attach preempt_notifier to user process in linux

Question

I am needing to identify whether a user process was ever preempted somehow, I understand we have hooks in preempt.h and sched.c which allow us to define preempt_notifiers which can in turn call sched_in and sched_out functions whenever a process is rescheduled or preempted.

But I still can't find out how can I attach a notifier to a particular process or pid in user space and then somehow log if this particular process was ever pre-empted. I'm assuming I have to write a module to do so, but how would I go about attaching a pid to a particular notifier?

Answer 1

The pseudo-file /proc/<pid>/status contains a line nonvoluntary_ctxt_switches: which seems to be the information that you're after.

Answer 2

The notifier is inherently per-process. When you register it, you are registering it for the current process. See the code in preempt_notifier_register(), it attaches the notifer to current->preempt_notifiers.