Unable to avoid child processes from inheriting the cpu affinity of parent

Question

I am wanting to affinitize the parent process to a particular core. In the code below, the variable core is a user supplied argument. Following that, I want to create NUM_CHILDREN processes and each one of them to be affinitized to one of the cores in a round robin manner. The child processes break out of the loop and do some more work (not shown in the code).

int child_core = 0;
CPU_ZERO(&mask);
CPU_SET(core,&mask);

if (sched_setaffinity(0, len, &mask) < 0)
{
    perror("sched_setaffinity");
}

for(int i = 0 i < NUM_CHILDREN; i++)
{
    pID = fork();
    if (pID == 0)
    {
        /* child */
        CPU_ZERO(&mask);
        CPU_SET(child_core,&mask);
        len = sizeof(mask);

        if (sched_setaffinity(0, len, &mask) < 0)
        {
            perror("sched_setaffinity");
        }

        break;
    }

    /*parent does some work*/
    child_core = (child_core+1)%6
}

The problem I am facing is that running sar -P ALL 1 100 shows that only a single core ( the core to which the parent has been affinitized ) is being utilized. I am trying to follow the solution mentioned here: Cpu affinity inherited by child process

Can someone please tell me how I can have the child processes to be affinitized to the proper cores.

Answer 1

I think your method needs to have the parent process increment the counter for child processes, and your affinity code needs to be executed for all processes.

Try this:

/* your call args and return type  may vary, just an illustration */
void doSetup()
{

    int child_core = 0;

    for(int i = 0 i < NUM_CHILDREN; i++)
    {
        pid_t pID = fork();
        if (pID == 0)
        {
        /* child */                 
        break;
        }
        else
        {
         /* parent only */
         child_core = (child_core+1)%6   
        }       
    }



  /* all processes */

    cpu_set_t mask;
    CPU_ZERO(&mask);
    CPU_SET(child_core,&mask);
    size_t len = sizeof(mask);

    if (sched_setaffinity(0, len, &mask) < 0)
    {
            perror("sched_setaffinity");
    }
    else
    {
      /* child process tasks calls here */
    }

    return;

}

There are some more examples and discussion at this link at IBM DeveloperWorks

Hope this helps.