[RFC/RFT][PATCH v2] cpuidle: New timer events oriented governor for tickless systems

[Rafael J. Wysocki]

From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>

The venerable menu governor does some thigns that are quite
questionable in my view.  First, it includes timer wakeups in
the pattern detection data and mixes them up with wakeups from
other sources which in some cases causes it to expect what
essentially would be a timer wakeup in a time frame in which
no timer wakeups are possible (becuase it knows the time until
the next timer event and that is later than the expected wakeup
time).  Second, it uses the extra exit latency limit based on
the predicted idle duration and depending on the number of tasks
waiting on I/O, even though those tasks may run on a different
CPU when they are woken up.  Moreover, the time ranges used by it
for the sleep length correction factors depend on whether or not
there are tasks waiting on I/O, which again doesn't imply anything
in particular, and they are not correlated to the list of available
idle states in any way whatever.  Also,  the pattern detection code
in menu may end up considering values that are too large to matter
at all, in which cases running it is a waste of time.

A major rework of the menu governor would be required to address
these issues and the performance of at least some workloads (tuned
specifically to the current behavior of the menu governor) is likely
to suffer from that.  It is thus better to introduce an entirely new
governor without them and let everybody use the governor that works
better with their actual workloads.

The new governor introduced here, the timer events oriented (TEO)
governor, uses the same basic strategy as menu: it always tries to
find the deepest idle state that can be used in the given conditions.
However, it applies a different approach to that problem.  First, it
doesn't use "correction factors" for the time till the closest timer,
but instead it tries to correlate the measured idle duration values
with the available idle states and use that information to pick up
the idle state that is most likely to "match" the upcoming CPU idle
interval.  Second, it doesn't take the number of "I/O waiters" into
account at all and the pattern detection code in it tries to avoid
taking timer wakeups into account.  It also only uses idle duration
values less than the current time till the closest timer (with the
tick excluded) for that purpose.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
---

The v2 is a re-write of major parts of the original patch.

The approach the same in general, but the details have changed significantly
with respect to the previous version.  In particular:
* The decay of the idle state metrics is implemented differently.
* There is a more "clever" pattern detection (sort of along the lines
  of what the menu does, but simplified quite a bit and trying to avoid
  including timer wakeups).
* The "promotion" from the "polling" state is gone.
* The "safety net" wakeups are treated as the CPU might have been idle
  until the closest timer.

I'm running this governor on all of my systems now without any
visible adverse effects.

Overall, it selects deeper idle states more often than menu on average, but
that doesn't seem to make a significant difference in the majority of cases.

In this preliminary revision it overtakes menu as the default governor
for tickless systems (due to the higher rating), but that is likely
to change going forward.  At this point I'm mostly asking for feedback
and possibly testing with whatever workloads you can throw at it.

The patch should apply on top of 4.19, although I'm running it on
top of my linux-next branch.  This version hasn't been run through
benchmarks yet and that likely will take some time as I will be
traveling quite a bit during the next few weeks.

---
 drivers/cpuidle/Kconfig            |   11 
 drivers/cpuidle/governors/Makefile |    1 
 drivers/cpuidle/governors/teo.c    |  491 +++++++++++++++++++++++++++++++++++++
 3 files changed, 503 insertions(+)

