Linux's Load Balancer Still Needs To Be Better Adapted For Intel Hybrid CPUs

Over the past year since launching Intel Alder Lake processors, Intel engineers have made a number of improvements to the Linux kernel for better dealing with the hybrid processor approach mixing P and E cores. While Alder Lake is running great with recent kernels and the P vs. E core selection for tasks on Linux is better than it was at launch, there still are areas for improvement as raised by Intel engineers this week.

Last week was the big Linux kernel patch series working on "classes of tasks" for hybrid CPUs and properly implementing Thread Director support on Linux. This week at Linux Plumbers Conference it was also raised how Linux's Energy Aware Scheduling could be adapted for Intel hybrid CPUs as where right now EAS is just tailored to Arm big.LITTLE designs.

Also at LPC 2022 this week, Intel engineers Rui Zhang and Yu Chen raised the Intel hybrid Linux work still remaining. This additional talk was about how the Linux load balancer is less than ideal for Intel's hybrid processors. In particular, the frequency max used for calculating the frequency scale is a global value and not something core-specific, with P / E cores having different maximum frequency values. The frequency maximum value can also be incorrect based on turbo mode, thermal/power throttling, etc. Currently the frequency max value also can't be adjusted at run-time.

Those interested in the topic can find the complete slide deck outlining the current Linux load balancing problems for Intel hybrid CPUs and possible improvements that could be made. I'll certainly post as usual when any new patches materialize and are ready for testing/benchmarking.