[PATCH RFC] cpuidle: consolidate calls to time capture

[Daniel Lezcano]

A few years ago, we changed the code in cpuidle to replace ktime_get()
by a local_clock() to get rid of potential seq lock in the path and an
extra latency.

Meanwhile, the code evolved and we are getting the time in some other
places like the power domain governor and in the future break even
deadline proposal.

Unfortunately, as the time must be compared across the CPU, we have no
other option than using the ktime_get() again. Hopefully, we can
factor out all the calls to local_clock() and ktime_get() into a
single one when the CPU is entering idle as the value will be reuse in
different places.

We can assume the time to go through the code path distance is small
enough between ktime_get() call in the cpuidle_enter() function and
the other users inspecting the value.

Signed-off-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
---
 drivers/base/power/domain_governor.c | 4 +++-
 drivers/cpuidle/cpuidle.c            | 6 +++---
 include/linux/cpuidle.h              | 1 +
 3 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/base/power/domain_governor.c b/drivers/base/power/domain_governor.c
index daa8c7689f7e..bee97f7b7b8d 100644
--- a/drivers/base/power/domain_governor.c
+++ b/drivers/base/power/domain_governor.c
@@ -279,8 +279,10 @@ static bool cpu_power_down_ok(struct dev_pm_domain *pd)
 		}
 	}
 
+	dev = per_cpu(cpuidle_devices, smp_processor_id());
+
 	/* The minimum idle duration is from now - until the next wakeup. */
-	idle_duration_ns = ktime_to_ns(ktime_sub(domain_wakeup, ktime_get()));
+	idle_duration_ns = ktime_to_ns(ktime_sub(domain_wakeup, dev->idle_start));
 	if (idle_duration_ns <= 0)
 		return false;
 
diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
index c149d9e20dfd..9db14581759b 100644
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -206,7 +206,7 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
 
 	struct cpuidle_state *target_state = &drv->states[index];
 	bool broadcast = !!(target_state->flags & CPUIDLE_FLAG_TIMER_STOP);
-	ktime_t time_start, time_end;
+	ktime_t time_end;
 
 	/*
 	 * Tell the time framework to switch to a broadcast timer because our
@@ -228,14 +228,14 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
 	sched_idle_set_state(target_state);
 
 	trace_cpu_idle_rcuidle(index, dev->cpu);
-	time_start = ns_to_ktime(local_clock());
+	dev->idle_start = ktime_get();
 
 	stop_critical_timings();
 	entered_state = target_state->enter(dev, drv, index);
 	start_critical_timings();
 
 	sched_clock_idle_wakeup_event();
-	time_end = ns_to_ktime(local_clock());
+	time_end = ktime_get();
 	trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, dev->cpu);
 
 	/* The cpu is no longer idle or about to enter idle. */
diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
index ec2ef63771f0..112494658e01 100644
--- a/include/linux/cpuidle.h
+++ b/include/linux/cpuidle.h
@@ -89,6 +89,7 @@ struct cpuidle_device {
 	unsigned int		poll_time_limit:1;
 	unsigned int		cpu;
 	ktime_t			next_hrtimer;
+	ktime_t			idle_start;
 
 	int			last_state_idx;
 	u64			last_residency_ns;
-- 
2.17.1