If the kernel was supposed to do as you say, and only check if the process should be disconnected upon explicit transition between the user and the kernel, the following cycle will lock the kernel of your computer:
while (1);
Obviously, this does not happen on regular desktop operating systems. The reason this is continuity is that when after the process is completed, the kernel receives an alarm during the cutoff, performs the steps and forcibly switches contexts as necessary.
Continuity, in principle, can work for kernel processes. However, I'm not sure what keventd does is more likely that it voluntarily gives up its temporary fragment on a regular basis (see sched_yield , sched_yield user space for the same effect), especially since the kernel can be configured to be unmanageable. This is the prerogative of the core process.