1 files changed, 138 insertions, 282 deletions

diff --git a/drivers/cpuidle/governors/teo.c b/drivers/cpuidle/governors/teo.c
index 7244f71c59c5..8fe5e1b47ef9 100644
--- a/drivers/cpuidle/governors/teo.c
+++ b/drivers/cpuidle/governors/teo.c
@@ -2,38 +2,35 @@
 /*
  * Timer events oriented CPU idle governor
  *
- * TEO governor:
  * Copyright (C) 2018 - 2021 Intel Corporation
  * Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
- *
- * Util-awareness mechanism:
- * Copyright (C) 2022 Arm Ltd.
- * Author: Kajetan Puchalski <kajetan.puchalski@arm.com>
  */
 
 /**
  * DOC: teo-description
  *
  * The idea of this governor is based on the observation that on many systems
- * timer events are two or more orders of magnitude more frequent than any
- * other interrupts, so they are likely to be the most significant cause of CPU
- * wakeups from idle states.  Moreover, information about what happened in the
- * (relatively recent) past can be used to estimate whether or not the deepest
- * idle state with target residency within the (known) time till the closest
- * timer event, referred to as the sleep length, is likely to be suitable for
- * the upcoming CPU idle period and, if not, then which of the shallower idle
- * states to choose instead of it.
+ * timer interrupts are two or more orders of magnitude more frequent than any
+ * other interrupt types, so they are likely to dominate CPU wakeup patterns.
+ * Moreover, in principle, the time when the next timer event is going to occur
+ * can be determined at the idle state selection time, although doing that may
+ * be costly, so it can be regarded as the most reliable source of information
+ * for idle state selection.
  *
- * Of course, non-timer wakeup sources are more important in some use cases
- * which can be covered by taking a few most recent idle time intervals of the
- * CPU into account.  However, even in that context it is not necessary to
- * consider idle duration values greater than the sleep length, because the
- * closest timer will ultimately wake up the CPU anyway unless it is woken up
- * earlier.
+ * Of course, non-timer wakeup sources are more important in some use cases,
+ * but even then it is generally unnecessary to consider idle duration values
+ * greater than the time time till the next timer event, referred as the sleep
+ * length in what follows, because the closest timer will ultimately wake up the
+ * CPU anyway unless it is woken up earlier.
  *
- * Thus this governor estimates whether or not the prospective idle duration of
- * a CPU is likely to be significantly shorter than the sleep length and selects
- * an idle state for it accordingly.
+ * However, since obtaining the sleep length may be costly, the governor first
+ * checks if it can select a shallow idle state using wakeup pattern information
+ * from recent times, in which case it can do without knowing the sleep length
+ * at all.  For this purpose, it counts CPU wakeup events and looks for an idle
+ * state whose target residency has not exceeded the idle duration (measured
+ * after wakeup) in the majority of relevant recent cases.  If the target
+ * residency of that state is small enough, it may be used right away and the
+ * sleep length need not be determined.
  *
  * The computations carried out by this governor are based on using bins whose
  * boundaries are aligned with the target residency parameter values of the CPU
@@ -54,105 +51,65 @@
  * sleep length and the idle duration measured after CPU wakeup fall into the
  * same bin (that is, the CPU appears to wake up "on time" relative to the sleep
  * length).  In turn, the "intercepts" metric reflects the relative frequency of
- * situations in which the measured idle duration is so much shorter than the
- * sleep length that the bin it falls into corresponds to an idle state
- * shallower than the one whose bin is fallen into by the sleep length (these
- * situations are referred to as "intercepts" below).
+ * non-timer wakeup events for which the measured idle duration falls into a bin
+ * that corresponds to an idle state shallower than the one whose bin is fallen
+ * into by the sleep length (these events are also referred to as "intercepts"
+ * below).
  *
- * In addition to the metrics described above, the governor counts recent
- * intercepts (that is, intercepts that have occurred during the last
- * %NR_RECENT invocations of it for the given CPU) for each bin.
+ * The governor also counts "intercepts" with the measured idle duration below
+ * the tick period length and uses this information when deciding whether or not
+ * to stop the scheduler tick.
  *
  * In order to select an idle state for a CPU, the governor takes the following
  * steps (modulo the possible latency constraint that must be taken into account
  * too):
  *
