Chapter 15: Time Management
Before discussing these functions, let us therefore consider which situations are faced by the kernel
depending on the selected configuration:
❑ A low-resolution system without dynamic ticks always uses a periodic tick. Support for one-shot
operations is not included in the kernel at all.
❑ Low-resolution systems with dynamic ticks use the tick device in one-shot mode.
❑ High-resolution systems always use one-shot mode independent of whether they work with
dynamic ticks or not.
All systems initially work in low-resolution mode and without dynamic ticks; they switch to a different
combination only later when the required hardware is initialized. I therefore focus on the low-resolution,
periodic tick case here. The more advanced options are discussed in Sections 15.4.5 (high-resolution
timers) and 15.5 (dynamic ticks). Some corrections are also required for broadcast mode; Section 15.6
covers them in more detail.
Before examining the low-resolution case without dynamic ticks, I would like to point out that
Figure 15-9 provides an overview of the tick handler functions that are used for the various possible
combinations. Note that which broadcast function is chosen for a system without dynamic ticks depends
on the mode of the underlying tick device. The details are given below.
HZ-based dynamic ticks
tick_handle_oneshot_broadcast
tick_handle_oneshot_broadcast
tick_handle_periodic_broadcast
tick_handle_periodic (low-res)
hrtimer_interrupt (high-res)
tick_nohz_handler (low-res)
hrtimer_interrupt (high-res)
broadcast
event_handler
Figure 15-9: Tick event and broadcast handler functions for all possible combinations of low- and
high-resolution mode, and with/without dynamic ticks.
Let us finally turn our attention totick_setup_periodic. The code flow diagram is shown in
Figure 15-10.
Set event device to periodic mode
Set event device to one-shot mode
Program next clock event
Yes
No
tick_setup_periodic
tick_set_periodic_handler
Event device supports periodic events?
Figure 15-10: Code flow diagram fortick_setup_periodic.
