linux时间子系统（四）

2.3 系统调用    timekeeper提供一系列的系统调用，使得用户空间可以获取想要的时间。下面简单的介绍一下clock_gettime系统调用 SYSCALL_DEFINE2(clock_gettime, const clockid_t, which_clock,                 struct timespec __user *,tp) {         struct k_clock *kc = clockid_to_kclock(which_clock);         struct timespec kernel_tp;         int error;           if (!kc)                 return -EINVAL;           error = kc->clock_get(which_clock, &kernel_tp);           if (!error && copy_to_user(tp, &kernel_tp, sizeof (kernel_tp)))                 error = -EFAULT;           return error; }   static __init int init_posix_timers(void) {         struct k_clock clock_realtime = {                 .clock_getres   = hrtimer_get_res,                 .clock_get      = posix_clock_realtime_get,                 .clock_set      = posix_clock_realtime_set,                 .clock_adj      = posix_clock_realtime_adj,                 .nsleep         = common_nsleep,                 .nsleep_restart = hrtimer_nanosleep_restart,                 .timer_create   = common_timer_create,                 .timer_set      = common_timer_set,                 .timer_get      = common_timer_get,                 .timer_del      = common_timer_del,         };         struct k_clock clock_monotonic = {                 .clock_getres   = hrtimer_get_res,                 .clock_get      = posix_ktime_get_ts,                 .nsleep         = common_nsleep,                 .nsleep_restart = hrtimer_nanosleep_restart,                 .timer_create   = common_timer_create,                 .timer_set      = common_timer_set,                 .timer_get      = common_timer_get,                 .timer_del      = common_timer_del,         };         struct k_clock clock_monotonic_raw = {                 .clock_getres   = hrtimer_get_res,                 .clock_get      = posix_get_monotonic_raw,         };         struct k_clock clock_realtime_coarse = {                 .clock_getres   = posix_get_coarse_res,                 .clock_get      = posix_get_realtime_coarse,         };         struct k_clock clock_monotonic_coarse = {                 .clock_getres   = posix_get_coarse_res,                 .clock_get      = posix_get_monotonic_coarse,         };         struct k_clock clock_boottime = {                 .clock_getres   = hrtimer_get_res,                 .clock_get      = posix_get_boottime,                 .nsleep         = common_nsleep,                 .nsleep_restart = hrtimer_nanosleep_restart,                 .timer_create   = common_timer_create,                 .timer_set      = common_timer_set,                 .timer_get      = common_timer_get,                 .timer_del      = common_timer_del,         };         posix_timers_register_clock(CLOCK_REALTIME, &clock_realtime);         posix_timers_register_clock(CLOCK_MONOTONIC, &clock_monotonic);         posix_timers_register_clock(CLOCK_MONOTONIC_RAW, &clock_monotonic_raw);         posix_timers_register_clock(CLOCK_REALTIME_COARSE, &clock_realtime_coarse);         posix_timers_register_clock(CLOCK_MONOTONIC_COARSE, &clock_monotonic_coarse);         posix_timers_register_clock(CLOCK_BOOTTIME, &clock_boottime);         posix_timers_cache = kmem_cache_create("posix_timers_cache",                                         sizeof (struct k_itimer), 0, SLAB_PANIC, NULL);         idr_init(&posix_timers_id);         return 0; } #define CLOCK_REALTIME                  0 #define CLOCK_MONOTONIC                 1 #define CLOCK_PROCESS_CPUTIME_ID        2 #define CLOCK_THREAD_CPUTIME_ID         3 #define CLOCK_MONOTONIC_RAW             4 #define CLOCK_REALTIME_COARSE           5 #define CLOCK_MONOTONIC_COARSE          6 #define CLOCK_BOOTTIME                  7 #define CLOCK_REALTIME_ALARM            8 #define CLOCK_BOOTTIME_ALARM            9    系统在初始化是会调用init_posix_timers等函数来初始化clock_gettime系统调用所需要的相关数据结构。这里，调用clock_gettime获取时间时，需要的是k_clock结构中的clock_get回调函数。对于clock_gettime的which_clock参数，系统支持获取包括xtime,boot time,monotonic time,raw monotonic time以及进程或者线程运行时间等共十种方式。对于获取xtime和monotonic time，which_clock有两种设置，分别是带_COARSE和不带两种方式。 2.3.1 _COARSE作用 static int posix_clock_realtime_get(clockid_t which_clock, struct timespec *tp) {         ktime_get_real_ts(tp);         return 0; }   #define ktime_get_real_ts(ts)   getnstimeofday(ts) void getnstimeofday(struct timespec *ts) {         unsigned long seq;         s64 nsecs;           WARN_ON(timekeeping_suspended);           do {                 seq = read_seqcount_begin(&xtime_seq);                   *ts = xtime;                 nsecs = timekeeping_get_ns();                   /* If arch requires, add in gettimeoffset() */                 nsecs += arch_gettimeoffset();           } while (read_seqcount_retry(&xtime_seq, seq));           timespec_add_ns(ts, nsecs); }        static inline s64 timekeeping_get_ns(void) {         cycle_t cycle_now, cycle_delta;         struct clocksource *clock;           /* read clocksource: */         clock = timekeeper.clock;         cycle_now = clock->read(clock);           /* calculate the delta since the last update_wall_time: */         cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;           /* return delta convert to nanoseconds using ntp adjusted mult. */         return clocksource_cyc2ns(cycle_delta, timekeeper.mult,                                   timekeeper.shift); }   static int posix_get_realtime_coarse(clockid_t which_clock, struct timespec *tp) {         *tp = current_kernel_time();         return 0; } struct timespec current_kernel_time(void) {         struct timespec now;         unsigned long seq;           do {                 seq = read_seqcount_begin(&xtime_seq);                                                                  now = xtime;         } while (read_seqcount_retry(&xtime_seq, seq));                  return now; }    从函数实现可以看到，当带_COARSE后缀时，函数直接返回xtime。而不带_COARSE后缀时，函数得首先统计当期时刻和上次更新xtime时的时间差，将时间差与xtime之和返回。从这点看，带后缀的比不带后缀的效率要高。当需要的时间不需要太精确时，可以使用带_COARSE后缀的参数来获取时间，这样可以略微提升应用的运行速度。当获取时间的操作很频繁时，作用尤其明显。