Linux 7.3-rc2 Scheduler Fix For Cache Aware Load Scheduling Misfits On Hybrid CPUs

Ahead of the Linux 7.3-rc2 kernel being released later today, a set of scheduler fixes have been sent out for this week's kernel test release. Most notable with the scheduler fixes are dealing with possible misfits for Cache-Aware Load Scheduling on hybrid CPUs like Intel Core (Ultra) processors. Cache Aware Scheduling is a wonderful Linux kernel feature for grouping together related tasks on CPU cores that can share the same last level cache (LLC). Cache Aware Scheduling can leaad to better cache locality and in turn lower latency and avoiding the costs of cache bouncing. But in the case of hybrid CPUs with big and little cores, there can be trade-offs if the grouped tasks end up getting all stuck to the E/little cores. Intel engineer Tim Chen worked out a fix for misfits with Cache Aware Scheduling on hybrid processors. He explained in that patch: "Cache-aware load balancing biases tasks toward their preferred LLC. On asymmetric CPU capacity systems (e.g. big.LITTLE) the destination LLC may contain CPUs that are too small to run the task. Pulling the task there turns it into a misfit, trading a cache-locality gain for a capacity loss that's more detrimental to performance. Guard both cache-aware migration entry points against this: - can_migrate_llc_task(): forbid the LLC migration when the task fits its source CPU but would not fit the destination CPU. - alb_break_llc(): veto the active balance under the same condition so the runnable task is not pushed onto a CPU that cannot accommodate it. Both checks are gated with checks for hybrid processors, so symmetric systems are unaffected. Tasks that already do not fit their source CPU are left to the existing LLC policy, since the move cannot make their fitness worse (this also preserves misfit up-migration to bigger CPUs). Additionally, if there are misfit tasks found in the load balancing classification phase, prioritize misfit task migrations over LLC load aggregation on asymmetric systems. A better fitting CPU will boost performance more than better cache locality." Good for overall system performance. That patch fixing things up is part of today's scheduler pull request.