On Tue, Jan 14, 2025 at 6:51 PM Valentin Schneider vschneid@redhat.com wrote:

vunmap()'s issued from housekeeping CPUs are a relatively common source of interference for isolated NOHZ_FULL CPUs, as they are hit by the flush_tlb_kernel_range() IPIs.

Given that CPUs executing in userspace do not access data in the vmalloc range, these IPIs could be deferred until their next kernel entry.

Deferral vs early entry danger zone

This requires a guarantee that nothing in the vmalloc range can be vunmap'd and then accessed in early entry code.

In other words, it needs a guarantee that no vmalloc allocations that have been created in the vmalloc region while the CPU was idle can then be accessed during early entry, right?