Compilation

Rust builds automatically download dependencies from crates.io; however, some build environments might not allow internet access requiring all dependencies to be available offline. The fetch target and offline option are provided for such cases.

The following downloads all Rust dependencies into $HOME/cargo-deps.

$ cd $SCX
$ meson setup build -Dcargo_home=$HOME/cargo-deps
$ meson compile -C build fetch

The following builds the schedulers without accessing the internet. The build directory doesn't have to be the same one. The only requirement is that the cargo_home option points to a directory which contains the content generated from the previous step.

$ cd $SCX
$ meson setup build -Dcargo_home=$HOME/cargo-deps -Doffline=true -Dbuildtype=release
$ meson compile -C build

Working with Rust Sub-projects

Each Rust sub-project is its own self-contained cargo project. When buildng as a part of this repository, meson invokes cargo with the appropriate options and environment variables to sync the build environment. When building separately by running cargo build directly in a sub-project directory, it will automatically figure out build environment. Please take a look at the scx_utils::BpfBuilder documentation for details.

For example, the following builds and runs the scx_rusty scheduler:

$ cd $SCX/scheds/rust/scx_rusty
$ cargo build --release
$ cargo run --release

Here too, the build step is not strictly necessary as it's implied by run.

Note that Rust userspace schedulers are published on crates.io and can be built and installed without cloning this repository as long as the necessary toolchains are available. Simply run:

$ cargo install scx_rusty

and scx_rusty will be built and installed as ~/.cargo/bin/scx_rusty.

systemd services

See: services

Kernel Feature Status

The kernel feature is not yet upstream and can be found in the sched_ext repository. The followings are important branches:

• sched_ext: The main development branch. This branch periodically pulls from the bpf-next tree to stay in sync with the kernel and BPF developments.
• sched_ext-release-*: sched_ext backports on top of released kernels. We plan to maintain backports for a few recent kernel releases until sched_ext is merged upstream. Currently maintained backports:
  • sched_ext-release-v6.6
• sched_ext-vN: Patchsets posted upstream. The v4 LKML thread has high-level discussions.
  • RFC: LMKL thread
  • `sched_ext-v2': LKML thread
  • `sched_ext-v3': LKML thread
  • `sched_ext-v4': LKML thread
  • `sched_ext-v5': LKML thread

Breaking Changes

A list of the breaking changes in the sched_ext kernel tree and the associated commits for the schedulers in this repo.

Getting in Touch

We aim to build a friendly and approachable community around sched_ext. You can reach us through the following channels:

• github: https://github.com/sched-ext/scx
• Slack: https://schedextworkspace.slack.com
• Reddit: https://reddit.com/r/sched_ext

We also hold weekly office hours every monday. Please see the #office-hours channel on slack for details. To join the slack community, you can use this link.

Additional Resources

There are blog posts and articles about sched_ext, which helps you to explore sched_ext in various ways. Followings are some examples:

• LWN: The extensible scheduler class (February, 2023)
• arighi's blog: Implement your own kernel CPU scheduler in Ubuntu with sched-ext (July, 2023)
• Changwoo's blog: sched_ext: a BPF-extensible scheduler class (Part 1) (December, 2023)