How can I get a process to core mapping in C?

Question

What library function can I call to get mapping of processes to cores or given a process id tell me what core it's running on, it ran last time, or scheduled to run. So something like this:

core 1:  14232,42323
core 2:  42213,63434,434
core 3:  34232,34314
core 4:  42325,6353,1434,4342
core 5:  43432,64535,14345,34233
core 6:  23242,53422,4231,34242
core 7:  78789
core 8:  23423,23124,5663

I sched_getcpu returns the core number of calling process. If there was a function that given a process id, would return the core number that would be good too but I have not found one. sched_getaffinity is not useful either; It just tells you given a process what cores it can run on which is not what I'm interested in.

Answer 1

Yes, the virtual file /proc/[pid]/stat seems to have this info: man 5 proc:

/proc/[pid]/stat
      Status  information  about  the  process.   This is used by ps(1).  It is
      defined in /usr/src/linux/fs/proc/array.c.
      (...fields description...)

      processor %d (since Linux 2.2.8)
                      CPU number last executed on.

on my dual core:

cat /proc/*/stat | awk '{printf "%-32s %d\n", $2 ":", $(NF-5)}'
(su):                            0
(bash):                          0
(tail):                          1
(hd-audio0):                     1
(chromium-browse):               0
(bash):                          1
(upstart-socket-):               1
(rpcbind):                       1

..though I can't say if it's pertinent and/or accurate..

Answer 2

I don't know that you can get information about what CPU any particular process is running on, but if you look in /proc, you'll find one entry for each running process. Under that, in /proc/<pid>/cpuset you'll find information about the set of CPUs that can be used to run that process.

Answer 3

Your question does not have any precise answer. The scheduler can migrate a process from one processor core to another at any time (and it is actually doing that). So by the time you got the answer it may be already wrong. And a process is usually not tied to any particular core (unless its CPU affinity has been set e.g. with sched_setaffinity(2), which is unusual; see also cpuset(7) for more).

Why are you asking? Why does that matter?

You probably want to dig inside /proc, see proc(5) man page.

^{In other words, if the kernel does give that information, it is thru /proc/ but I guess that information is not available because it does not make any sense.}

NB. The kernel will schedule processes on the various processor cores much better than you can do, so even with a warehouse, you should not care about the core running some pid.