AMD Posts Linux Patches For Capacity Aware Scheduling On Hybrid CPUs

On Saturday a new set of Linux kernel driver patches were posted by AMD for enabling Capacity Aware Scheduling on AMD heterogeneous/hybrid processors. The AMD P-State CPU frequency scaling driver currently does not export per-CPU capacity metrics today and thus the kernel scheduler is treating all cores as equal. This is poor behavior with the performance vs. efficiency core differences today and all the more so with the upcoming low-power core type believed to be for some Zen 6 Ryzen SKUs. This weekend's patches have AMD p-State publish asymmetric per-CPU capacities and enables it by default on capable hardware. Or the new behavior can be turned off with the new amd_case=disable boot parameter to disable AMD Capacity Aware Scheduling. "AMD heterogeneous processors combine multiple core types (e.g. performance and efficiency cores) whose maximum sustained performance and frequency differ. On such platforms the Linux scheduler relies on per-CPU capacity values to place and balance tasks across cores. Today amd-pstate does not export any per-CPU capacity information. As a result the scheduler sees every CPU as having identical capacity and treats performance and efficiency cores as interchangeable. Tasks are therefore placed and balanced without regard to core strength, which hurts both throughput and energy efficiency on hybrid systems." Those interested can see this patch series for implementing Capacity Aware Scheduling on AMD hybrid processors. This should help with better task placement on Ryzen performance vs. efficiency cores and will be fun to benchmark as time allows. We'll see if these patches get reviewed and queued in time for the upcoming Linux v7.5 kernel cycle.