Linux系统时间的设定以及自带的timesync时间同步

1.三个阶段的系统时间设定

1.1 内核启动阶段

这里是在menuconfig文件配置RTC设定系统时间选项。

     CONFIG_RTC_HCTOSYS_DEVICE="rtc1"，或者直接在deconfig中添加选项。

     没选就不会设置，我猜默认1970-XX-XXX。（因为很久以前看的，没有笔记习惯，所以具体逻辑忘记了）

 

1.2 systemd启动

int clock_apply_epoch(ClockChangeDirection *ret_attempted_change) {
        struct stat st;
        struct timespec ts;
        usec_t epoch_usec, now_usec;

        /* NB: we update *ret_attempted_change in *all* cases, both
         * on success and failure, to indicate what we intended to do! */

        assert(ret_attempted_change);

        if (stat(EPOCH_FILE, &st) < 0) {
                if (errno != ENOENT)
                        log_warning_errno(errno, "Cannot stat " EPOCH_FILE ": %m");

                epoch_usec = (usec_t) TIME_EPOCH * USEC_PER_SEC;
        } else
                epoch_usec = timespec_load(&st.st_mtim);

        now_usec = now(CLOCK_REALTIME);
        if (now_usec < epoch_usec)
                *ret_attempted_change = CLOCK_CHANGE_FORWARD;
        else if (now_usec > usec_add(epoch_usec, CLOCK_VALID_RANGE_USEC_MAX))
                *ret_attempted_change = CLOCK_CHANGE_BACKWARD;
        else {
                *ret_attempted_change = CLOCK_CHANGE_NOOP;
                return 0;
        }

        if (clock_settime(CLOCK_REALTIME, timespec_store(&ts, epoch_usec)) < 0)
                return -errno;

        return 1;
}

 

    systemd版本应该是250。

    默认的情况下，在编译systemd的时候会记录一个编译时间。

   没有特别设置而且是git下来的情况应该是git log中的最近的一次提交记录。

   如果不是git 的，使用的是NEW文件的修改时间。

   参照：meson.build（systemd的Build文件）

   

   上面记录的时间 > RTC的时间  就会重新设置系统时间。

   上面记录的时间 < RTC的时间  保持不变（具体还会判断一下可以超过太多也是不行的）。 

   参照：src/shared/clock-util.c（int clock_apply_epoch(ClockChangeDirection *ret_attempted_change)）    

 

1.3 timesyncd启动

 

static int load_clock_timestamp(uid_t uid, gid_t gid) {
        _cleanup_close_ int fd = -1;
        usec_t min = TIME_EPOCH * USEC_PER_SEC;
        usec_t ct;
        int r;

        /* Let's try to make sure that the clock is always
         * monotonically increasing, by saving the clock whenever we
         * have a new NTP time, or when we shut down, and restoring it
         * when we start again. This is particularly helpful on
         * systems lacking a battery backed RTC. We also will adjust
         * the time to at least the build time of systemd. */

        fd = open(CLOCK_FILE, O_RDWR|O_CLOEXEC, 0644);
        if (fd >= 0) {
                struct stat st;
                usec_t stamp;

                /* check if the recorded time is later than the compiled-in one */
                r = fstat(fd, &st);
                if (r >= 0) {
                        stamp = timespec_load(&st.st_mtim);
                        if (stamp > min)
                                min = stamp;
                }

                if (geteuid() == 0) {
                        /* Try to fix the access mode, so that we can still
                           touch the file after dropping privileges */
                        r = fchmod_and_chown(fd, 0644, uid, gid);
                        if (r < 0)
                                log_warning_errno(r, "Failed to chmod or chown %s, ignoring: %m", CLOCK_FILE);
                }

        } else {
                r = mkdir_safe_label(STATE_DIR, 0755, uid, gid,
                                     MKDIR_FOLLOW_SYMLINK | MKDIR_WARN_MODE);
                if (r < 0) {
                        log_debug_errno(r, "Failed to create state directory, ignoring: %m");
                        goto settime;
                }

                /* create stamp file with the compiled-in date */
                r = touch_file(CLOCK_FILE, false, min, uid, gid, 0644);
                if (r < 0)
                        log_debug_errno(r, "Failed to create %s, ignoring: %m", CLOCK_FILE);
        }

settime:
        ct = now(CLOCK_REALTIME);
        if (ct < min) {
                struct timespec ts;
                char date[FORMAT_TIMESTAMP_MAX];

                log_info("System clock time unset or jumped backwards, restoring from recorded timestamp: %s",
                         format_timestamp(date, sizeof(date), min));

                if (clock_settime(CLOCK_REALTIME, timespec_store(&ts, min)) < 0)
                        log_error_errno(errno, "Failed to restore system clock, ignoring: %m");
        }

        return 0;
}

   这里需要启用ntp服务。

   timedatectl set-ntp true

   没启用ntp时，是不会设置系统时间的。

   设置系统时间的逻辑和systemd相同，也是通过比较决定是否设定。    

1.4默认的ntp服务器

    默认的ntp服务器是google的，国内访问不了，注意改成国内的。

    /etc/systemd/timesysncd.conf

    ntp.aliyun.com