The extensible scheduler framework "sched_ext" that allows for custom schedulers as BPF programs has been working on sub-scheduler support in recent kernel cycles. With Linux 7.3, sub-scheduler support is now deemed feature complete.

In the works over the past year has been sub-scheduler support for allowing multiple scheduler instances to be attached to the cgroup hierarchy for dealing with different workloads on the system. Each application domain running on the Linux system could have its own BPF scheduler that would be tailored to its individual needs/behavior. After the past several kernel cycles building out the sub-scheduler capabilities, it's now considered feature complete for use.

Tejun Heo elaborated in the sched_ext pull request that was merged this week for Linux 7.3:

"Most of this cycle completes the enqueue-path support for hierarchical sub-scheduling, which makes sub-scheduler support feature complete: a root BPF scheduler can now hand a cgroup subtree over to a nested sub-scheduler together with revocable CPU grants, and the sub-scheduler owns all scheduling decisions for its tasks on those CPUs.

Development volume was high and a number of changes plugging holes in the new support landed late in the cycle. Also included are core scheduling fixes that were completed too late for the v7.2 release and are routed through this pull request.

- Sub-scheduler CPU delegation:

- Parent schedulers now grant and revoke per-CPU capabilities (enqueueing, preemption, CPU frequency control) on their children, enforced on every path a scheduler can reach a CPU through. Previously only dispatching could be delegated; this lets sub-schedulers fully schedule their CPUs.

- Rescue execution: a task whose scheduler doesn't have access to the CPUs the task needs to run on starved until the watchdog ejected the whole scheduler. The kernel now runs such tasks directly on a small bandwidth budget, turning a scheduler-killing failure into bounded degradation.

- Cgroup integration: tasks migrating across a sub-scheduler boundary weren't re-homed to the new owner, causing wrong-scheduler scheduling and a use-after-free. Sub-schedulers now take over their cgroup subtree and receive its cgroup callbacks.

- Arena objects now cross the kernel/BPF boundary as typed pointer arguments, translated transparently by the BPF tree's new arena argument support, replacing untyped arguments with manual translation.

- scx_qmap now demonstrates full hierarchical sub-scheduling.

- Robustness improvements: the abort path is now NMI-safe, fixing deadlocks when errors are raised from NMI context and making hardlockup recovery direct. Reenqueue loops that could monopolize a CPU ahead of the watchdog now eject the offending scheduler, and stalls are blamed on the scheduler actually responsible.

- Hardening: BPF-writable arena memory is validated before kernel use, and task slice and vtime writes got explicit synchronization rules, closing corruption vectors open to buggy or malicious schedulers.

- Core scheduling: sched_ext dispatching can drop the rq lock inside the core-wide pick, which let interleaving selections corrupt each other's state and hard-hang the machine. The selection now restarts when the lock was released. The task ordering callback was also invoked with its arguments swapped, and the default ordering is updated to work across sub-scheduler boundaries. The fixes are marked for stable."

The sched_ext changes are merged as of Thursday to Linux 7.3 Git.