C Program to get CPU usage for a PID and all its children

Question

I have a C program that parses the /proc//stat directory to calculate the average CPU utilization over a period of 5 seconds:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#define ITERATIONS 5

int main(int argc, char *argv[])
{
    if (argc != 2) {
        printf( "usage: %s <PID>\n", argv[0] );
        return(-1);
    }

    long double a[4], b[4];
    long double pidTime = 0.0;
    long int clk;
    int i;
    FILE *fp;
    char stat[1024];

    clk = sysconf(_SC_CLK_TCK);

    if (clk == -1) {
        printf("Could not determine clock ticks per second");
        return(-1);
    }

    char *pidPath = malloc(strlen("/proc/stat/")+strlen(argv[1])+1);
    if (pidPath == NULL) {
        printf("Could not allocate memory for str\n");
        return(-1);
    } else {
        strcpy(pidPath, "/proc/");
        strcat(pidPath, argv[1]);
        strcat(pidPath, "/stat");
    }

    for(i = 0; i < ITERATIONS; i++)
    {
        fp = fopen(pidPath,"r");
        if (fp == NULL) {
            perror(pidPath);
            return(-1);
        } else {
            fgets(stat, sizeof(stat), fp);
            sscanf(stat,"%*d %*s %*c %*d %*d %*d %*d %*d %*u %*lu %*lu %*lu %*lu %Lf %Lf %Lf %Lf %*ld %*ld %*ld %*ld %*llu",&a[0],&a[1],&a[2],&a[3]);
            fclose(fp);
            sleep(1);
        }

        fp = fopen(pidPath,"r");
        if (fp == NULL) {
            perror(pidPath);
            return(-1);
        } else {
            fgets(stat, sizeof(stat), fp);
           sscanf(stat,"%*d %*s %*c %*d %*d %*d %*d %*d %*u %*lu %*lu %*lu %*lu %Lf %Lf %Lf %Lf %*ld %*ld %*ld %*ld %*llu",&b[0],&b[1],&b[2],&b[3]);
            fclose(fp);
        }

        pidTime += (((b[0]+b[1]+b[2]+b[3]) - (a[0]+a[1]+a[2]+a[3])));
    }

    pidTime = (pidTime / (clk * ITERATIONS));
    printf("pidCPU=%Lf\n", pidTime);
    printf("%ld", clk);
    free(pidPath);
    return(0);
}

From what I understand the relevant fields in stat are:

int utime; /** user mode jiffies **/
int stime; /** kernel mode jiffies **/
int cutime; /** user mode jiffies with childs **/
int cstime; /** kernel mode jiffies with childs **/

For a single process, this works great, but when I have a process that forks, or is multithreaded, this breaks down. Do the cutime and cstime counters only work when the parent is waiting for the child processes? How can I calculate the total usage of the process tree rooted at PID?

@Ascam I should, I think I was just being lazy at this point. You're right though. — anthozep, Aug 09 '16 at 16:07
Anyway, just following logic, since you're tracking a PID, you'll only be given infos about **this** PID. Each child process being a... **process**, you refer to them by their own ID. My final point is: yes, it seems normal that you're getting only some `wait` activity watching parents. — , Aug 09 '16 at 16:15

score 1 · Accepted Answer · edited May 23 '17 at 12:01

1

Yes, the parent needs to wait for the CPU time of the children to be added in (see manual entry for getrusage link). Also see this answer for more details.

edited May 23 '17 at 12:01

Community

1
1

answered Aug 09 '16 at 15:04

Jerzy

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So is there a way to get the process tree from /proc? If so, I could potentially make this a recursive solution – anthozep Aug 09 '16 at 16:08
On some kernels, you may be able to get the children from `/proc/**pid**/task/**taskid**/children` file [see link](https://lkml.org/lkml/2011/12/28/59). Otherwise, you can do reverse lookup from the parent pid. The parent pid is listed in `/proc/*pid*/status` on line `PPid`. You can try from a shell with: `fgrep PPid /proc/[0-9]*/status 2>/dev/null | fgrep $$ | cut -d/ -f3` which should list subprocesses of your shell (`$$`). Note that the grep/cut will itself show up in this list, but these processes will be gone by the time your prompt returns. – Jerzy Aug 15 '16 at 09:37

C Program to get CPU usage for a PID and all its children

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