How is each process pinned to a specific core by scheduler (Linux)

Question

I am now studying about the scheduler of Linux. Regarding CPU core affinity, I would like to know the following:

1) How is each process(thread) pinned to a core?

there is a system call sched_setaffinity to change the core affinity on which a process is executed. But internally, when a process(or a thread) is generated, how does the default Linux scheduler assign the process(thread) to a specific core? I modified sched_setaffinity system call to dump information about the task being moved from one core to another.

printk(KERN_INFO "%d %d %ld %lu %s\n", current->pid, current->tgid,
                                       current->state, current->cpus_allowed,
                                       current->comm);

It seems that there is no dump of the above information in /var/log/messages. So the default scheduler pins each process in a different way, but I cannot figure out how.

2) Is it possible to get core ID by PID or other information?

This is what I want to implement inside of Linux kernel. In task_struct, there is a member called cpus_allowed. But this is a mask for setting affinity, not core ID. I want to retrieve a data identifying the core on which specified process is running.

Thanks,

Answer 1

Each CPU has its own runqueue, AFAIK we can find out current CPU of a process by looking for which runqueue it belongs to. Given task_struct *p, we can get its runqueue by struct rq = task_rq(p), and struct rq has a field named cpu, I guess this should be the answer.

I have not tried this in practice, just read some code online, and am not quite sure it it will work or not. Wish it could help you.