- * 1. Find the deepest CPU idle state whose target residency does not exceed
- *    the current sleep length (the candidate idle state) and compute 3 sums as
- *    follows:
- *
- *    - The sum of the "hits" and "intercepts" metrics for the candidate state
- *      and all of the deeper idle states (it represents the cases in which the
- *      CPU was idle long enough to avoid being intercepted if the sleep length
- *      had been equal to the current one).
+ * 1. Find the deepest enabled CPU idle state (the candidate idle state) and
+ *    compute 2 sums as follows:
  *
- *    - The sum of the "intercepts" metrics for all of the idle states shallower
- *      than the candidate one (it represents the cases in which the CPU was not
- *      idle long enough to avoid being intercepted if the sleep length had been
- *      equal to the current one).
+ *    - The sum of the "hits" metric for all of the idle states shallower than
+ *      the candidate one (it represents the cases in which the CPU was likely
+ *      woken up by a timer).
  *
- *    - The sum of the numbers of recent intercepts for all of the idle states
- *      shallower than the candidate one.
+ *    - The sum of the "intercepts" metric for all of the idle states shallower
+ *      than the candidate one (it represents the cases in which the CPU was
+ *      likely woken up by a non-timer wakeup source).
  *
- * 2. If the second sum is greater than the first one or the third sum is
- *    greater than %NR_RECENT / 2, the CPU is likely to wake up early, so look
- *    for an alternative idle state to select.
+ * 2. If the second sum computed in step 1 is greater than a half of the sum of
+ *    both metrics for the candidate state bin and all subsequent bins(if any),
+ *    a shallower idle state is likely to be more suitable, so look for it.
  *
- *    - Traverse the idle states shallower than the candidate one in the
+ *    - Traverse the enabled idle states shallower than the candidate one in the
  *      descending order.
  *
- *    - For each of them compute the sum of the "intercepts" metrics and the sum
- *      of the numbers of recent intercepts over all of the idle states between
- *      it and the candidate one (including the former and excluding the
- *      latter).
- *
- *    - If each of these sums that needs to be taken into account (because the
- *      check related to it has indicated that the CPU is likely to wake up
- *      early) is greater than a half of the corresponding sum computed in step
- *      1 (which means that the target residency of the state in question had
- *      not exceeded the idle duration in over a half of the relevant cases),
- *      select the given idle state instead of the candidate one.
- *
- * 3. By default, select the candidate state.
- *
- * Util-awareness mechanism:
+ *    - For each of them compute the sum of the "intercepts" metrics over all
+ *      of the idle states between it and the candidate one (including the
+ *      former and excluding the latter).
  *
- * The idea behind the util-awareness extension is that there are two distinct
- * scenarios for the CPU which should result in two different approaches to idle
- * state selection - utilized and not utilized.
+ *    - If this sum is greater than a half of the second sum computed in step 1,
+ *      use the given idle state as the new candidate one.
  *
- * In this case, 'utilized' means that the average runqueue util of the CPU is
- * above a certain threshold.
+ * 3. If the current candidate state is state 0 or its target residency is short
+ *    enough, return it and prevent the scheduler tick from being stopped.
  *
- * When the CPU is utilized while going into idle, more likely than not it will
- * be woken up to do more work soon and so a shallower idle state should be
- * selected to minimise latency and maximise performance. When the CPU is not
- * being utilized, the usual metrics-based approach to selecting the deepest
- * available idle state should be preferred to take advantage of the power
- * saving.
- *
- * In order to achieve this, the governor uses a utilization threshold.
- * The threshold is computed per-CPU as a percentage of the CPU's capacity
- * by bit shifting the capacity value. Based on testing, the shift of 6 (~1.56%)
- * seems to be getting the best results.
- *
- * Before selecting the next idle state, the governor compares the current CPU
- * util to the precomputed util threshold. If it's below, it defaults to the
- * TEO metrics mechanism. If it's above, the closest shallower idle state will
- * be selected instead, as long as is not a polling state.
+ * 4. Obtain the sleep length value and check if it is below the target
+ *    residency of the current candidate state, in which case a new shallower
+ *    candidate state needs to be found, so look for it.
  */
 
