Linux: Why do we need an idle task?

There are two main reasons for having the idle task in the Linux kernel design:

• Historical reason: In the older days, CPUs were not capable of idling in a lower power state, so they would constantly execute the no-operation (nop^[1]) instruction. Now, with the advances in the CPU design, most processors will leverage a variant of the halt (HLT^[2]) instruction that will allow them to go to a lower power mode.
• Convenience: Instead of coding up and handling a special case for what to do when none of the processes are runnable i.e. the run queue is empty, we instead schedule the idle task, which is always runnable.

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1. No operation code ↩
2. Halt instruction for 8086 based archs. ↩

Linux: No entries seen in /proc/modules on loading new kernel

Issue: No entries are seen in /proc/modules on loading a new kernel (typically built from scratch¹).
Solution: The solution is pretty simple. This is happening because the kernel has not loaded the module for which you would like to see the symbols. You can double confirm this by running:

$lsmod  > modulelist.txt
$cat modulelist.txt

If you do not see your module here, set the following while rebuilding your kernel:

make LSMOD="modulelist.txt" localmodconfig

Using this, reboot into your new kernel and your module should get loaded.

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1. Quick ref on rebuilding kernel from source↩︎