Writing a Minimal Scheduler with eBPF, sched_ext, and C

Today, eBPF is used for software-defined networking, observability, monitoring tools, and more. Previously, creating these was labor-intensive and had a high barrier to entry. With the new scheduler extensions, we can now add custom schedulers to this list. Sched_ext allows us to write schedulers with custom policies directly in eBPF.
In this talk, we’ll develop a basic FIFO (First-In-First-Out) scheduler in C to show you how to get started with writing your own. If you’re interested in diving deeper into eBPF, join us for a quick hands-on intro to custom scheduling!

Transcript

1. Writing a Minimal Scheduler with eBPF, sched_ext, and C timetableworld.com Johannes Bechberger (SAP SE)
2. Sport Sleep Work Cook https://mostlynerdless.de/blog/2013/12/08/real-life-practice-in-lottery-scheduling/
3. CPU 1 ... CPU 2 ... Task 1 Task 2
4. CPU Time Task 1 Task 2
5. CPU 1 Time Task 1 Task 2
6. CPU 1 Time Task 1 Task 2
7. CPU 1 Time 2 Task 1 Task 2
8. CPU 1 Time 2 1 Task 1 Task 2
9. CPU 1 Time 2 1 1 Task 1 Task 2
10. CPU 1 Time 2 1 2 1 Task 1 Task 2
11. Task 1 Task 2 CPU 1 Time 2 1 2 1 1
12. Terminology Process ∪ Thread = Task
13. Typical Scheduler Goals Fairness
14. Typical Scheduler Goals Resource Utilization
15. Typical Scheduler Goals Overhead
16. Typical Scheduler Goals Responsiveness
17. Clash IO-bound CPU-bound vs
18. Problem: Only a few are implemented on your system
19. David Vernet, https://archives.kernel-recipes.org/wp-content/uploads/2025/01/Sched_Ext.pdf
20. David Vernet, https://archives.kernel-recipes.org/wp-content/uploads/2025/01/Sched_Ext.pdf
21. Let's create our own
22. Let's create our own Has someone done this before in this room?
23. Let's create our own How hard can it be?
24. Let's create our own +
25. eBPF is a crazy technology, it’s like putting JavaScript into the Linux kernel Brendan Gregg “ https://www.youtube.com/watch?v=tDacjrSCeq4
26. eBPF is a crazy technology, it’s like putting JavaScript into the Linux kernel Brendan Gregg “ https://www.facesofopensource.com/brendan-gregg/
27. Courtesy of Mohammed Aboullaite eBPF runtime
28. Courtesy of Mohammed Aboullaite eBPF runtime
29. Courtesy of Mohammed Aboullaite
30. Here comes sched-ext
31. 1.Ease of experimentation and exploration 2.Customization 3.Rapid scheduler deployments https://lwn.net/Articles/978911/ “
32. https://slowroads.io/ Example Application
33. Structure • Idea • Implementation • Demo • Modifying values
34. Structure • Idea • Implementation • Demo • Modifying values Having fun with schedulers
35. Structure • Idea • Implementation • Demo • Modifying values Having fun with schedulers github.com/parttimenerd/minimal-scheduler Try it yourself
36. First-Come, First-Served Scheduler Run as long as you want, we won’t stop you
37. Task 1 Task 2 CPU 1 Time 2 1 2 1 1
38. Task 1 Task 2 CPU 1 Time
39. fcfs.bpf.c
40. Making errors is normal !!/** * @timeout_ms: The maximum amount of time, in milliseconds, that a * runnable task should be able to wait before being scheduled. The * maximum timeout may not exceed the default timeout of 30 seconds. * * Defaults to the maximum allowed timeout value of 30 seconds. !*/ u32 timeout_ms; https://github.com/torvalds/linux/blob/master/kernel/sched/ext.c
41. First-Come, First-Out Scheduler The early bird eats the time slice
42. Task 1 Task 2 CPU 1 Time 2 1 2 1 1
43. Task 1 CPU 1 Local Queue CPU 2 Local Queue Global Queue Scheduler ... ... Scheduler dance
44. fifo.bpf.c
45. Lottery Scheduler Are you the lucky task who gets the time slice?
46. Lottery Scheduler Task 1 CPU 1 CPU 2 Scheduling Queue treated as a lottery bowl ... ... Draw randomly from queue Ask for new task Return finished task Enqueue task for the first time Task 2 2 1 3 4 7 6 1
47. lottery.bpf.c
48. VRuntime-based Scheduler • Tracks virtual runtime (vruntime) of tasks (time on CPU) • Task with shortest vruntime runs first • Use a simple priority queue runtime
49. VRuntime-based Scheduler You already run quite a long time, lets choose another task
50. Andrea Righi, https://fosdem.org/2025/schedule/event/fosdem-2025-4618-level-up-your-linux-gaming-how-schedext-can-save-your-fps/
51. Andrea Righi, https://fosdem.org/2025/schedule/event/fosdem-2025-4618-level-up-your-linux-gaming-how-schedext-can-save-your-fps/
52. vtime.bpf.c
53. Andrea Righi, https://fosdem.org/2025/schedule/event/fosdem-2025-4618-level-up-your-linux-gaming-how-schedext-can-save-your-fps/
54. vtime.bpf.c A basic version of the Completely Fair Scheduler
55. What else can we do?
56. Implement good schedulers
57. David Vernet, https://archives.kernel-recipes.org/wp-content/uploads/2025/01/Sched_Ext.pdf
58. https://github.com/sched-ext/scx
59. https://www.phoronix.com/news/Linux-Rust-Sched-To-eBPF
61. Experiments
62. Paper: https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/tr-2007-149.pdf
63. An erratic scheduler
64. An erratic scheduler https://lwn.net/SubscriberLink/1007689/922423e440f5e68a/
65. sapmachine.io
66. Can we write them in Java?
67. https://github.com/parttimenerd/hello-ebpf
68. An erratic scheduler https://lwn.net/SubscriberLink/1007689/922423e440f5e68a/
69. An erratic scheduler https://lwn.net/SubscriberLink/1007689/922423e440f5e68a/ Written in Java
71. One that reacts to sound https://github.com/parttimenerd/sound-of-scheduling
72. eBPF is a crazy technology, it’s like putting JavaScript into the Linux kernel Brendan Gregg “ https://www.youtube.com/watch?v=tDacjrSCeq4
73. One the produces sound https://github.com/parttimenerd/loudness-scheduler
74. Do you have policy ideas?
75. timetableworld.com Johannes Bechberger mostlynerdless.de OpenJDK Developer, SAP Fin. https://github.com/parttimenerd/minimal-scheduler sapmachine.io