 #include <linux/cpuidle.h>
 #include <linux/jiffies.h>
 #include <linux/kernel.h>
-#include <linux/sched.h>
 #include <linux/sched/clock.h>
-#include <linux/sched/topology.h>
 #include <linux/tick.h>
 
 #include "gov.h"
 
 /*
- * The number of bits to shift the CPU's capacity by in order to determine
- * the utilized threshold.
- *
- * 6 was chosen based on testing as the number that achieved the best balance
- * of power and performance on average.
- *
- * The resulting threshold is high enough to not be triggered by background
- * noise and low enough to react quickly when activity starts to ramp up.
+ * Idle state exit latency threshold used for deciding whether or not to check
+ * the time till the closest expected timer event.
  */
-#define UTIL_THRESHOLD_SHIFT 6
+#define LATENCY_THRESHOLD_NS	(RESIDENCY_THRESHOLD_NS / 2)
 
 /*
  * The PULSE value is added to metrics when they grow and the DECAY_SHIFT value
@@ -161,66 +118,37 @@
 #define PULSE		1024
 #define DECAY_SHIFT	3
 
-/*
- * Number of the most recent idle duration values to take into consideration for
- * the detection of recent early wakeup patterns.
- */
-#define NR_RECENT	9
-
 /**
  * struct teo_bin - Metrics used by the TEO cpuidle governor.
  * @intercepts: The "intercepts" metric.
  * @hits: The "hits" metric.
- * @recent: The number of recent "intercepts".
  */
 struct teo_bin {
 	unsigned int intercepts;
 	unsigned int hits;
-	unsigned int recent;
 };
 
 /**
  * struct teo_cpu - CPU data used by the TEO cpuidle governor.
- * @time_span_ns: Time between idle state selection and post-wakeup update.
  * @sleep_length_ns: Time till the closest timer event (at the selection time).
  * @state_bins: Idle state data bins for this CPU.
  * @total: Grand total of the "intercepts" and "hits" metrics for all bins.
- * @next_recent_idx: Index of the next @recent_idx entry to update.
- * @recent_idx: Indices of bins corresponding to recent "intercepts".
- * @tick_hits: Number of "hits" after TICK_NSEC.
- * @util_threshold: Threshold above which the CPU is considered utilized
+ * @tick_intercepts: "Intercepts" before TICK_NSEC.
+ * @short_idles: Wakeups after short idle periods.
+ * @artificial_wakeup: Set if the wakeup has been triggered by a safety net.
  */
 struct teo_cpu {
-	s64 time_span_ns;
 	s64 sleep_length_ns;
 	struct teo_bin state_bins[CPUIDLE_STATE_MAX];
 	unsigned int total;
-	int next_recent_idx;
-	int recent_idx[NR_RECENT];
-	unsigned int tick_hits;
-	unsigned long util_threshold;
+	unsigned int tick_intercepts;
+	unsigned int short_idles;
+	bool artificial_wakeup;
 };
 
 static DEFINE_PER_CPU(struct teo_cpu, teo_cpus);
 
 /**
- * teo_cpu_is_utilized - Check if the CPU's util is above the threshold
- * @cpu: Target CPU
- * @cpu_data: Governor CPU data for the target CPU
- */
-#ifdef CONFIG_SMP
-static bool teo_cpu_is_utilized(int cpu, struct teo_cpu *cpu_data)
-{
-	return sched_cpu_util(cpu) > cpu_data->util_threshold;
-}
-#else
-static bool teo_cpu_is_utilized(int cpu, struct teo_cpu *cpu_data)
-{
-	return false;
-}
-#endif
-
-/**
  * teo_update - Update CPU metrics after wakeup.
  * @drv: cpuidle driver containing state data.
  * @dev: Target CPU.
@@ -232,23 +160,17 @@ static void teo_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
 	s64 target_residency_ns;
 	u64 measured_ns;
 
