linux kernel timer : dynamic ticks

linux kernel timer : dynamic ticks

quoted from : Professional Linux kernel architecture by Wolfgang Mauerer, 933-934


1. if NO active tasks are on the run queue, the kernel picks a special task-- theidle tasks to run.

2. whenever the idle task is selected to run, the periodic tick is DISABLED until the next timer will expire.

3. The tick is re-enabled again after this time span, or when an interrupt occurs.

4. In the meantime, the CPU can enjoy a well-deserved sleep.

5. Note that only classical timers need to be considered for this purpose.

High-resoultion timers are NOT bound by the tick frequency, and are aslo NOT implemented on top of periodic ticks.

6. One-shot clocks is a prerequisite for dynamic tick. The feature of a dymanic tick is that the tick mechanism can be stoped and restarted as necessary. The purely periodic timers are not suitable.

7. Periodic ticks mean a tick implementation that does NOT use dymanic ticks.

8. dynamic ticks are implemented differently on high-res and low-res timers.

9. struct tick_sched

http://lxr.linux.no/#linux+v2.6.34.1/include/linux/tick.h#L48

**
30 * struct tick_sched - sched tick emulation and no idle tick control/stats
31 * @sched_timer: hrtimer to schedule the periodic tick in high
32 * resolution mode
33 * @idle_tick: Store the last idle tick expiry time when the tick
34 * timer is modified for idle sleeps. This is necessary
35 * to resume the tick timer operation in the timeline
36 * when the CPU returns from idle
37 * @tick_stopped: Indicator that the idle tick has been stopped
38 * @idle_jiffies: jiffies at the entry to idle for idle time accounting
39 * @idle_calls: Total number of idle calls
40 * @idle_sleeps: Number of idle calls, where the sched tick was stopped
41 * @idle_entrytime: Time when the idle call was entered
42 * @idle_waketime: Time when the idle was interrupted
43 * @idle_exittime: Time when the idle state was left
44 * @idle_sleeptime: Sum of the time slept in idle with sched tick stopped
45 * @sleep_length: Duration of the current idle sleep
46 * @do_timer_lst: CPU was the last one doing do_timer before going idle
47 */
48struct tick_sched {
49 struct hrtimer sched_timer;
50 unsigned long check_clocks;
51 enum tick_nohz_mode nohz_mode;
52 ktime_t idle_tick;
53 int inidle;
54 int tick_stopped;
55 unsigned long idle_jiffies;
56 unsigned long idle_calls;
57 unsigned long idle_sleeps;
58 int idle_active;
59 ktime_t idle_entrytime;
60 ktime_t idle_waketime;
61 ktime_t idle_exittime;
62 ktime_t idle_sleeptime;
63 ktime_t idle_lastupdate;
64 ktime_t sleep_length;
65 unsigned long last_jiffies;
66 unsigned long next_jiffies;
67 ktime_t idle_expires;
68 int do_timer_last;
69};