Index: linux-pm/drivers/cpuidle/governors/teo.c
===================================================================
--- /dev/null
+++ linux-pm/drivers/cpuidle/governors/teo.c
@@ -0,0 +1,491 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Timer events oriented CPU idle governor
+ *
+ * Copyright (C) 2018 Intel Corporation
+ * Author: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
+ *
+ * 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 source 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 time to the closest timer is
+ * likely to be suitable for the upcoming idle time of the CPU and, if not, then
+ * which of the shallower idle states to choose.
+ *
+ * Of course, non-timer wakeup sources are more important in some use cases and
+ * they can be covered by detecting patterns among recent idle time intervals
+ * of the CPU.  However, even in that case it is not necessary to take idle
+ * duration values greater than the time till the closest timer into account, as
+ * the patterns that they may belong to produce average values close enough to
+ * the time till the closest timer (sleep length) anyway.
+ *
+ * Thus this governor estimates whether or not the upcoming idle time of the CPU
+ * is likely to be significantly shorter than the sleep length and selects an
+ * idle state for it in accordance with that, as follows:
+ *
+ * - If there is a pattern of 5 or more recent non-timer wakeups earlier than
+ *   the closest timer event, expect one more of them to occur and use the
+ *   average of the idle duration values corresponding to them to select an
+ *   idle state for the CPU.
+ *
+ * - Otherwise, find the state on the basis of the sleep length and state
+ *   statistics collected over time:
+ *
+ *   o Find the deepest idle state whose target residency is less than or euqal
+ *     to the sleep length.
+ *
+ *   o Select it if it matched both the sleep length and the idle duration
+ *     measured after wakeup in the past more often than it matched the sleep
+ *     length, but not the idle duration (i.e. the measured idle duration was
+ *     significantly shorter than the sleep length matched by that state).
+ *
+ *   o Otherwise, select the shallower state with the greatest matched "early"
+ *     wakeups metric.
+ */
+
+#include <linux/cpuidle.h>
+#include <linux/jiffies.h>
+#include <linux/kernel.h>
+#include <linux/sched/clock.h>
+#include <linux/tick.h>
+
+/*
+ * The SPIKE value is added to metrics when they grow and the DECAY_SHIFT value
+ * is used for decreasing metrics on a regular basis.
+ */
+#define SPIKE		1024
+#define DECAY_SHIFT	3
+
+/*
+ * Number of the most recent idle duration values to take into consideration for
+ * the detection of wakeup patterns.
+ */
+#define INTERVALS	8
+/*
+ * Minimum number of recent idle duration values needed to compute a "typical"
+ * one.
+ */
+#define COUNT_LIMIT	5
+
+/**
+ * struct teo_idle_state - Idle state data used by the TEO cpuidle governor.
+ * @early_hits: "Early" CPU wakeups matched by this state.
+ * @hits: "On time" CPU wakeups matched by this state.
+ * @misses: CPU wakeups "missed" by this state.
+ *
+ * A CPU wakeup is "matched" by a given idle state if the idle duration measured
+ * after the wakeup is between the target residency of that state and the target
+ * residnecy of the next one (or if this is the deepest available idle state, it
+ * "matches" a CPU wakeup when the measured idle duration is at least equal to
+ * its target residency).
+ *
+ * Also, from the TEO governor prespective, a CPU wakeup from idle is "early" if
+ * it occurs significantly earlier than the closest expected timer event (that
+ * is, early enough to match an idle state shallower than the one matching the
+ * time till the closest timer event).  Otherwise, the wakeup is "on time", or
+ * it is a "hit".
+ *
+ * A "miss" occurs when the given state doesn't match the wakeup, but it matches
+ * the time till the closest timer event used for idle state selection.
+ */
+struct teo_idle_state {
+	unsigned int early_hits;
+	unsigned int hits;
+	unsigned int misses;
+};
+
+/**
+ * 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).
