unsigned long timestamp = jiffies
unsigned long nextTick = jiffies + 1
unsigned long 5sLater = jiffies + 5*HZ

/*
 *  These inlines deal with timer wrapping correctly. You are
 *  strongly encouraged to use them
 *  1. Because people otherwise forget
 *  2. Because if the timer wrap changes in future you won't have to
 * alter your driver code.
 *
 * time_after(a,b) returns true if the time a is after time b.
 *
 * Do this with "<0" and ">=0" to only test the sign of the result. A
 * good compiler would generate better code (and a really good compiler
 * wouldn't care). Gcc is currently neither.
 */
#define time_after(a,b) \
 (typecheck(unsigned long, a) && \
  typecheck(unsigned long, b) && \
  ((long)(b) - (long)(a) < 0))
#define time_before(a,b)time_after(b,a)

#define time_after_eq(a,b)  \
 (typecheck(unsigned long, a) && \
  typecheck(unsigned long, b) && \
  ((long)(a) - (long)(b) >= 0))
#define time_before_eq(a,b) time_after_eq(b,a)

/*
 * Calculate whether a is in the range of [b, c].
 */
#define time_in_range(a,b,c) \
 (time_after_eq(a,b) && \
     time_before_eq(a,c))

/*
 * Calculate whether a is in the range of [b, c).
 */
#define time_in_range_open(a,b,c) \
 (time_after_eq(a,b) && \
  time_before(a,c))

/* Same as above, but does so with platform independent 64bit types.
 * These must be used when utilizing jiffies_64 (i.e. return value of
 * get_jiffies_64() */
#define time_after64(a,b)   \
 (typecheck(__u64, a) && \
     typecheck(__u64, b) && \
     ((__s64)(b) - (__s64)(a) < 0))
#define time_before64(a,b)  time_after64(b,a)

#define time_after_eq64(a,b)\
 (typecheck(__u64, a) && \
  typecheck(__u64, b) && \
     ((__s64)(a) - (__s64)(b) >= 0))
#define time_before_eq64(a,b)   time_after_eq64(b,a)

/*
 * These four macros compare jiffies and 'a' for convenience.
 */

/* time_is_before_jiffies(a) return true if a is before jiffies */
#define time_is_before_jiffies(a) time_after(jiffies, a)

/* time_is_after_jiffies(a) return true if a is after jiffies */
#define time_is_after_jiffies(a) time_before(jiffies, a)

/* time_is_before_eq_jiffies(a) return true if a is before or equal to jiffies*/
#define time_is_before_eq_jiffies(a) time_after_eq(jiffies, a)

/* time_is_after_eq_jiffies(a) return true if a is after or equal to jiffies*/
#define time_is_after_eq_jiffies(a) time_before_eq(jiffies, a)

/*
 * The IDs of the various system clocks (for POSIX.1b interval timers):
 */
#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

unsigned long  delay  =  jiffies + 100;
While (time_before (jiffies , delay ));

sleep()               // s
usleep()            // us
nanosleep()         // ns
clock_nanosleep()   // POSIX规范接口，更高级，提供各种类型的定时器

REAL：记录墙上时间的流逝（SIGALARM）
VIRTUAL：只记录进程空间的时间流逝（SIGVTALARM）
PROF：在进程执行以及内核为进程服务时，记录时间的流逝（SIGPROF）

int timer_create(clockid_t clockid,
          struct sigevent *restrict evp,
          timer_t *restrict timerid);
int timer_getoverrun(timer_t timerid);
int timer_gettime(timer_t timerid, struct itimerspec *value);
int timer_settime(timer_t timerid, int flags,
              const struct itimerspec *restrict value,
              struct itimerspec *restrict ovalue);