Looking at the source of
strace I found the use of the clone flag
CLONE_IDLETASK which is described there as:
#define CLONE_IDLETASK 0x00001000 /* kernel-only flag */
After looking deeper into it I found that, although that flag is not covered in
man clone it is actually used by the kernel during the boot process to create idle processes (all of which should have PID 0) for each CPU on the machine. i.e. a machine with 8 CPUs will have at least 7 (see question below) such processes "running" (note quotes).
Now, this leads me to a couple of question about what that "idle" process actually do. My assumption is that it executes NOP operation continuously until its timeframe ends and the kernel assigns a real process to run or assign the idle process once again (if the CPU is not being used). Yet, that's a complete guess. So:
On a machine with, say, 8 CPUs will 7 such idle processes be created? (and one CPU will be held by the kernel itself whilst no performing userspace work?)
Is the idle process really just an infinite stream of NOP operations? (or a loop that does the same).
Is CPU usage (say
uptime) simply calculated by how long the idle process was on the CPU and how long it was not there during a certain period of time?
P.S. It is likely that a good deal of this question is due to the fact that I do not fully understand how a CPU works. i.e. I understand the assembly, the timeframes and the interrupts but I do not know how, for example, a CPU may use more or less energy depending on what it is executing. I would be grateful if someone can enlighten me on that too.