+ * @states: Idle states data corresponding to this CPU.
+ * @last_state: Idle state entered by the CPU last time.
+ * @interval_idx: Index of the most recent saved idle interval.
+ * @intervals: Saved idle duration values.
+ * @intervals_sq: Saved idle duration values squared.
+ */
+struct teo_cpu {
+	u64 time_span_ns;
+	u64 sleep_length_ns;
+	struct teo_idle_state states[CPUIDLE_STATE_MAX];
+	int last_state;
+	int interval_idx;
+	unsigned int intervals[INTERVALS];
+	unsigned int max_duration:1;
+};
+
+static DEFINE_PER_CPU(struct teo_cpu, teo_cpus);
+
+/**
+ * teo_update - Update CPU data after wakeup.
+ * @drv: cpuidle driver containing state data.
+ * @dev: Target CPU.
+ */
+static void teo_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
+{
+	struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);
+	unsigned int sleep_length_us = ktime_to_us(cpu_data->sleep_length_ns);
+	int i, idx_hit = -1, idx_timer = -1;
+	unsigned int measured_us;
+
+	if (cpu_data->max_duration) {
+		measured_us = sleep_length_us;
+	} else {
+		measured_us = dev->last_residency;
+		i = cpu_data->last_state;
+		if (measured_us >= 2 * drv->states[i].exit_latency)
+			measured_us -= drv->states[i].exit_latency;
+		else
+			measured_us /= 2;
+	}
+
+	/*
+	 * Decay the "early hits" metric for all of the states and find the
+	 * states matching the sleep length and the measured idle duration.
+	 */
+	for (i = 0; i < drv->state_count; i++) {
+		unsigned int early_hits = cpu_data->states[i].early_hits;
+
+		cpu_data->states[i].early_hits -= early_hits >> DECAY_SHIFT;
+
+		if (drv->states[i].target_residency <= measured_us)
+			idx_hit = i;
+
+		if (drv->states[i].target_residency <= sleep_length_us)
+			idx_timer = i;
+	}
+
+	/*
+	 * Update the "hits" and "misses" data for the state matching the sleep
+	 * length.  If it matches the measured idle duration too, this is a hit,
+	 * so increase the "hits" metric for it then.  Otherwise, this is a
+	 * miss, so increase the "misses" metric for it.  In the latter case
+	 * also increase the "early hits" metric for the state that actually
+	 * matches the measured idle duration.
+	 */
+	if (idx_timer >= 0) {
+		unsigned int hits = cpu_data->states[idx_timer].hits;
+		unsigned int misses = cpu_data->states[idx_timer].misses;
+
+		hits -= hits >> DECAY_SHIFT;
+		misses -= misses >> DECAY_SHIFT;
+
+		if (idx_timer > idx_hit) {
+			misses += SPIKE;
+			if (idx_hit >= 0)
+				cpu_data->states[idx_hit].early_hits += SPIKE;
+		} else {
+			hits += SPIKE;
+		}
+
+		cpu_data->states[idx_timer].misses = misses;
+		cpu_data->states[idx_timer].hits = hits;
+	}
+
+	/*
+	 * Save idle duration values corresponding to non-timer wakeups for
+	 * pattern detection.
+	 *
+	 * If the total time span between idle state selection and the "reflect"
+	 * callback is greater than or equal to the sleep length determined at
+	 * the idle state selection time, the wakeup is likely to be due to a
+	 * timer event.
+	 */
+	if (cpu_data->time_span_ns >= cpu_data->sleep_length_ns)
+		measured_us = UINT_MAX;
+
+	cpu_data->intervals[cpu_data->interval_idx++] = measured_us;
+	if (cpu_data->interval_idx > INTERVALS)
+		cpu_data->interval_idx = 0;
+}
+
+/**
+ * teo_idle_duration - Estimate the duration of the upcoming CPU idle time.
+ * @drv: cpuidle driver containing state data.
+ * @cpu_data: Governor data for the target CPU.
+ * @sleep_length_us: Time till the closest timer event in microseconds.
+ */
+unsigned int teo_idle_duration(struct cpuidle_driver *drv,
+			       struct teo_cpu *cpu_data,
+			       unsigned int sleep_length_us)
+{
+	u64 sum, sq_sum, max, limit;
+	unsigned int count;
+
+	/*
+	 * If the sleep length is below the target residency of idle state 1,
+	 * the only viable choice is to select the first available (enabled)
+	 * idle state, so return immediately in that case.
+	 */
+	if (sleep_length_us < drv->states[1].target_residency)
+		return sleep_length_us;
+
+	/*
+	 * The purpose of this function is to check if there is a pattern of
+	 * wakeups indicating that it would be better to select a state
+	 * shallower than the deepest one matching the sleep length or the
