cpuidle: Export the next timer expiration for CPUs

To be able to predict the sleep duration for a CPU entering idle, it
is essential to know the expiration time of the next timer.  Both the
teo and the menu cpuidle governors already use this information for
CPU idle state selection.

Moving forward, a similar prediction needs to be made for a group of
idle CPUs rather than for a single one and the following changes
implement a new genpd governor for that purpose.

In order to support that feature, add a new function called
tick_nohz_get_next_hrtimer() that will return the next hrtimer
expiration time of a given CPU to be invoked after deciding
whether or not to stop the scheduler tick on that CPU.

Make the cpuidle core call tick_nohz_get_next_hrtimer() right
before invoking the ->enter() callback provided by the cpuidle
driver for the given state and store its return value in the
per-CPU struct cpuidle_device, so as to make it available to code
outside of cpuidle.

Note that at the point when cpuidle calls tick_nohz_get_next_hrtimer(),
the governor's ->select() callback has already returned and indicated
whether or not the tick should be stopped, so in fact the value
returned by tick_nohz_get_next_hrtimer() always is the next hrtimer
expiration time for the given CPU, possibly including the tick (if
it hasn't been stopped).

Co-developed-by: Lina Iyer <lina.iyer@linaro.org>
Co-developed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
[ rjw: Subject & changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 17 insertions, 2 deletions

diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
index 7f108309e871..0f4b7c45df3e 100644
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -328,9 +328,23 @@ int cpuidle_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 int cpuidle_enter(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 		  int index)
 {
+	int ret = 0;
+
+	/*
+	 * Store the next hrtimer, which becomes either next tick or the next
+	 * timer event, whatever expires first. Additionally, to make this data
+	 * useful for consumers outside cpuidle, we rely on that the governor's
+	 * ->select() callback have decided, whether to stop the tick or not.
+	 */
+	WRITE_ONCE(dev->next_hrtimer, tick_nohz_get_next_hrtimer());
+
 	if (cpuidle_state_is_coupled(drv, index))
-		return cpuidle_enter_state_coupled(dev, drv, index);
-	return cpuidle_enter_state(dev, drv, index);
+		ret = cpuidle_enter_state_coupled(dev, drv, index);
+	else
+		ret = cpuidle_enter_state(dev, drv, index);
+
+	WRITE_ONCE(dev->next_hrtimer, 0);
+	return ret;
 }
 
 /**
@@ -511,6 +525,7 @@ static void __cpuidle_device_init(struct cpuidle_device *dev)
 {
 	memset(dev->states_usage, 0, sizeof(dev->states_usage));
 	dev->last_residency = 0;
+	dev->next_hrtimer = 0;
 }
 
 /**