Linux uptime实现详解
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Linux uptime的实现
上源码
源码是长这个样子的, 小而美!
static int uptime_proc_show(struct seq_file *m, void *v){ struct timespec uptime; struct timespec idle; u64 idletime; u64 nsec; u32 rem; int i; idletime = 0; for_each_possible_cpu(i) idletime += (__force u64) kcpustat_cpu(i).cpustat[CPUTIME_IDLE]; get_monotonic_boottime(&uptime); nsec = cputime64_to_jiffies64(idletime) * TICK_NSEC; idle.tv_sec = div_u64_rem(nsec, NSEC_PER_SEC, &rem); idle.tv_nsec = rem; seq_printf(m, "%lu.%02lu %lu.%02lu\n", (unsigned long) uptime.tv_sec, (uptime.tv_nsec / (NSEC_PER_SEC / 100)), (unsigned long) idle.tv_sec, (idle.tv_nsec / (NSEC_PER_SEC / 100))); return 0;}static int uptime_proc_open(struct inode *inode, struct file *file){ return single_open(file, uptime_proc_show, NULL);}static const struct file_operations uptime_proc_fops = { .open = uptime_proc_open, .read = seq_read, .llseek = seq_lseek, .release = single_release,};static int __init proc_uptime_init(void){ proc_create("uptime", 0, NULL, &uptime_proc_fops); return 0;}fs_initcall(proc_uptime_init);
此时使用命令读取uptime是这样的:
cat /proc/uptime47.69 22.13
这就是刚刚开机不久后的开机时间47秒, CPU空闲22秒
增加调试信息
为了理清楚内核的时间的关系, 加了调试打印, 即get_xtime_and_monotonic_and_sleep_offset() 及其后面的seq_printf
static int uptime_proc_show(struct seq_file *m, void *v){ struct timespec uptime; struct timespec idle; u64 idletime; u64 nsec; u32 rem; int i; struct timespec xtime; struct timespec wtime; struct timespec stime; idletime = 0; for_each_possible_cpu(i) idletime += (__force u64) kcpustat_cpu(i).cpustat[CPUTIME_IDLE]; get_xtime_and_monotonic_and_sleep_offset(&xtime, &wtime, &stime); seq_printf(m, "xtime: %lu.%02lu\n", (unsigned long) xtime.tv_sec, (xtime.tv_nsec / (NSEC_PER_SEC / 100))); seq_printf(m, "wtime: %lu.%02lu\n", (unsigned long) wtime.tv_sec, (wtime.tv_nsec / (NSEC_PER_SEC / 100))); seq_printf(m, "stime: %lu.%02lu\n", (unsigned long) stime.tv_sec, (stime.tv_nsec / (NSEC_PER_SEC / 100))); do_posix_clock_monotonic_gettime(&uptime); seq_printf(m, "mtime: %lu.%02lu\n", (unsigned long) uptime.tv_sec, (uptime.tv_nsec / (NSEC_PER_SEC / 100))); monotonic_to_bootbased(&uptime); nsec = cputime64_to_jiffies64(idletime) * TICK_NSEC; idle.tv_sec = div_u64_rem(nsec, NSEC_PER_SEC, &rem); idle.tv_nsec = rem; seq_printf(m, "%lu.%02lu %lu.%02lu\n", (unsigned long) uptime.tv_sec, (uptime.tv_nsec / (NSEC_PER_SEC / 100)), (unsigned long) idle.tv_sec, (idle.tv_nsec / (NSEC_PER_SEC / 100))); return 0;}
此时的输出变成了这个样子:
xtime: 1262311849.44wtime: 3032655555.40stime: 0.00mtime: 108.85108.85 76.50
内核时间维护
- 硬件时钟中断是时间更新的根本来源
- 内核使用全局的一个数据结构来维护时间
static struct timekeeper timekeeper;
删除了无效的结构, 只关注主要的结构成员:
struct timekeeper { ...... /* Current CLOCK_REALTIME time in seconds */ u64 xtime_sec; /* Clock shifted nano seconds */ u64 xtime_nsec; /* * wall_to_monotonic is what we need to add to xtime (or xtime corrected * for sub jiffie times) to get to monotonic time. Monotonic is pegged * at zero at system boot time, so wall_to_monotonic will be negative, * however, we will ALWAYS keep the tv_nsec part positive so we can use * the usual normalization. * * wall_to_monotonic is moved after resume from suspend for the * monotonic time not to jump. We need to add total_sleep_time to * wall_to_monotonic to get the real boot based time offset. * * - wall_to_monotonic is no longer the boot time, getboottime must be * used instead. */ struct timespec wall_to_monotonic; /* Offset clock monotonic -> clock realtime */ ktime_t offs_real; /* time spent in suspend */ struct timespec total_sleep_time; /* Offset clock monotonic -> clock boottime */ ktime_t offs_boot; /* The raw monotonic time for the CLOCK_MONOTONIC_RAW posix clock. */ ......
- **xtime_sec 和xtime_nsec:** 是根本, 记录的是当前的时间相对于1970-01-01 00:00:00 +0000 (UTC)的差值. 硬件时间更新即是此值.- **wall_to_monotonic:** 记录了一个差值(其实是一个负值), 通过xtime_sec + wall_to_monotonic 可计算出当前的开机时间以来的一个时间值(未记入计算机的时间), 即调用do_posix_clock_monotonic_gettime()返回的结果- **total_sleep_time: ** 记录了系统的休眠时间, 即通过调用do_posix_clock_monotonic_gettime()返回的结果
总结
其实内核主要维护xtime, wall_to_monotonic 和 total_sleep_time三个值即可完成对系统时间的正确维护, 当然这里没有考虑NTP时间同步的因素, 如果增加了NTP时间同步会繁杂一些.
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