The Linux kernel's extensible scheduler class " [1]sched_ext " to allow for custom CPU scheduling policies as BPF programs continues enabling new functionality. Queued up in the sched_ext development code ahead of next month's Linux 7.1 cycle is the new SCX_ENQ_IMMED capability for enabling tighter control over when tasks land on a CPU.

The new SCX_ENQ_IMMED flag for sched_ext controls task enqueue behavior by ensuring that a task is enqueued to a local Dispatch Queue (DSQ) if it's able to start running immediately. In other words, if it can be executed immediately.

Longtime Linux developer Tejun Heo worked on this SCX_ENQ_IMMED flag for sched_ext and explained in [2]the patch :

"Add SCX_ENQ_IMMED enqueue flag for local DSQ insertions. Once a task is dispatched with IMMED, it either gets on the CPU immediately and stays on it, or gets reenqueued back to the BPF scheduler. It will never linger on a local DSQ behind other tasks or on a CPU taken by a higher-priority class.

...

This enables tighter scheduling latency control by preventing tasks from piling up on local DSQs. It also enables opportunistic CPU sharing across sub-schedulers - without this, a sub-scheduler can stuff the local DSQ of a shared CPU, making it difficult for others to use."

With the patch now part of sched_ext's "for-7.1" Git branch, you can expect to see SCX_ENQ_IMMED submitted as part of next month's Linux 7.1 merge window.



[1] https://www.phoronix.com/search/sched_ext

[2] https://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git/commit/?h=for-7.1&id=98d709cba3193f0bec54da4cd76ef499ea2f1ef7