Another idea how one could solve this problem by a clever kernel interface. This idea is probably a better than the approach in my post above, since this enables to migrate threads from big to little cores and vice versa except for critical regions.

Add an interface to the kernel/scheduler

void lock_cacheline_size(...)

void unlock_cacheline_size(...)

Calling lock_cacheline_size tells that from now on the thread must stay on a core with the same cache line size. Consider it as locking a mutex on the cache line size.

After this you can run critical code that depends on the fact that the cache line size stays constant for the whole time.

After you are finished with it, you call unlock_cacheline_size (as a kind of unlocking the mutex on the cache line size) - from now on the scheduler is free again to migrate the code again to cores with different cache line sizes.