getcpu() - system call for kernel 2.6.18

[superfox_il_volpone]

Hi guys,
first message here. I'd like to retrieve the current cpu id where the thread is running.
I know the getcpu() system call, but it requires kernel 2.6.19 while my kernel is 2.6.18 :oO. Isthere any alternative?

regards
-s.fox

[Rubberman]

You might find something in /proc/<pid> where <pid> is the task's process id. As you noted, getcpu() is only valid for kernels from 2.6.19 and later. So, there is nothing there for RHEL 5 or other distributions stuck at the 2.6.18 or earlier kernel. Sorry I can't be more specific than that. I munged through my /proc and didn't come up with anything that is clearly the cpu associated with a specific process.

I'm going to boot up my CentOS 5 VM and see if I can dig up more for you.

[Rubberman]

Ok. No joy in Mudville! Sorry, but I can't find anything in /proc for 2.6.18 that will help. There are some entries that I don't understand, so that doesn't mean the data isn't there, but I just don't know where it is!

[Rubberman]

So, after some more Googling and checking, here is a work around. You can use the command "ps -aeF | grep <pid>" and look at the PSR column to find the current cpu the task is running on. This definitely works for 2.6.18 kernels. Next step is to look at the source for ps to find how it determines which CPU is running the process. Once you do that, you should be able to create your own library function, or use the appropriate library function, to get the CPU where the process is running. Without spending a lot more time, that is as definitive an answer as I can get you. I hope this helps!

[superfox_il_volpone]

As you may have figured out, I am stuck with CentOS 5. This is a kind of workaround:

// for kernel < 2.6.19
pid_t pid = syscall(SYS_getpid);
std::stringstream stringbuffer;
int cpu_id;
stringbuffer << "cat /proc/" << pid << "/stat | awk '{print $39}'";
FILE* file = popen(stringbuffer.str().c_str(), "r");
if (fscanf(file, "%d", &cpu_id) == EOF){
    cpu_id = -1;
}
 pclose(file);

return cpu_id;

Thanks for your help.
Regards
- sfox