Linux Kernel Architecture

Chapter 15: Time Management

1 because I do not consider dynamic tick mode for now. The previously discussed functiondo_timer

is used to handle all duties of the global timer. Recall that this includes updating the globaljiffies64

variable.

CPU responsible for global tick?

Set next tick after tick_period

return HRTIMER_RESTART

tick_sched_timer

tick_do_update_jiffies64

update_process_times

profile_tick

Figure 15-17: Code flow diagram fortick_sched_timer.

When the per-CPU periodic tick tasks have been performed inupdate_process_times(see Section 15.8)

andprofile_tick, the time for the next event is computed, andhrtimer_forwardprograms the timer

accordingly. By returningHRTIMER_RESTART, the timer is automatically re-queued and activated when

the next tick is due.

15.4.5 Switching to High-Resolution Timers

High-resolution timers are not enabled from the very beginning, but can only be activated when suitable

high-resolution clock sources have been initialized and added to the generic clock framework. Low-

resolution ticks, however, are provided (nearly) from the very beginning. In the following, I discuss how

the kernel switches from low- to high-resolution mode.

The high-resolution queue is processed byhrtimer_run_queuewhen low-resolution timers are active.

Before the queues are run, the function provides checks if a clock event device suitable for high resolution

timers is present in the system. In this case, the switch to high resolution mode is performed:

kernel/hrtimer.c

void hrtimer_run_queues(void)

{

...

if (tick_check_oneshot_change(!hrtimer_is_hres_enabled()))

if (hrtimer_switch_to_hres())

return;

...

}

tick_check_oneshot_changesignalizes that high-resolution timers can be used if a clock that

supports one-shot mode and fulfills the resolution requirements for high-res timers, that is, if the flag

CLOCK_SOURCE_VALID_FOR_HRESis set.hrtimer_switch_to_hresperforms the actual switch. The

required steps are summarized in Figure 15-18.