The long standing consensus in the High-Performance Computing (HPC) Operating Systems (OS) community is that lightweight kernel (LWK) based OSes have the potential to outperform Linux at extreme scale. To explore if LWKs live up to their expectation we developed IHK/McKernel, a lightweight multi-kernel OS designed for HPC, and deployed it on two high-end supercomputers to compare its performance against Linux. Oakforest-PACS, an Intel Xeon Phi (x86) based supercomputer, runs a moderately tuned Linux distribution. Fugaku, the world's fastest supercomputer at the time of writing this paper, is based on Fujitsu's A64FX (aarch64) CPU that runs a highly tuned Linux environment.