The wfi Instruction

In some cases, it might be useful for the operating system to turn off the CPUs to conserve power, heating, and batter life (if applicable). Therefore, instruction set architectures (ISA) such as RISC-V come equipped with the wfi (wait-for-interrupt) instruction. wfi puts the CPU core on which it is executing into a low power state until an interrupt is received. At that point, the core is awakened and it can resume executing from where i left off. This is very similar to the way condition variables work, but instead, we are dealing with the physical hardware on the machine.

Implementation

In order to implement this feature, we need to first be able to call an assembly instruction from our kernel code implementation. Luckily, the compiler allows you to do by using the asm directive as follows:

static inline void
__wfi(void)
{
  asm volatile("wfi");
}

When you call the function __wfi() from the kernel code, the compiler will replace the body of the function with the assembly instruction wfi. Your job in this step to figure out where and when you must call the __wfi() function so that you turn off the CPU cores and fix the problems that we have identified with the xv6 scheduler.

Testing

To test if our implementation can solve the inefficiency problem in xv6, you can make use of the htop utility to measure the CPU utilization when the xv6 kernel is running. If htop is not installed on your Linux machine, you can install it using your favorite package manager. In the case of Ubuntu (or any Debian-based distribution), you can do something like the following:

$ sudo apt install -y htop

After that, you can use htop to visualize the CPU utilization on all the available cores in your system and verify if your implementation has solved the inefficiency problem or not.