/proc/pid/stat的第22个字段为进程在内核启动之后，再经过该时间启动，以clock tick为单位。
(22) starttime %llu The time the process started after system boot. In kernels before Linux 2.6, this value was expressed in jiffies. Since Linux 2.6, the value is expressed in clock ticks (divide by sysconf(_SC_CLK_TCK)). The format for this field was %lu before Linux 2.6.
//fs/proc/array.c
static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
struct pid *pid, struct task_struct *task, int whole)
{
//...
/* Temporary variable needed for gcc-2.96 */
/* convert timespec -> nsec*/
start_time =
(unsigned long long)task->real_start_time.tv_sec * NSEC_PER_SEC
+ task->real_start_time.tv_nsec;
/* convert nsec -> ticks */
start_time = nsec_to_clock_t(start_time);
//...
}
u64 nsec_to_clock_t(u64 x)
{
#if (NSEC_PER_SEC % USER_HZ) == 0
return div_u64(x, NSEC_PER_SEC / USER_HZ);
#elif (USER_HZ % 512) == 0
return div_u64(x * USER_HZ / 512, NSEC_PER_SEC / 512);
#else
/*
* max relative error 5.7e-8 (1.8s per year) for USER_HZ <= 1024,
* overflow after 64.99 years.
* exact for HZ=60, 72, 90, 120, 144, 180, 300, 600, 900, ...
*/
return div_u64(x * 9, (9ull * NSEC_PER_SEC + (USER_HZ / 2)) / USER_HZ);
#endif
}
即:
` /proc/$pid/stat/starttime = process starttime - kernel boot`
/proc/$pid/stat/starttime
/------------------\
______|__________________|________________|___________
kernel boot process start now
所以，进程进程的墙上时间为kernel boot + /proc/$pid/stat/starttime。
进程的运行时长= now - (kernel boot + /proc/$pid/stat/starttime)
/proc/stat中的btime为内核启动的墙上时间。
- procfs
ps -o lstart -p $pid可以得到进程的启动时间：
# ps -o lstart -p 10420
STARTED
Mon May 22 17:53:42 2017
static int pr_lstart(char *restrict const outbuf, const proc_t *restrict const pp){
time_t t;
t = time_of_boot + pp->start_time / Hertz;
return snprintf(outbuf, COLWID, "%24.24s", ctime(&t));
}
其中pp->start_time为/proc/$pid/stat/starttime， Hertz为USER_HZ，为固定值100。
参考http://man7.org/linux/man-pages/man5/proc.5.html