linux時間子系統（九）

3.4.3 模擬tick事件

     當系統切換到高精度模式後，tick_device被高精度定時器系統接管，再也不按期地產生tick事件。內核在3.0.30版本中尚未完全的廢除jiffies機制，系統仍是依賴按期到來的tick事件，完成進程調度和時間更新等操做，大量存在的低精度定時器仍然依賴於jiffies計數。因此，儘管tick_device被接管，高精度定時器系統仍然須要繼續提供按期的tick事件。爲了完成這個需求，因爲高精度模式已經啓用，內核定義了一個hrtimer，把它的到期時間設定爲一個jiffy的時間，當着個hrtimer到期時，在這個hrtimer的到期函數中，進行和原來的tick_device一樣的操做，而後把該hrtimer的到期時間順延一個jiffy週期。如此反覆循環，能夠完美的模擬原有tick_device的功能。
     在kernel/time/tick-sched.c中，內核定義了一個per_cpu的全局變量:tick_cpu_sched，從而爲每一個cpu提供了一個tick_sched結構。該結構主要用於管理NO_HZ配置下的tickless處理，由於模擬tick事件與tickless有很強的相關性，因此高精度定時器也利用了該結構的如下字段，用來完成模擬tick時間的操做：

 

struct tick_sched {

        struct hrtimer                  sched_timer; /* 主要用於模擬tick時間的hrtimer。*/

        unsigned long                   check_clocks; /* 第0位代表是否有符合要求的高精度定時器。*/

        enum tick_nohz_mode             nohz_mode; /* 用於表示當前的工做模式。*/

......

}

   內核使用函數tick_setup_sched_timer，該函數的做用就是設置一個用於模擬tick事件的hrtimer。

 

 

void tick_setup_sched_timer(void)

{

        struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);

        ktime_t now = ktime_get();

 

        /*

         * Emulate tick processing via per-CPU hrtimers:

         */

        hrtimer_init(&ts->sched_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS); /* 初始化該cpu所屬的tick_sched結構中的sched_timer字段。*/

        ts->sched_timer.irqsafe = 1;

        ts->sched_timer.function = tick_sched_timer; /* 把該hrtimer的回調函數設置爲tick_sched_timer.*/

 

        /* Get the next period (per cpu) */

        hrtimer_set_expires(&ts->sched_timer, tick_init_jiffy_update()); /* 設置到期時間爲下一個jiffy時刻。*/

 

        for (;;) {

                hrtimer_forward(&ts->sched_timer, now, tick_period);

                hrtimer_start_expires(&ts->sched_timer,

                                      HRTIMER_MODE_ABS_PINNED);

                /* Check, if the timer was already in the past */

                if (hrtimer_active(&ts->sched_timer))

                        break;

                now = ktime_get();

        }

 

#ifdef CONFIG_NO_HZ

        if (tick_nohz_enabled) {

                ts->nohz_mode = NOHZ_MODE_HIGHRES; /* 將工做模式設置爲NOHZ_MODE_HIGHRES模式,代表利用高精度模式實現NO_HZ。*/

                printk(KERN_INFO "Switched to NOHz mode on CPU #%d\n", smp_processor_id());

        }

#endif

}

 

static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer)

{               

        struct tick_sched *ts =

                container_of(timer, struct tick_sched, sched_timer);

        struct pt_regs *regs = get_irq_regs();

        ktime_t now = ktime_get();

        int cpu = smp_processor_id();

        static int i=0;

 

#ifdef CONFIG_NO_HZ

        /*

         * Check if the do_timer duty was dropped. We don't care about

         * concurrency: This happens only when the cpu in charge went

         * into a long sleep. If two cpus happen to assign themself to

         * this duty, then the jiffies update is still serialized by

         * xtime_lock.

         */

        if (unlikely(tick_do_timer_cpu == TICK_DO_TIMER_NONE))

                tick_do_timer_cpu = cpu;

#endif

 

        /* Check, if the jiffies need an update */

        if (tick_do_timer_cpu == cpu) /* 在smp系統中，只有一個cpu負責jiffies計數，時間更新等全局操做。因此斷定當前cpu

   是不是負責更新jiffies和時間，若是是，則執行更新操做。*/

                tick_do_update_jiffies64(now);

 

        /*

         * Do not call, when we are not in irq context and have

         * no valid regs pointer

         */

        if (regs) { /* 利用regs指針確保當前是在中斷上下文，而後調用update_precess_timer。*/

                /*

                 * When we are idle and the tick is stopped, we have to touch

                 * the watchdog as we might not schedule for a really long

                 * time. This happens on complete idle SMP systems while

                 * waiting on the login prompt. We also increment the "start of

                 * idle" jiffy stamp so the idle accounting adjustment we do

                 * when we go busy again does not account too much ticks.

                 */

                if (ts->tick_stopped) {

                        touch_softlockup_watchdog();

                        ts->idle_jiffies++;

                }

                update_process_times(user_mode(regs));

                profile_tick(CPU_PROFILING);

        }

 

        hrtimer_forward(timer, now, tick_period); /* 把hrtimer的到期時間推動一個tick週期。*/

 

        return HRTIMER_RESTART; /*返回HRTIMER_RESTART代表該hrtimer須要再次啓動，以便產生下一個tick事件。*/

}

     對比模擬tick時間的hrtimer的回調函數tick_sched_timer和切換前tick_device的回調函數tick_handle_periodic，他們幾乎完成了同樣的工做。

 

3.5 hrtimer的使用

     咱們可使用系統調用timer_create/timer_delete/timer_gettime/timer_settime設置精度達到ns的定時器。不過每一個進程只能有一個。

     系統提供getitimer/setitimer系統調用，提供精度爲us級別的定時器。