linux中的jiffies变量.doc

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(232)-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 wont have to * alter your driver code. * * time_after(a,b) returns true if the time a is after time b. * * Do this with =0 to only test the sign of the result. A * good compiler would generate better code (and a really good compiler * wouldnt 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_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_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; /* Dont 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 dont overflow early, * but even this doesnt overflow in hundreds of years * in 64 bits, so. */ x *= TICK_NSEC; do_div(x, (NSEC_PER_SEC / USER_HZ); #endif return x; 在中 /* * do_div() is NOT a C function. It wants to return * two values (the quotient and the remainder), but * since that doesnt 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