-	if (cpu_data->time_span_ns >= cpu_data->sleep_length_ns) {
+	cpu_data->short_idles -= cpu_data->short_idles >> DECAY_SHIFT;
+
+	if (cpu_data->artificial_wakeup) {
 		/*
-		 * One of the safety nets has triggered or the wakeup was close
-		 * enough to the closest timer event expected at the idle state
-		 * selection time to be discarded.
+		 * If one of the safety nets has triggered, assume that this
+		 * might have been a long sleep.
 		 */
 		measured_ns = U64_MAX;
 	} else {
 		u64 lat_ns = drv->states[dev->last_state_idx].exit_latency_ns;
 
-		/*
-		 * The computations below are to determine whether or not the
-		 * (saved) time till the next timer event and the measured idle
-		 * duration fall into the same "bin", so use last_residency_ns
-		 * for that instead of time_span_ns which includes the cpuidle
-		 * overhead.
-		 */
 		measured_ns = dev->last_residency_ns;
 		/*
 		 * The delay between the wakeup and the first instruction
@@ -256,14 +178,16 @@ static void teo_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
 		 * time, so take 1/2 of the exit latency as a very rough
 		 * approximation of the average of it.
 		 */
-		if (measured_ns >= lat_ns)
+		if (measured_ns >= lat_ns) {
 			measured_ns -= lat_ns / 2;
-		else
+			if (measured_ns < RESIDENCY_THRESHOLD_NS)
+				cpu_data->short_idles += PULSE;
+		} else {
 			measured_ns /= 2;
+			cpu_data->short_idles += PULSE;
+		}
 	}
 
-	cpu_data->total = 0;
-
 	/*
 	 * Decay the "hits" and "intercepts" metrics for all of the bins and
 	 * find the bins that the sleep length and the measured idle duration
@@ -275,8 +199,6 @@ static void teo_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
 		bin->hits -= bin->hits >> DECAY_SHIFT;
 		bin->intercepts -= bin->intercepts >> DECAY_SHIFT;
 
-		cpu_data->total += bin->hits + bin->intercepts;
-
 		target_residency_ns = drv->states[i].target_residency_ns;
 
 		if (target_residency_ns <= cpu_data->sleep_length_ns) {
@@ -286,33 +208,7 @@ static void teo_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
 		}
 	}
 
-	i = cpu_data->next_recent_idx++;
-	if (cpu_data->next_recent_idx >= NR_RECENT)
-		cpu_data->next_recent_idx = 0;
-
-	if (cpu_data->recent_idx[i] >= 0)
-		cpu_data->state_bins[cpu_data->recent_idx[i]].recent--;
-
-	/*
-	 * If the deepest state's target residency is below the tick length,
-	 * make a record of it to help teo_select() decide whether or not
-	 * to stop the tick.  This effectively adds an extra hits-only bin
-	 * beyond the last state-related one.
-	 */
-	if (target_residency_ns < TICK_NSEC) {
-		cpu_data->tick_hits -= cpu_data->tick_hits >> DECAY_SHIFT;
-
-		cpu_data->total += cpu_data->tick_hits;
-
-		if (TICK_NSEC <= cpu_data->sleep_length_ns) {
-			idx_timer = drv->state_count;
-			if (TICK_NSEC <= measured_ns) {
-				cpu_data->tick_hits += PULSE;
-				goto end;
-			}
-		}
-	}
-
+	cpu_data->tick_intercepts -= cpu_data->tick_intercepts >> DECAY_SHIFT;
 	/*
 	 * If the measured idle duration falls into the same bin as the sleep
 	 * length, this is a "hit", so update the "hits" metric for that bin.
@@ -321,14 +217,13 @@ static void teo_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
 	 */
 	if (idx_timer == idx_duration) {
 		cpu_data->state_bins[idx_timer].hits += PULSE;
-		cpu_data->recent_idx[i] = -1;
 	} else {
 		cpu_data->state_bins[idx_duration].intercepts += PULSE;
-		cpu_data->state_bins[idx_duration].recent++;
-		cpu_data->recent_idx[i] = idx_duration;
+		if (TICK_NSEC <= measured_ns)
+			cpu_data->tick_intercepts += PULSE;
 	}
 
