在科学应用中,jiffies通常表示各种时间间隔。在linux中jiffies定义如下:
extern unsigned long volatile jiffies;
jiffies变量总是无符号长整数。因此,在32位体系结构上是32位,在64位体系结构上是64位。对于32位无符号长整数,取值范围为0~0xffffffff(2^32-1=4294967295),如果jiffies到达最大值后还要继续增加的话,它的值会回绕到0.
1. 一个回绕的例子:
jiffies_error.c
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#include <stdio.h>
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#include <stdlib.h>
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unsigned long jiffies = 4294967294;
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unsigned long timeout;
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int main(int argc, char* argv[])
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{
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timeout = jiffies + 1;
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if(timeout > jiffies) {
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printf("no timeout\n");
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}else {
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printf("timeout\n");
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}
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jiffies++;
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jiffies++;
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if(timeout > jiffies) {
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printf("wrap around: no timeout\n");
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}else {
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printf("wrap around: no timeout\n");
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}
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return 0;
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}
编译运行:
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gwwu@hz-dev2.wgw.com:~/test/jiffies>gcc -g jiffies_error.c -o jiffies_error
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gwwu@hz-dev2.wgw.com:~/test/jiffies>./jiffies_error
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no timeout
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wrap around: no timeout--------------but in fact, it is wrap around, and jiffies should be large than timeout
2. 为了解决回绕问题,linux内核提供了四个宏来帮助比较jiffies,
2.1 内核中的版本如下:
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/*
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* These inlines deal with timer wrapping correctly. You are
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* strongly encouraged to use them
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* 1. Because people otherwise forget
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* 2. Because if the timer wrap changes in future you won't have to
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* alter your driver code.
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*
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* time_after(a,b) returns true if the time a is after time b.
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*
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* Do this with "<0" and ">=0" to only test the sign of the result. A
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* good compiler would generate better code (and a really good compiler
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* wouldn't care). Gcc is currently neither.
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*/
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#define time_after(a,b) \
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(typecheck(unsigned long, a) && \
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typecheck(unsigned long, b) && \
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((long)((b) - (a)) < 0))
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#define time_before(a,b) time_after(b,a)
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#define time_after_eq(a,b) \
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(typecheck(unsigned long, a) && \
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typecheck(unsigned long, b) && \
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((long)((a) - (b)) >= 0))
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#define time_before_eq(a,b) time_after_eq(b,a)
2.2 简化版如下:
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#define time_after(a,b) ((long)(b)-(long)(a) < 0)
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#define time_before(a,b) ((long)(a)-(long)(b) < 0)
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#define time_after_eq(a,b) ((long)(a)-(long)(b) >= 0)
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#define time_before_eq(a,b) ((long)(b)-(long)(a) >= 0)
time_after(a,b)和time_after_eq(a,b), 当a超过b时,返回真,否则返回假
time_before(a,b)和time_before_eq(a,b),当b超过a时,返回真,否则返回假。
2.3 解决回绕问题的原理
下面研究一下time_after(a,b),time_before(a,b)解决回绕问题的原理。
在理想的情况下,时间是可以不停增长的,后来的时间值一定比前面的值大。然后计算机的世界不是一个理想的世界,所有的值都有其位数限制的。
在32位平台上,
1)unsigned long的位数为32位,从0到0xffffffff值都是递增的。
十六进制: 0-------------------0xffffffff
十进制: 0-------------------
4294967295
2)long的位数为32位。按照二进制补码的表示方式,从0到0x7fffffff的区间,值是逐渐递增的。
从0x80000000到0xFFFFFFFF这个区间,值也是递增的。
十六进制: 0x0-----------0x7fffffff----0x80000000-------------0xffffffff
十进制: 0---------2147483647----(-2147483648)----------(-1)
我们假设时间值a,b都是unsigned long, 而且
b发生在a之后, 表示b一定要大于a。如果单纯用unsigned long类型进行a和b大小的比较的话,当a和b的值超过0xffffffff会回绕成0。会导致比较失败。
time_after,time_before使用了有符号long型来解决这个问题。
这里有以下几种情况:
1)a和b都在区间0x0-----------0x7fffffff
在前提条件(a和b的绝对时间差值不能太大,防止b回绕后比a小)下 ,(long)(a)-(long)(b) < 0必然成立。
2)a在区间
0x0-----------0x7fffffff,b在区间0x80000000-------------0xffffffff
当a-b > 0x80000000时,(long)(a)-(long)(b) < 0必然成立
举例,假设b = 0x90000000, a = 0x00000001,则 (long)(a)-(long)(b) = 1879048193(0x70000001 < 0x80000000)
假设b = 0x90000000, a = 0x20000001,则 (long)(a)-(long)(b) = -1879048191(0x9000001 > 0x80000000)
3)a和b都在区间
0x80000000-------------0xffffffff
在前提条件(a和b的绝对时间差值不能太大,防止b回绕后比a小)下 ,(long)(a)-(long)(b) < 0必然成立。
4)a在区间
0x80000000-------------0xffffffff,b在区间0x0-----------0x7fffffff
当a-b > 0x80000000时,此时说明a与b靠的比较近,(long)(a)-(long)(b) < 0必然成立
举例,假设a = 0x90000000, b = 0x00000001,则 (long)(a)-(long)(b) = -1879048193(0x8fffffff > 0x80000000)
假设a =
0x90000000, b = 0x20000001,则 (long)(a)-(long)(b) = 1879048191(0x6ffffffff < 0x80000000)
所以linux内核采用有符号数的减法来避免回绕的问题。前提是,a和b两个的时间差不能太大,而一般情况下,linux定时器不会有这么大的差出现,所以不成问题。
2.4 举例
jiffies_ok.c
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#include <stdio.h>
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#include <stdlib.h>
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#define time_before(unknown,known) (long)((long)(unknown) - (long)(known)<0)
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unsigned long jiffies = 4294967294;
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unsigned long timeout;
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int main(int argc, char* argv[])
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{
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timeout = jiffies + 1;
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printf("time_before=%d\n",time_before(jiffies,timeout));
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jiffies++;
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jiffies++;
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printf("wrap around: time_before=%d\n",time_before(jiffies,timeout));
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return 0;
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}
编译运行:
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gwwu@hz-dev2.wgw.com:~/test/jiffies>gcc -g jiffies_ok.c -o jiffies_ok
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gwwu@hz-dev2.wgw.com:~/test/jiffies>./jiffies_ok
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time_before=1 ----no timeout
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wrap around: time_before=0 --------------timeout
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