cpuidle: Use nanoseconds as the unit of time

Currently, the cpuidle subsystem uses microseconds as the unit of
time which (among other things) causes the idle loop to incur some
integer division overhead for no clear benefit.

In order to allow cpuidle to measure time in nanoseconds, add two
new fields, exit_latency_ns and target_residency_ns, to represent the
exit latency and target residency of an idle state in nanoseconds,
respectively, to struct cpuidle_state and initialize them with the
help of the corresponding values in microseconds provided by drivers.
Additionally, change cpuidle_governor_latency_req() to return the
idle state exit latency constraint in nanoseconds.

Also meeasure idle state residency (last_residency_ns in struct
cpuidle_device and time_ns in struct cpuidle_driver) in nanoseconds
and update the cpuidle core and governors accordingly.

However, the menu governor still computes typical intervals in
microseconds to avoid integer overflows.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Doug Smythies <dsmythies@telus.net>
Tested-by: Doug Smythies <dsmythies@telus.net>

1 files changed, 13 insertions, 12 deletions

diff --git a/drivers/cpuidle/governors/ladder.c b/drivers/cpuidle/governors/ladder.c
index b0126b8c32fe..8e9058c4ea63 100644
--- a/drivers/cpuidle/governors/ladder.c
+++ b/drivers/cpuidle/governors/ladder.c
@@ -27,8 +27,8 @@ struct ladder_device_state {
 	struct {
 		u32 promotion_count;
 		u32 demotion_count;
-		u32 promotion_time;
-		u32 demotion_time;
+		u64 promotion_time_ns;
+		u64 demotion_time_ns;
 	} threshold;
 	struct {
 		int promotion_count;
@@ -68,9 +68,10 @@ static int ladder_select_state(struct cpuidle_driver *drv,
 {
 	struct ladder_device *ldev = this_cpu_ptr(&ladder_devices);
 	struct ladder_device_state *last_state;
-	int last_residency, last_idx = dev->last_state_idx;
+	int last_idx = dev->last_state_idx;
 	int first_idx = drv->states[0].flags & CPUIDLE_FLAG_POLLING ? 1 : 0;
-	int latency_req = cpuidle_governor_latency_req(dev->cpu);
+	s64 latency_req = cpuidle_governor_latency_req(dev->cpu);
+	s64 last_residency;
 
 	/* Special case when user has set very strict latency requirement */
 	if (unlikely(latency_req == 0)) {
@@ -80,13 +81,13 @@ static int ladder_select_state(struct cpuidle_driver *drv,
 
 	last_state = &ldev->states[last_idx];
 
-	last_residency = dev->last_residency - drv->states[last_idx].exit_latency;
+	last_residency = dev->last_residency_ns - drv->states[last_idx].exit_latency_ns;
 
 	/* consider promotion */
 	if (last_idx < drv->state_count - 1 &&
 	    !dev->states_usage[last_idx + 1].disable &&
-	    last_residency > last_state->threshold.promotion_time &&
-	    drv->states[last_idx + 1].exit_latency <= latency_req) {
+	    last_residency > last_state->threshold.promotion_time_ns &&
+	    drv->states[last_idx + 1].exit_latency_ns <= latency_req) {
 		last_state->stats.promotion_count++;
 		last_state->stats.demotion_count = 0;
 		if (last_state->stats.promotion_count >= last_state->threshold.promotion_count) {
@@ -98,11 +99,11 @@ static int ladder_select_state(struct cpuidle_driver *drv,
 	/* consider demotion */
 	if (last_idx > first_idx &&
 	    (dev->states_usage[last_idx].disable ||
-	    drv->states[last_idx].exit_latency > latency_req)) {
+	    drv->states[last_idx].exit_latency_ns > latency_req)) {
 		int i;
 
 		for (i = last_idx - 1; i > first_idx; i--) {
-			if (drv->states[i].exit_latency <= latency_req)
+			if (drv->states[i].exit_latency_ns <= latency_req)
 				break;
 		}
 		ladder_do_selection(dev, ldev, last_idx, i);
@@ -110,7 +111,7 @@ static int ladder_select_state(struct cpuidle_driver *drv,
 	}
 
 	if (last_idx > first_idx &&
-	    last_residency < last_state->threshold.demotion_time) {
+	    last_residency < last_state->threshold.demotion_time_ns) {
 		last_state->stats.demotion_count++;
 		last_state->stats.promotion_count = 0;
 		if (last_state->stats.demotion_count >= last_state->threshold.demotion_count) {
@@ -150,9 +151,9 @@ static int ladder_enable_device(struct cpuidle_driver *drv,
 		lstate->threshold.demotion_count = DEMOTION_COUNT;
 
 		if (i < drv->state_count - 1)
-			lstate->threshold.promotion_time = state->exit_latency;
+			lstate->threshold.promotion_time_ns = state->exit_latency_ns;
 		if (i > first_idx)
-			lstate->threshold.demotion_time = state->exit_latency;
+			lstate->threshold.demotion_time_ns = state->exit_latency_ns;
 	}
 
 	return 0;