Why does linux kernel need idle thread?

Question

Rather than "do nothing" if there is nothing to do (including SMP), why linux kernel runs idle thread?

Answer 1

When the scheduler decides to switch to the idle task, at this point, the dynamic tick begins to work, by disabling periodic tick until the next timer expires. The tick will be reenabled after this time span or when an interrupt occurs at some time.

In the mean time, the CPU is going to a well-deserved sleep, in an architecture-specific way, therefore saving your power. Take a look at the definition of cpu_idle() in arch/x86/kernel/process.c.

/*
 * The idle thread. There's no useful work to be
 * done, so just try to conserve power and have a
 * low exit latency (ie sit in a loop waiting for
 * somebody to say that they'd like to reschedule)
 */
void cpu_idle(void)

Answer 2

What do you mean by "do nothing"??

When the CPU is powered up there is a rather long list of things that happen. Once powered up the CPU cannot "do nothing". It has to do something because there is a voltage and a periodic clock signal. You can power it down again and do ABSOLUTELY nothing but then you have to go through the long list of things to get a steady clock signal when you need it again.

So the idle thread is a thread that does the bare minimum. i.e. if multiplying two floating point numbers required the least number of cycles and the least number of electronic circuitry; then the idle thread would be multiplying two floats all the time. In addition as Wang said the Linux kernel (in some configurations) monitors when cores start executing idle thread and also switches them to a lower frequency, disabling any sort of periodic OS house keeping. This results in a bit of latency when the core is needed but then there is much less power being used.