Why Does the Kernel Enter an Idle Loop and How Does It Wake Up?

The kernel's primary responsibility is process scheduling; when a process blocks, the kernel schedules another runnable process.

If no processes are ready to run, the kernel enters an idle state. The idle logic continuously loops, executing the hlt assembly instruction to halt the CPU until an interrupt occurs.

while (1) {
    while (!need_resched()) { // check if any other process can run
        asm("hlt"); // no runnable process, halt the CPU
    }
    schedule_idle(); // a process became runnable, switch to it
}

When an interrupt arrives—e.g., a new TCP packet—the kernel wakes up, a blocked process becomes runnable, and need_resched() returns true. The idle loop exits the hlt state, calls schedule_idle(), and switches from the idle (PID 0, named "swapper") to the target process.

After the newly scheduled process finishes and all processes block again, the kernel returns to the idle loop, repeating the cycle.