Linux Kernel Architecture

Chapter 15: Time Management

When the handler has finished, theHRTIMER_STATE_CALLBACKflag can be removed again. If the timer

requested to be put back into the queue, thenenqueue_hrtimerfulfills this request.

15.4.4 Periodic Tick Emulation

The clock event handler in high-resolution mode ishrtimer_interrupt. This implies that

tick_handle_periodicdoes not provide the periodic tick anymore. An equivalent functionality thus

needs be made available based on high-resolution timers. The implementation is (nearly) identical

between the situations with and without dynamic ticks. The generic framework for dynamic ticks is

discussed in Section 15.5; the required components are covered here only cursorily.

Essentially,tick_schedis a special data structure to manage all relevant information about periodic

ticks, and one instance per CPU is provided by the global variabletick_cpu_sched.

tick_setup_sched_timeris called to activate the tick emulation layer when the kernel switches to

high-resolution mode. One high-resolution timer is installed per CPU. The required instance ofstruct

hrtimeris kept in the per-CPU variabletick_sched:

<tick.h>

struct tick_sched {

struct hrtimer sched_timer;

...

}

The functiontick_sched_timeris used as the callback handler. To avoid a situation in which all CPUs

are engaged in running the periodic tick handlers at the same time, the kernel distributes the acceleration

time as shown in Figure 15-16. Recall that the length of a tick period (in nanoseconds) istick_period.

The ticks are spread across the first half of this period. Assume that the first tick starts at time 0. If the

system containsNCPUs, the remaining periodic ticks are started at times
,2
,3
,...The offset
is

given bytick_period/(2N).

CPU 0

CPU 1

CPU 2

CPU 3

CPU N

periodic

tick occurs

0 tick_period/2 tick_period time

Figure 15-16: Distributing periodic tick handlers in high-resolution mode.

The tick timer is registered like every other regular high-resolution timer. The function displays some

similarities totick_periodic, but is slightly more complicated. The code flow diagram is shown in

Figure 15-17.

If the CPU that is currently executing the timer is responsible to provide the global tick (recall that this

duty has already been distributed in low-resolution mode at boot time), thentick_do_update_jiffies64

computes the number of jiffies that have passed since the last update — in our case, this will always be