linux中的jiffies变量

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：

1. /*
2. * The 64-bit value is not atomic - you MUST NOT read it
3. * without sampling the sequence number in xtime_lock.
4. * get_jiffies_64() will do this for you as appropriate.
5. */
6. extern u64 __jiffy_data jiffies_64;
7. 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。

1. #if (BITS_PER_LONG < 64)
2. u64 get_jiffies_64(void);
3. #else
4. static inline u64 get_jiffies_64(void)
5. {
6. return (u64)jiffies;
7. }
8. #endif

1. 在<Time.c(kernel)>中
2. #if (BITS_PER_LONG < 64)
3. u64 get_jiffies_64(void)
4. {
5. unsigned long seq;
6. u64 ret;
7. do {
8. seq = read_seqbegin(&xtime_lock);
9. ret = jiffies_64;
10. } while (read_seqretry(&xtime_lock, seq));
11. return ret;
12. }

• 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：

1. /*
2. * These inlines deal with timer wrapping correctly. You are
3. * strongly encouraged to use them
4. * 1. Because people otherwise forget
5. * 2. Because if the timer wrap changes in future you won't have to
6. * alter your driver code.
7. *
8. * time_after(a,b) returns true if the time a is after time b.
9. *
10. * Do this with "<0" and ">=0" to only test the sign of the result. A
11. * good compiler would generate better code (and a really good compiler
12. * wouldn't care). Gcc is currently neither.
13. */
14. #define time_after(a,b) /
15. (typecheck(unsigned long, a) && /
16. typecheck(unsigned long, b) && /
17. ((long)(b) - (long)(a) < 0))
18. #define time_before(a,b) time_after(b,a)
19. #define time_after_eq(a,b) /
20. (typecheck(unsigned long, a) && /
21. typecheck(unsigned long, b) && /
22. ((long)(a) - (long)(b) >= 0))
23. #define time_before_eq(a,b) time_after_eq(b,a)
24. /* Same as above, but does so with platform independent 64bit types.
25. * These must be used when utilizing jiffies_64 (i.e. return value of
26. * get_jiffies_64() */
27. #define time_after64(a,b) /
28. (typecheck(__u64, a) && /
29. typecheck(__u64, b) && /
30. ((__s64)(b) - (__s64)(a) < 0))
31. #define time_before64(a,b) time_after64(b,a)
32. #define time_after_eq64(a,b) /
33. (typecheck(__u64, a) && /
34. typecheck(__u64, b) && /
35. ((__s64)(a) - (__s64)(b) >= 0))
36. #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。

1. 在<Time.c(kernel)>中
2. /*
3. * Convert jiffies/jiffies_64 to clock_t and back.
4. */
5. clock_t jiffies_to_clock_t(long x)
6. {
7. #if (TICK_NSEC % (NSEC_PER_SEC / USER_HZ)) == 0
8. return x / (HZ / USER_HZ);
9. #else
10. u64 tmp = (u64)x * TICK_NSEC;
11. do_div(tmp, (NSEC_PER_SEC / USER_HZ));
12. return (long)tmp;
13. #endif
14. }
15. unsigned long clock_t_to_jiffies(unsigned long x)
16. {
17. #if (HZ % USER_HZ)==0
18. if (x >= ~0UL / (HZ / USER_HZ))
19. return ~0UL;
20. return x * (HZ / USER_HZ);
21. #else
22. u64 jif;
23. /* Don't worry about loss of precision here .. */
24. if (x >= ~0UL / HZ * USER_HZ)
25. return ~0UL;
26. /* .. but do try to contain it here */
27. jif = x * (u64) HZ;
28. do_div(jif, USER_HZ);
29. return jif;
30. #endif
31. }
32. u64 jiffies_64_to_clock_t(u64 x)
33. {
34. #if (TICK_NSEC % (NSEC_PER_SEC / USER_HZ)) == 0
35. do_div(x, HZ / USER_HZ);
36. #else
37. /*
38. * There are better ways that don't overflow early,
39. * but even this doesn't overflow in hundreds of years
40. * in 64 bits, so..
41. */
42. x *= TICK_NSEC;
43. do_div(x, (NSEC_PER_SEC / USER_HZ));
44. #endif
45. return x;
46. }

1. 在<Div64.h(include/asm-i385)>中
2. /*
3. * do_div() is NOT a C function. It wants to return
4. * two values (the quotient and the remainder), but
5. * since that doesn't work very well in C, what it
6. * does is:
7. *
8. * - modifies the 64-bit dividend _in_place_
9. * - returns the 32-bit remainder
10. *
11. * This ends up being the most efficient "calling
12. * convention" on x86.
13. */
14. #define do_div(n,base) ({ /
15. unsigned long __upper, __low, __high, __mod, __base; /
16. __base = (base); /
17. asm("":"=a" (__low), "=d" (__high):"A" (n)); /
18. __upper = __high; /
19. if (__high) { /
20. __upper = __high % (__base); /
21. __high = __high / (__base); /
22. } /
23. asm("divl %2":"=a" (__low), "=d" (__mod):"rm" (__base), "0" (__low), "1" (__upper)); /
24. asm("":"=A" (n):"a" (__low),"d" (__high)); /
25. __mod; /
26. })

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。