全局变量jiffies用来记录自系统启动以来产生的节拍的总数。启动时，内核将该变量初始化为0，此后，每次时钟中断处理程序都会增加该变量的值。一秒内时钟中断的次数等于Hz，所以jiffies一秒内增加的值也就是Hz。
系统运行时间以秒为单位，等于jiffies/Hz。
注意，jiffies类型为无符号长整型(unsigned long)，其他任何类型存放它都不正确。
将以秒为单位的时间转化为jiffies：
seconds * Hz
将jiffies转化为以秒为单位的时间：
jiffies / Hz
相比之下，内核中将秒转换为jiffies用的多些。
jiffies的内部表示jiffies定义于文件<linux/Jiffies.h>中：
/* * The 64-bit value is not atomic - you MUST NOT read it * without sampling the sequence number in xtime_lock. * get_jiffies_64() will do this for you as appropriate. */ extern u64 __jiffy_data jiffies_64; extern unsigned long volatile __jiffy_data jiffies;ld(1)脚本用于连接主内核映像（在x86上位于arch/i386/kernel/vmlinux.lds.S中），然后用jiffies_64变量的初值覆盖jiffies变量。因此jiffies取整个jiffies_64变量的低32位。
访问jiffies的代码只会读取jiffies_64的低32位，通过get_jiffies_64()函数就可以读取整个64位的值。在64位体系结构上，jiffies_64和jiffies指的是同一个变量。
#if (BITS_PER_LONG < 64) u64 get_jiffies_64(void); #else static inline u64 get_jiffies_64(void) { return (u64)jiffies; } #endif 在<Time.c(kernel)>中 #if (BITS_PER_LONG < 64) u64 get_jiffies_64(void) { unsigned long seq; u64 ret; do { seq = read_seqbegin(&xtime_lock); ret = jiffies_64; } while (read_seqretry(&xtime_lock, seq)); return ret; } jiffies的回绕wrap around当jiffies的值超过它的最大存放范围后就会发生溢出。对于32位无符号长整型，最大取值为(2^32)-1,即429496795。如果节拍计数达到了最大值后还要继续增加，它的值就会回绕到0。
内核提供了四个宏来帮助比较节拍计数，它们能正确的处理节拍计数回绕的问题：
/* * 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) /* 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) 用户空间和HZ问题提出：
在2.6以前的内核中，如果改变内核中的HZ值会给用户空间中某些程序造成异常结果。因为内核是以节拍数/秒的形式给用户空间导出这个值的，应用程序便依赖这个特定的HZ值。如果在内核中改变了HZ的定义值，就打破了用户空间的常量关系---用户空间并不知道新的HZ值。
解决方法：
内核更改所有导出的jiffies值。内核定义了USER_HZ来代表用户空间看到的HZ值。在x86体系结构上，由于HZ值原来一直是100，所以USER_HZ值就定义为100。内核可以使用宏jiffies_to_clock_t()将一个有HZ表示的节拍计数转换为一个由USER_HZ表示的节拍计数。
在<Time.c(kernel)>中 /* * Convert jiffies/jiffies_64 to clock_t and back. */ clock_t jiffies_to_clock_t(long x) { #if (TICK_NSEC % (NSEC_PER_SEC / USER_HZ)) == 0 return x / (HZ / USER_HZ); #else u64 tmp = (u64)x * TICK_NSEC; do_div(tmp, (NSEC_PER_SEC / USER_HZ)); return (long)tmp; #endif } unsigned long clock_t_to_jiffies(unsigned long x) { #if (HZ % USER_HZ)==0 if (x >= ~0UL / (HZ / USER_HZ)) return ~0UL; return x * (HZ / USER_HZ); #else u64 jif; /* Don't worry about loss of precision here .. */ if (x >= ~0UL / HZ * USER_HZ) return ~0UL; /* .. but do try to contain it here */ jif = x * (u64) HZ; do_div(jif, USER_HZ); return jif; #endif } u64 jiffies_64_to_clock_t(u64 x) { #if (TICK_NSEC % (NSEC_PER_SEC / USER_HZ)) == 0 do_div(x, HZ / USER_HZ); #else /* * There are better ways that don't overflow early, * but even this doesn't overflow in hundreds of years * in 64 bits, so.. */ x *= TICK_NSEC; do_div(x, (NSEC_PER_SEC / USER_HZ)); #endif return x; } 在<Div64.h(include/asm-i385)>中 /* * do_div() is NOT a C function. It wants to return * two values (the quotient and the remainder), but * since that doesn't work very well in C, what it * does is: * * - modifies the 64-bit dividend _in_place_ * - returns the 32-bit remainder * * This ends up being the most efficient "calling * convention" on x86. */ #define do_div(n,base) ({ / unsigned long __upper, __low, __high, __mod, __base; / __base = (base); / asm("":"=a" (__low), "=d" (__high):"A" (n)); / __upper = __high; / if (__high) { / __upper = __high % (__base); / __high = __high / (__base); / } / asm("divl %2":"=a" (__low), "=d" (__mod):"rm" (__base), "0" (__low), "1" (__upper)); / asm("":"=A" (n):"a" (__low),"d" (__high)); / __mod; / })用户空间期望HZ=USER_HZ，但是如果它们不相等，则由宏完成转换。
