为什么 sum_exec_runtime 是 0 ?

Question

我想得到 thread 时间。我的环境是 linux-4.13.4， Ubuntu 16.04

关于sum_exec_runtime

sum_exec_runtime

Total time process ran on CPU
In real time
Nano second units (10^−9)
Updated in __update_curr(), called from update_curr()

增加 syscall 来记录获取时间

struct task_struct {
    ...
    ...
    struct sched_entity        se;    
    // TODO: to get start time and end time
    u64 vm_start_time;  
    u64 vm_end_time;
    ...
    ...
};

增加2个系统调用

asmlinkage long sys_start_vm_timer(int __user *vm_tid);

asmlinkage long sys_end_vm_timer(int __user vm_tid, unsigned long long __user *time);

SYSCALL_DEFINE1(start_vm_timer,
                int __user, vm_tid){
        (find_task_by_vpid(vm_tid))->vm_start_time =
        (find_task_by_vpid(vm_tid))->se.sum_exec_runtime;
        return 0;        
}

SYSCALL_DEFINE2(end_timer,
        int __user, tid,
        unsigned long long __user, *time){
    u64 vm_time;
    (find_task_by_vpid(vm_tid))->vm_end_time =
    (find_task_by_vpid(vm_tid))->se.sum_exec_runtime;
    printk("-------------------\n");
    printk("end_vm_time: vm_elapsed_time = %llu \n", ((find_task_by_vpid(vm_tid))->vm_end_time - (find_task_by_vpid(vm_tid))->vm_start_time) );
    vm_time = ((find_task_by_vpid(vm_tid))->vm_end_time - (find_task_by_vpid(vm_tid))->vm_start_time);
    copy_to_user(time, &vm_time, sizeof(unsigned long long));
    return 0;
}

Test

测试 for loop.

#include <stdio.h>
#include <linux/kernel.h>
#include <sys/syscall.h>
#include <unistd.h>
#include <time.h>
#include <sys/time.h>
#include <stdlib.h>

int main(){

    int tid = syscall(SYS_gettid);
    printf("tid = %d \n", tid);
    printf("My process ID : %d\n", getpid());
    printf("My parent's ID: %d\n", getppid());

    struct timeval start, end;
    unsigned long long elapsedTime = 0;

    gettimeofday(&start, NULL);

    syscall(336, tid);

    int i = 0;
    int j = 0;
    for(i = 0; i < 65535; i++){
        j += 1;
    }

    syscall(337, tid, &elapsedTime);

    gettimeofday(&end, NULL);

    printf("thread time = %llu microseconds \n", elapsedTime/1000);
    printf("gettimeofday = %ld microseconds \n", ((end.tv_sec * 1000000 + end.tv_usec)- (start.tv_sec * 1000000 + start.tv_usec)));
    return 0;
}

## Unexpected Result and why?

wxf@wxf:/home/wxf/cPrj$ ./thread_time 
tid = 6905 
My process ID : 6905
My parent's ID: 6595
thread time = 0 microseconds 
gettimeofday = 422 microseconds 

From dmesg,

[63287.065285] tid = 6905 
[63287.065288] start_vm_timer = 0 
[63287.065701] tid = 6905 
[63287.065702] -------------------
[63287.065703] end_vm_timer = 0 
[63287.065704] end_vm_time: vm_elapsed_time = 0 

###应该要差不多一样啊，可是为什么是 0？