As shown in bellowing chart, there should be a gap between
tick_nohz_idle_enter(step '1') and tick_nohz_get_sleep_length
(step '3') when idle, which comprise of the execution time of
the code behind step '1' and a series of notify call(step2,4).
Yes, for most of the scenarios, it would NOT be a problem.
However,all time consumed by the steps above will cause the
sleep_length is not very precised in the scenario where the
idle state is shallow(imagin an idle state last less than 500us
under a 200Mhz's P-state). Change it in this series of patch.
Please refer to bellowing chart for detailed information
current approach:
static void cpu_idle_loop(void)
{
while (1) {
tick_nohz_idle_enter();
--->__tick_nohz_idle_enter
--->tick_nohz_stop_sched_tick
{
...
ts->sleep_length = ktime_sub(dev->next_event, now);
/*1*/
...
}
while (!need_resched()) {
...
local_irq_disable();
arch_cpu_idle_enter();
--->idle_notifier_call_chain(IDLE_START);
/*2*/
cpuidle_idle_call();
--->next_state = cpuidle_select(drv, dev);
/*3*/
--->static int arm_enter_idle_state(...)
{
...
ret = cpu_pm_enter();
/*4*/
}
}
---------|----------------------|----------------------------|------------------------------------|--------------------------------------------|--------------------
1.now 2.IDLE_START 3.select idle state
4.CPU_PM_ENTER next_event
(sleep_length)
modified approach:
static void cpu_idle_loop(void)
{
while (1) {
tick_nohz_idle_enter();
--->__tick_nohz_idle_enter
--->tick_nohz_stop_sched_tick
{
...
...
}
while (!need_resched()) {
...
local_irq_disable();
arch_cpu_idle_enter();
--->idle_notifier_call_chain(IDLE_START);
/*1*/
cpuidle_idle_call();
--->ret = cpu_pm_enter();
/*2*/
--->tick_nohz_get_sleep_length
{
ts->sleep_length = ktime_sub(dev->next_event,
now); /*3*/
}
--->next_state = cpuidle_select(drv, dev);
/*4*/
--->static int arm_enter_idle_state(...)
}
----------------|---------------------------------------|---------------------|------------------|--------------------------------------------|--------------------
1.IDLE_START 2.CPU_PM_ENTER
3.now 4.select idle state next_event
(sleep_length)
Signed-off-by: Zhaoyang Huang <[email protected]>
---
kernel/time/tick-sched.c | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 536ada8..ee3be3d 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -975,6 +975,11 @@ void tick_nohz_irq_exit(void)
ktime_t tick_nohz_get_sleep_length(void)
{
struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
+ struct clock_event_device *dev =
__this_cpu_read(tick_cpu_device.evtdev);
+ ktime_t now;
+
+ now = ktime_get();
+ ts->sleep_length = ktime_sub(dev->next_event, now);
return ts->sleep_length;
}
--
1.9.1
