Linux Process Deep Dive-1: Knowledge of PID 0

PID 0

An Idle Process used to swap all pages of processs to/from memory/backing store.
“Swap Process”
It was there, so Linux always have CPU to execute something
If you try to execute ps command on the clean Ubuntu Linux, you’ll find there was no process with “PID0”
Idle systemcall not supported since kernal 2.3.13

Then how can we prove that PID0 really exist?

Demo

Using bpftrace for tracking the kernel function hrtimer_wakeup
hrtimer_wakeup mean to:
- Waking up processes
- Move them into runnable process

hrtimer_wakeup source code

##Refer to https://elixir.bootlin.com/linux/latest/source/kernel/time/hrtimer.c#L2000

static enum hrtimer_restart hrtimer_wakeup(struct hrtimer *timer)
{
	struct hrtimer_sleeper *t =
		container_of(timer, struct hrtimer_sleeper, timer);
	struct task_struct *task = t->task;

	t->task = NULL;
	if (task)
		wake_up_process(task);

	return HRTIMER_NORESTART;
}

I open a new Amazon EC2 Instance with Ubuntu 22 LTS AMI for testing.

Command

1	sudo bpftrace -e 'kfunc:hrtimer_wakeup {printf("%s:%d\n",curtask->comm,curtask->pid); }'

It shows there are only 1 instance of swapper: swapper/0 with PID0.

If you open a new VM with 3 to 4 vCPU, then you execute the command again, You will find that there are 3 or4 swapper instances with PID0, so there may have swapper/0, swapper/1, swapper/2 appears.

PPID

If you try to execute the command ps -eaf on your local system, you may find there are a column named PPID, It means the Parrent Process ID.

In Linux systems, A PPID is always assigned to every process ID. It tells us which process started a particular process. Therefore, the PPID value of 0 for the init process indicates that the init process has no parent.