Dynamic tick/tickless kernel
If, however, a CPU leaves the idle state upon an interrupt (the architecture code calls handle_IRQ(), which indirectly calls tick_irq_enter()), this CPU tick device is enabled (first in one-shot mode), and before it performs any task, the tick_nohz_irq_ enter() function is called to ensure that jiffies are up to date so that the interrupt handler does not have to deal with a stale jiffy value, and then it resumes the periodic tick, which is kept active until the next call to tick_nohz_idle_stop_tick() (which is essentially called from do_idle()).
Using standard kernel low-precision (low-res) timers CONFIG_HZ
Jiffies and HZ zcat /proc/config.gz |grep CONFIG_HZ ; #Jiffies每秒增加HZ次，一次tick 就是 1/HZ 秒
Kernel timer APIs
定时器：struct timer_list { struct hlist_node entry; unsigned long expires; void (*function)(struct timer_list *); u32 flags; );
void timer_setup( struct timer_list *timer, void (*function)( struct timer_list *), unsigned int flags); #动态设置一个定时器
#define DEFINE_TIMER(_name, _function) [...] #静态设置定时器的宏
int mod_timer(struct timer_list *timer, unsigned long expires); #修改一个定时器，重设它的到期时间，return 0 如果timer先前是不活动的，timer pending 或在执行func则return 1；
void add_timer(struct timer_list *timer)； #启动定时器，需要先把定时器设置好；比如 my_timer.expires = jiffies + ((12 * HZ) / 10); /* 1.2s */ add_timer(&my_timer);
删除或取消定时器：
int del_timer(struct timer_list *timer); #return 0 if timer is inactive，else 1
int del_timer_sync(struct timer_list *timer); #会等待每个核心的定时器的func完成后才return，非原子操作专用
int timer_pending(const struct timer_list *timer); #检查timer 状态
