For a long time, an 8K kernel stack was large enough. However, since 2014, the default stack size has increased to 16K [1]. To conserve memory at Google, we maintained 8K stacks via a custom patch while verifying that our workload could fit within this limit. As we qualify new workloads and kernels, we find it more difficult to keep the stacks at 8K. Therefore, we will increase the stack size to the mainline value of 16K. However, this translates to a significant increase in memory usage, potentially counted in petabytes. With virtually mapped stacks [2], it's possible to implement auto-growth on faults. Ideally, the vast majority of kernel threads could fit into 4K or 8K stacks, with only a small number requiring deeper stacks that would expand as needed. The complication is that new pages must always be available from within an interrupt context. To ensure this, pages must be accessible to kernel threads in an atomic and lockless manner. This could be achieved by using a per-CPU supply of pages dedicated to handling kernel-stack faults. [1] https://lwn.net/Articles/600644 [2] https://lwn.net/Articles/692608
