Doubts about a "kernel process"

Question

Actually one of the concepts that I don't really understand is the 'kernel porocess' or kernel thread. Searching in the web and for similar questions in SOO I found the following: What is a Kernel thread? where the answer was:

A kernel thread is a kernel task running only in kernel mode

As far as I know a process to go into 'kernel mode' has to issue a system call. So I don't understand how these threads are running in this mode all the time. Can somebody help me to understand how this works?

Answer 1

Kernel threads are threads that can be created only by another kernel thread (already in "kernel mode" - maybe some driver could spawn them to do some cleanup or monitoring for them), so there's no user thread to start with (and to switch context with syscall), and these threads are started inside the kernel address space, so they don't need anything extra to go to kernel mode.

Created kernel thread runs with the normal priority and scheduler capatibilities as user thread. From kthread_create_on_node (http://lxr.free-electrons.com/source/kernel/kthread.c#L269):

310         task = create->result;
311         if (!IS_ERR(task)) {
312                 static const struct sched_param param = { .sched_priority = 0 };
313                 va_list args;
314 
315                 va_start(args, namefmt);
316                 vsnprintf(task->comm, sizeof(task->comm), namefmt, args);
317                 va_end(args);
318                 /*
319                  * root may have changed our (kthreadd's) priority or CPU mask.
320                  * The kernel thread should not inherit these properties.
321                  */
322                 sched_setscheduler_nocheck(task, SCHED_NORMAL, &param);
323                 set_cpus_allowed_ptr(task, cpu_all_mask);
324         }