-end:
+	cpu_data->total -= cpu_data->total >> DECAY_SHIFT;
 	cpu_data->total += PULSE;
 }
 
@@ -376,17 +271,12 @@ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 	struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);
 	s64 latency_req = cpuidle_governor_latency_req(dev->cpu);
 	ktime_t delta_tick = TICK_NSEC / 2;
-	unsigned int tick_intercept_sum = 0;
 	unsigned int idx_intercept_sum = 0;
 	unsigned int intercept_sum = 0;
-	unsigned int idx_recent_sum = 0;
-	unsigned int recent_sum = 0;
 	unsigned int idx_hit_sum = 0;
 	unsigned int hit_sum = 0;
 	int constraint_idx = 0;
 	int idx0 = 0, idx = -1;
-	bool alt_intercepts, alt_recent;
-	bool cpu_utilized;
 	s64 duration_ns;
 	int i;
 
@@ -395,10 +285,14 @@ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 		dev->last_state_idx = -1;
 	}
 
-	cpu_data->time_span_ns = local_clock();
 	/*
-	 * Set the expected sleep length to infinity in case of an early
-	 * return.
+	 * Set the sleep length to infinity in case the invocation of
+	 * tick_nohz_get_sleep_length() below is skipped, in which case it won't
+	 * be known whether or not the subsequent wakeup is caused by a timer.
+	 * It is generally fine to count the wakeup as an intercept then, except
+	 * for the cases when the CPU is mostly woken up by timers and there may
+	 * be opportunities to ask for a deeper idle state when no imminent
+	 * timers are scheduled which may be missed.
 	 */
 	cpu_data->sleep_length_ns = KTIME_MAX;
 
@@ -411,32 +305,6 @@ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 	if (!dev->states_usage[0].disable)
 		idx = 0;
 
-	cpu_utilized = teo_cpu_is_utilized(dev->cpu, cpu_data);
-	/*
-	 * If the CPU is being utilized over the threshold and there are only 2
-	 * states to choose from, the metrics need not be considered, so choose
-	 * the shallowest non-polling state and exit.
-	 */
-	if (drv->state_count < 3 && cpu_utilized) {
-		/*
-		 * If state 0 is enabled and it is not a polling one, select it
-		 * right away unless the scheduler tick has been stopped, in
-		 * which case care needs to be taken to leave the CPU in a deep
-		 * enough state in case it is not woken up any time soon after
-		 * all.  If state 1 is disabled, though, state 0 must be used
-		 * anyway.
-		 */
-		if ((!idx && !(drv->states[0].flags & CPUIDLE_FLAG_POLLING) &&
-		    teo_state_ok(0, drv)) || dev->states_usage[1].disable) {
-			idx = 0;
-			goto out_tick;
-		}
-		/* Assume that state 1 is not a polling one and use it. */
-		idx = 1;
-		duration_ns = drv->states[1].target_residency_ns;
-		goto end;
-	}
-
 	/* Compute the sums of metrics for early wakeup pattern detection. */
 	for (i = 1; i < drv->state_count; i++) {
 		struct teo_bin *prev_bin = &cpu_data->state_bins[i-1];
@@ -448,7 +316,6 @@ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 		 */
 		intercept_sum += prev_bin->intercepts;
 		hit_sum += prev_bin->hits;
-		recent_sum += prev_bin->recent;
 
 		if (dev->states_usage[i].disable)
 			continue;
@@ -464,7 +331,6 @@ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 		/* Save the sums for the current state. */
 		idx_intercept_sum = intercept_sum;
 		idx_hit_sum = hit_sum;
-		idx_recent_sum = recent_sum;
 	}
 
 	/* Avoid unnecessary overhead. */
@@ -482,74 +348,68 @@ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 		goto end;
 	}
 
