内核变量——Jiffies

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本文深入探讨了Linux内核中jiffies变量的作用，包括如何计算系统运行时间、jiffies变量的转换方法以及与时间相关的宏用法。通过实际代码示例，解释了如何利用jiffies进行定时和时间差计算。

全局变量jiffies表示自系统启动以来系统产生的嘀嗒数。当启动时，内核初始化该变量为0。每次时钟中断就会增1，所以系统运行时间可以计算为：jiffies/HZ秒。

jiffies变量定义如下：

extern unsigned long volatile jiffies;

将jiffies转换为秒：(jiffies / HZ)。将秒换算为jiffies：(seconds*HZ)。

jiffies比较相关的宏：

#define time_after(unknown, known) ((long)(known) - (long)(unknown) < 0)

#define time_before(unknown, known) ((long)(unknown) - (long)(known) < 0)

#define time_after_eq(unknown, known) ((long)(unknown) - (long)(known) >= 0)

#define time_before_eq(unknown, known) ((long)(known) - (long)(unknown) >= 0)

使用例子：

unsigned long timeout = jiffies + HZ/2;	/* timeout in 0.5s */
/* ... */
if (time_before(jiffies, timeout)) {
/* we did not time out, good ... */
} else {
/* we timed out, error ... */
}

代码参考：

typedef struct {
    unsigned long jiffies;
}os_time_t;

static inline osal_result os_clock_get_time(os_time_t *time){
    time->jiffies = jiffies;
    return OSAL_SUCCESS;
}

static inline osal_result os_clock_get_time_diff_msecs( os_time_t *time,
                                                        unsigned long *msecs)
{
    *msecs = (jiffies - time->jiffies) * 1000 / HZ;
    return OSAL_SUCCESS;
}

static inline osal_result os_clock_get_time_diff_secs(  os_time_t *time,
                                                        unsigned long *secs)
{
    *secs = (jiffies - time->jiffies) / HZ;
    return OSAL_SUCCESS;
}