How to find PID of all processes related to a given process?

Question

How can I go about finding the PID or other information about a process that is doing the work of another process? I'm talking about kworker threads, for example, or any other threads/processes that are doing work within the kernel for another process.

My dilemma is that I have a real-time scheduled process (SCHED_FIFO) running at sched prio 99, with CPU affinity bound to CPU 0, but when I inject CPU stress onto my machine, I notice that my important rt process is not able to preempt the other, non-important processes. I'm thinking this may be because the kworkers that do work for this important process do not inherit the priority that the main process has, even though I specify the -a option in taskset and chrt. My current idea is to manually taskset and chrt the kworker threads so they don't get preempted by other, non-rt processes.

Answer 1

You can get a tree of all the tasks started by a process if you know the PID of the starting process using ps.

$ ps f -g2305957
    PID TTY      STAT   TIME COMMAND
2305957 pts/20   Ss     0:00 bash
2310399 pts/20   S+     0:00  \_ stress -t 300 -c 16
2310400 pts/20   R+     0:24      \_ stress -t 300 -c 16
2310401 pts/20   R+     0:24      \_ stress -t 300 -c 16
2310402 pts/20   R+     0:24      \_ stress -t 300 -c 16
2310403 pts/20   R+     0:24      \_ stress -t 300 -c 16
2310404 pts/20   R+     0:24      \_ stress -t 300 -c 16
2310405 pts/20   R+     0:24      \_ stress -t 300 -c 16
2310406 pts/20   R+     0:23      \_ stress -t 300 -c 16
2310407 pts/20   R+     0:25      \_ stress -t 300 -c 16
2310408 pts/20   R+     0:22      \_ stress -t 300 -c 16
2310409 pts/20   R+     0:24      \_ stress -t 300 -c 16
2310410 pts/20   R+     0:25      \_ stress -t 300 -c 16
2310411 pts/20   R+     0:24      \_ stress -t 300 -c 16
2310412 pts/20   R+     0:23      \_ stress -t 300 -c 16
2310413 pts/20   R+     0:24      \_ stress -t 300 -c 16
2310414 pts/20   R+     0:25      \_ stress -t 300 -c 16
2310415 pts/20   R+     0:22      \_ stress -t 300 -c 16

This is stress running across 16 threads. Related to this answer.

Answer 2

pstree -p -H <PID>

The command shows you the complete processtree of your system with all PIDs and highlights your process.

Maybe that helps to find what is going on.