Speaker
Description
Transitioning a CPU into and out of idle has a non-negligible energy overhead. During low-utilization states, frequent background wakeups repeatedly pay this "wakeup tax." Because the Energy Model only prices active OPP power and not idle-state transition costs, EAS defaults to spreading tasks across a cluster rather than packing them.
Prototyping with sched_ext on a Pixel 9 running Linux 7.1 as a proof-of-concept, we measure the power and latency tradeoffs of batching non-urgent wakeups onto already-active CPUs. We show that reducing idle exits measurably lowers CPU power, and that the tail-latency penalty of packing comes from committing tasks to a specific CPU at wakeup time. Deferring CPU selection until dispatch time achieves the same power savings at a fraction of the latency cost.
Discussion points:
- Energy Aware Scheduling: Should the Energy Model expose an idle-exit cost term, or rely on a low-utilization packing heuristic?
- Cluster-scoped deferral: Can we support cluster-scoped task push-pull (similar to RT pushable tasks) instead of committing at select_task_rq()? Would this make it easier to batch?
- Classifying latency-tolerance: What task QoS hints and system-state signals should govern wakeup deferral?