内核中的jiffies变量

关于内核中的jiffies变量，可通过下图解释。

在内核中很多函数的参数都是都是以jiffies为单位的，如定时器中mod_timer(&timer, jiffies + 2*HZ);

jiffies代表当前发生中断的次数，2*Hz代表发生了发生了2*Hz次中断，那jiffies+2Hz就是未来时间点的中断次数，转换成时间就是2s后的时间点。

还有很多表示时间的用jiffies为单位的函数,如

/** * mod_timer - modify a timer's timeout * @timer: the timer to be modified * @expires: new timeout in jiffies * * mod_timer() is a more efficient way to update the expire field of an * active timer (if the timer is inactive it will be activated) * * mod_timer(timer, expires) is equivalent to: * *     del_timer(timer); timer->expires = expires; add_timer(timer); * * Note that if there are multiple unserialized concurrent users of the * same timer, then mod_timer() is the only safe way to modify the timeout, * since add_timer() cannot modify an already running timer. * * The function returns whether it has modified a pending timer or not. * (ie. mod_timer() of an inactive timer returns 0, mod_timer() of an * active timer returns 1.) */int mod_timer(struct timer_list *timer, unsigned long expires){expires = apply_slack(timer, expires);/* * This is a common optimization triggered by the * networking code - if the timer is re-modified * to be the same thing then just return: */if (timer_pending(timer) && timer->expires == expires)return 1;return __mod_timer(timer, expires, false, TIMER_NOT_PINNED);}EXPORT_SYMBOL(mod_timer);

/** * wait_event_interruptible_timeout - sleep until a condition gets true or a timeout elapses * @wq: the waitqueue to wait on * @condition: a C expression for the event to wait for * @timeout: timeout, in jiffies * * The process is put to sleep (TASK_INTERRUPTIBLE) until the * @condition evaluates to true or a signal is received. * The @condition is checked each time the waitqueue @wq is woken up. * * wake_up() has to be called after changing any variable that could * change the result of the wait condition. * * Returns: * 0 if the @timeout elapsed, -%ERESTARTSYS if it was interrupted by * a signal, or the remaining jiffies (at least 1) if the @condition * evaluated to %true before the @timeout elapsed. */#define wait_event_interruptible_timeout(wq, condition, timeout)\({\long __ret = timeout;\if (!___wait_cond_timeout(condition))\__ret = __wait_event_interruptible_timeout(wq,\condition, timeout);\__ret;\})

/** * schedule_delayed_work - put work task in global workqueue after delay * @dwork: job to be done * @delay: number of jiffies to wait or 0 for immediate execution * * After waiting for a given time this puts a job in the kernel-global * workqueue. */static inline bool schedule_delayed_work(struct delayed_work *dwork,unsigned long delay)

当然，也有相应的函数将我们常用的时间如ms转化成jiffies的，如msecs_to_jiffies

unsigned long msecs_to_jiffies(const unsigned int m){/* * Negative value, means infinite timeout: */if ((int)m < 0)return MAX_JIFFY_OFFSET;#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)/* * HZ is equal to or smaller than 1000, and 1000 is a nice * round multiple of HZ, divide with the factor between them, * but round upwards: */return (m + (MSEC_PER_SEC / HZ) - 1) / (MSEC_PER_SEC / HZ);#elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)/* * HZ is larger than 1000, and HZ is a nice round multiple of * 1000 - simply multiply with the factor between them. * * But first make sure the multiplication result cannot * overflow: */if (m > jiffies_to_msecs(MAX_JIFFY_OFFSET))return MAX_JIFFY_OFFSET;return m * (HZ / MSEC_PER_SEC);#else/* * Generic case - multiply, round and divide. But first * check that if we are doing a net multiplication, that * we wouldn't overflow: */if (HZ > MSEC_PER_SEC && m > jiffies_to_msecs(MAX_JIFFY_OFFSET))return MAX_JIFFY_OFFSET;return (MSEC_TO_HZ_MUL32 * m + MSEC_TO_HZ_ADJ32)>> MSEC_TO_HZ_SHR32;#endif}EXPORT_SYMBOL(msecs_to_jiffies);