It is often useful for the kernel to perform some operations in the background.The ker- nel accomplishes this via kernel threads—standard processes that exist solely in kernel- space.The significant difference between kernel threads and normal processes is that kernel threads do not have an address space. (Their mm pointer, which points at their address space, is NULL.) They operate only in kernel-space and do not context switch into user-space. Kernel threads, however, are schedulable and preemptable, the same as normal processes.
对kernel 来说在background执行一些操作是很有用的，比如xxx. 内核通过kernel threads 来满足这些需要。