-	tick_intercept_sum = intercept_sum +
-			cpu_data->state_bins[drv->state_count-1].intercepts;
-
 	/*
 	 * If the sum of the intercepts metric for all of the idle states
 	 * shallower than the current candidate one (idx) is greater than the
 	 * sum of the intercepts and hits metrics for the candidate state and
-	 * all of the deeper states, or the sum of the numbers of recent
-	 * intercepts over all of the states shallower than the candidate one
-	 * is greater than a half of the number of recent events taken into
-	 * account, a shallower idle state is likely to be a better choice.
+	 * all of the deeper states, a shallower idle state is likely to be a
+	 * better choice.
 	 */
-	alt_intercepts = 2 * idx_intercept_sum > cpu_data->total - idx_hit_sum;
-	alt_recent = idx_recent_sum > NR_RECENT / 2;
-	if (alt_recent || alt_intercepts) {
+	if (2 * idx_intercept_sum > cpu_data->total - idx_hit_sum) {
 		int first_suitable_idx = idx;
 
 		/*
 		 * Look for the deepest idle state whose target residency had
 		 * not exceeded the idle duration in over a half of the relevant
-		 * cases (both with respect to intercepts overall and with
-		 * respect to the recent intercepts only) in the past.
+		 * cases in the past.
 		 *
 		 * Take the possible duration limitation present if the tick
 		 * has been stopped already into account.
 		 */
 		intercept_sum = 0;
-		recent_sum = 0;
 
 		for (i = idx - 1; i >= 0; i--) {
 			struct teo_bin *bin = &cpu_data->state_bins[i];
 
 			intercept_sum += bin->intercepts;
-			recent_sum += bin->recent;
 
-			if ((!alt_recent || 2 * recent_sum > idx_recent_sum) &&
-			    (!alt_intercepts ||
-			     2 * intercept_sum > idx_intercept_sum)) {
+			if (2 * intercept_sum > idx_intercept_sum) {
 				/*
 				 * Use the current state unless it is too
 				 * shallow or disabled, in which case take the
 				 * first enabled state that is deep enough.
 				 */
 				if (teo_state_ok(i, drv) &&
-				    !dev->states_usage[i].disable)
+				    !dev->states_usage[i].disable) {
 					idx = i;
-				else
-					idx = first_suitable_idx;
-
+					break;
+				}
+				idx = first_suitable_idx;
 				break;
 			}
 
 			if (dev->states_usage[i].disable)
 				continue;
 
-			if (!teo_state_ok(i, drv)) {
+			if (teo_state_ok(i, drv)) {
 				/*
-				 * The current state is too shallow, but if an
-				 * alternative candidate state has been found,
-				 * it may still turn out to be a better choice.
+				 * The current state is deep enough, but still
+				 * there may be a better one.
 				 */
-				if (first_suitable_idx != idx)
-					continue;
-
-				break;
+				first_suitable_idx = i;
+				continue;
 			}
 
-			first_suitable_idx = i;
+			/*
+			 * The current state is too shallow, so if no suitable
+			 * states other than the initial candidate have been
+			 * found, give up (the remaining states to check are
+			 * shallower still), but otherwise the first suitable
+			 * state other than the initial candidate may turn out
+			 * to be preferable.
+			 */
+			if (first_suitable_idx == idx)
+				break;
 		}
 	}
 
@@ -561,38 +421,41 @@ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 		idx = constraint_idx;
 
