Since idle tasks are generally used to safely consume CPU time that is not required by other software, what would happen if there was no idle task? Would the RTOS just automatically create one? Also, what other purpose do idle tasks serve other than consuming time?
what would happen if there was no idle task? Would the RTOS just automatically create one?
I doubt there is any RTOS that would do that. If there would be no idle task, then the list of runnable tasks would be empty and the scheduler would probably crash. Generally the single most important reason for idle thread's existence is to make the list of runnable tasks "never empty". This simplifies the code of scheduler.
Also, what other purpose do idle tasks serve other than consuming time?
In some systems idle task can perform some low priority activities (for example some garbage collection). It can also switch the core to low-power mode, especially on embedded devices. In that case when the idle task is run it means that there is nothing more to do, so the core can be stopped and wait for the next event (hardware interrupt or timeout) without using too much power. When the next event arrives the core is awakened by hardware and the event is processed. Either some "normal" thread will start running, or - if there is still nothing more to do - idle thread will resume and again switch to low-power mode.