Linux Kernel Architecture

Chapter 15: Time Management

Set resolution in clock base

retrigger_next_event

hrtimer_switch_to_highres

tick_init_highres

tick_switch_to_oneshot(hrtimer_interrupt)

tick_setup_sched_timer

Figure 15-18: Code flow diagram forhrtimer_switch_to_hires.

tick_init_switch_to_highresis a wrapper function usingtick_switch_to_oneshotto set the clock

event device to one-shot mode. Additionally,hrtimer_interruptis installed as event handler. After-

ward the periodic tick emulation is activated withtick_init_highresas discussed above. Since the

resolution is now improved, this also needs to be reflected in the data structures:

kernel/hrtimer.c

static int hrtimer_switch_to_hres(void)

{

...

base->hres_active = 1;

base->clock_base[CLOCK_REALTIME].resolution = KTIME_HIGH_RES;

base->clock_base[CLOCK_MONOTONIC].resolution = KTIME_HIGH_RES;

...

}

Finally,retrigger_next_eventreprograms the clock event device to set the ball rolling. High-resolution

support is now active!

15.5 Dynamic Ticks

Periodic ticks have provided a notion of time to the Linux kernel for many of years. The approach is

simple and effective, but shows one particular deficiency on systems where power consumption does

matter: The periodic tick requires that the system is in an active state at a certain frequency. Longer

periods of rest are impossible because of this.

Dynamic ticks mend this problem. The periodic tick is only activated when some tasks actually do need

to be performed. Otherwise, it is temporarily disabled. Support for this technique can be selected at

compile time, and the resulting system is also referred to as atickless system.However,thisnameisnot

entirely accurate because the fundamental frequencyHZat which the periodic tick operates when it is

functional still provides a raster for time flow. Since the tick can be activated and deactivated according

to the current needs, the termdynamic ticksfits very well.

How can the kernel decide if the system has nothingto do? Recall from Chapter 2 that if no active tasks

are on the run queue, the kernel picks a special task — the idle task — to run. At this point, the dynamic

tick mechanism enters the game. Whenever the idle task is selected to run, the periodic tick is disabled