 	/*
-	 * If the CPU is being utilized over the threshold, choose a shallower
-	 * non-polling state to improve latency, unless the scheduler tick has
-	 * been stopped already and the shallower state's target residency is
-	 * not sufficiently large.
+	 * If either the candidate state is state 0 or its target residency is
+	 * low enough, there is basically nothing more to do, but if the sleep
+	 * length is not updated, the subsequent wakeup will be counted as an
+	 * "intercept" which may be problematic in the cases when timer wakeups
+	 * are dominant.  Namely, it may effectively prevent deeper idle states
+	 * from being selected at one point even if no imminent timers are
+	 * scheduled.
+	 *
+	 * However, frequent timers in the RESIDENCY_THRESHOLD_NS range on one
+	 * CPU are unlikely (user space has a default 50 us slack value for
+	 * hrtimers and there are relatively few timers with a lower deadline
+	 * value in the kernel), and even if they did happen, the potential
+	 * benefit from using a deep idle state in that case would be
+	 * questionable anyway for latency reasons.  Thus if the measured idle
+	 * duration falls into that range in the majority of cases, assume
+	 * non-timer wakeups to be dominant and skip updating the sleep length
+	 * to reduce latency.
+	 *
+	 * Also, if the latency constraint is sufficiently low, it will force
+	 * shallow idle states regardless of the wakeup type, so the sleep
+	 * length need not be known in that case.
 	 */
-	if (cpu_utilized) {
-		i = teo_find_shallower_state(drv, dev, idx, KTIME_MAX, true);
-		if (teo_state_ok(i, drv))
-			idx = i;
-	}
-
-	/*
-	 * Skip the timers check if state 0 is the current candidate one,
-	 * because an immediate non-timer wakeup is expected in that case.
-	 */
-	if (!idx)
-		goto out_tick;
-
-	/*
-	 * If state 0 is a polling one, check if the target residency of
-	 * the current candidate state is low enough and skip the timers
-	 * check in that case too.
-	 */
-	if ((drv->states[0].flags & CPUIDLE_FLAG_POLLING) &&
-	    drv->states[idx].target_residency_ns < RESIDENCY_THRESHOLD_NS)
+	if ((!idx || drv->states[idx].target_residency_ns < RESIDENCY_THRESHOLD_NS) &&
+	    (2 * cpu_data->short_idles >= cpu_data->total ||
+	     latency_req < LATENCY_THRESHOLD_NS))
 		goto out_tick;
 
 	duration_ns = tick_nohz_get_sleep_length(&delta_tick);
 	cpu_data->sleep_length_ns = duration_ns;
 
+	if (!idx)
+		goto out_tick;
+
 	/*
-	 * If the closest expected timer is before the terget residency of the
+	 * If the closest expected timer is before the target residency of the
 	 * candidate state, a shallower one needs to be found.
 	 */
 	if (drv->states[idx].target_residency_ns > duration_ns) {
@@ -607,7 +470,7 @@ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
 	 * total wakeup events, do not stop the tick.
 	 */
 	if (drv->states[idx].target_residency_ns < TICK_NSEC &&
-	    tick_intercept_sum > cpu_data->total / 2 + cpu_data->total / 8)
+	    cpu_data->tick_intercepts > cpu_data->total / 2 + cpu_data->total / 8)
 		duration_ns = TICK_NSEC / 2;
 
 end:
@@ -644,17 +507,16 @@ static void teo_reflect(struct cpuidle_device *dev, int state)
 	struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);
 
 	dev->last_state_idx = state;
-	/*
-	 * If the wakeup was not "natural", but triggered by one of the safety
-	 * nets, assume that the CPU might have been idle for the entire sleep
-	 * length time.
-	 */
 	if (dev->poll_time_limit ||
 	    (tick_nohz_idle_got_tick() && cpu_data->sleep_length_ns > TICK_NSEC)) {
+		/*
+		 * The wakeup was not "genuine", but triggered by one of the
+		 * safety nets.
+		 */
 		dev->poll_time_limit = false;
-		cpu_data->time_span_ns = cpu_data->sleep_length_ns;
+		cpu_data->artificial_wakeup = true;
 	} else {
-		cpu_data->time_span_ns = local_clock() - cpu_data->time_span_ns;
+		cpu_data->artificial_wakeup = false;
 	}
 }
 
@@ -667,14 +529,8 @@ static int teo_enable_device(struct cpuidle_driver *drv,
 			     struct cpuidle_device *dev)
 {
 	struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);
-	unsigned long max_capacity = arch_scale_cpu_capacity(dev->cpu);
-	int i;
 
 	memset(cpu_data, 0, sizeof(*cpu_data));
-	cpu_data->util_threshold = max_capacity >> UTIL_THRESHOLD_SHIFT;
-
-	for (i = 0; i < NR_RECENT; i++)
-		cpu_data->recent_idx[i] = -1;
 
 	return 0;
 }