Linux Kernel Architecture

Chapter 15: Time Management

Finally, the tick device is reprogrammed to provide the next event at the proper point in time. While the

method to set the timer differs between high- and low-resolution mode, the code jumps to the labelout

if programming is successful in both cases:

kernel/time/tick-sched.c

expires = ktime_add_ns(last_update, tick_period.tv64 *

delta_jiffies);

ts->idle_expires = expires;

if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {

hrtimer_start(&ts->sched_timer, expires,

HRTIMER_MODE_ABS);

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

if (hrtimer_active(&ts->sched_timer))

goto out;

} else if(!tick_program_event(expires, 0))

goto out;

tick_do_update_jiffies64(ktime_get());

}

raise_softirq_irqoff(TIMER_SOFTIRQ);

out:

ts->next_jiffies = next_jiffies;

ts->last_jiffies = last_jiffies;

ts->sleep_length = ktime_sub(dev->next_event, now);

}

If reprogramming failed, then too much time was spent in processing, and the expiration date already

liesinthepast.Inthiscase,tick_do_update_jiffies_64updates jiffies to the correct value, and the

timer softIRQTIMER_SOFTIRQis raised to process any pending timer-wheel timers. Note that the softIRQ

is also raised if some events are due in the current jiffy period.

Restarting Ticks

tick_nohz_restart_sched_tickis used to restart ticks. The code flow diagram is given by Figure 15-20.

Account idle time

Set tick_sched->tick_stopped = 0

Program next tick event

tick_nohz_restart_sched_tick

tick_do_update_jiffies64

Figure 15-20: Code flow diagram for

tick_nohz_restart_sched_tick.

Again, the implementation is complicated by various technical details, but the general principle is rather

simple. Our old acquaintancetick_do_updates_jiffies64is called first. After correctly accounting the