+	 * deepest one at all if the sleep lenght is long.  Larger idle duration
+	 * values are beyond the interesting range.
+	 *
+	 * Narrowing the range of interesting values down upfront also helps to
+	 * avoid overflows during the computation below.
+	 */
+	max = drv->states[drv->state_count-1].target_residency;
+	max = min_t(u64, sleep_length_us, max + (max >> 2));
+
+	/*
+	 * The limit here is the value to compare with the variance of the saved
+	 * recent idle duration values in order to decide whether or not it is
+	 * small.  Take 1/8 of the interesting range and the with a 10 us cap.
+	 */
+	limit = max_t(u64, max >> 3, 10);
+	limit *= limit;
+
+	do {
+		u64 cap = max;
+		int i;
+
+		/*
+		 * Compute the sum of the saved intervals below the cap and the
+		 * sum of of their squares.  Count them and find the maximum
+		 * interval below the cap.
+		 */
+		count = 0;
+		sum = 0;
+		sq_sum = 0;
+		max = 0;
+
+		for (i = 0; i < INTERVALS; i++) {
+			u64 val = cpu_data->intervals[i];
+
+			if (val >= cap)
+				continue;
+
+			count++;
+			sum += val;
+			sq_sum += val * val;
+			if (max < val)
+				max = val;
+		}
+
+		/*
+		 * If the number of intervals is too small to get a meaningful
+		 * result from them, return the original sleep length.
+		 */
+		if (count < COUNT_LIMIT)
+			return sleep_length_us;
+
+		/*
+		 * A pattern appears to be there if the variance is small
+		 * relative to the limit determined earlier.
+		 */
+	} while (count * sq_sum - sum * sum > count * count * limit);
+
+	return div64_u64(sum, count);
+}
+
+/**
+ * teo_select - Selects the next idle state to enter.
+ * @drv: cpuidle driver containing state data.
+ * @dev: Target CPU.
+ * @stop_tick: Indication on whether or not to stop the scheduler tick.
+ */
+static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
+		      bool *stop_tick)
+{
+	struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);
+	int latency_req = cpuidle_governor_latency_req(dev->cpu);
+	unsigned int sleep_length_us, duration_us;
+	unsigned int max_early_count;
+	int max_early_idx, idx, i;
+	ktime_t delta_tick;
+
+	if (cpu_data->last_state >= 0) {
+		teo_update(drv, dev);
+		cpu_data->last_state = -1;
+	}
+
+	cpu_data->time_span_ns = local_clock();
+
+	cpu_data->sleep_length_ns = tick_nohz_get_sleep_length(&delta_tick);
+	sleep_length_us = ktime_to_us(cpu_data->sleep_length_ns);
+
+	duration_us = teo_idle_duration(drv, cpu_data, sleep_length_us);
+
+	/*
+	 * If the time needed to enter and exit the idle state matching the
+	 * expected idle duration is comparable with the expected idle duration
+	 * itself, the time to spend in that state is likely to be small, so it
+	 * probably is better to select a shallower state then.  Tweak the
+	 * latency limit to enforce that.
+	 */
+	if (duration_us < latency_req)
+		latency_req = duration_us;
+
+	max_early_count = 0;
+	max_early_idx = -1;
+	idx = -1;
+
+	for (i = 0; i < drv->state_count; i++) {
+		struct cpuidle_state *s = &drv->states[i];
+		struct cpuidle_state_usage *su = &dev->states_usage[i];
+
+		if (s->disabled || su->disable) {
+			/*
+			 * If the "early hits" metric of a disabled state is
+			 * greater than the current maximum, it should be taken
+			 * into account, because it would be a mistake to select
+			 * a deeper state with lower "early hits" metric.  The
+			 * index cannot be changed to point to it, however, so
+			 * just increase the max count alone and let the index
+			 * still point to a shallower idle state.
+			 */
+			if (max_early_idx >= 0 &&
+			    max_early_count < cpu_data->states[i].early_hits)
+				max_early_count = cpu_data->states[i].early_hits;
+
+			continue;
+		}
+
+		if (idx < 0)
+			idx = i; /* first enabled state */
+
+		if (s->target_residency > duration_us) {
+			/*
+			 * If the next wakeup is expected to be "early", the
+			 * time frame of it is known already.
+			 */
+			if (duration_us < sleep_length_us)
+				break;
+
+			/*
+			 * If the "hits" metric of the state matching the sleep
+			 * length is greater than its "misses" metric, that is
+			 * the one to use.
+			 */
+			if (cpu_data->states[idx].hits >= cpu_data->states[idx].misses)
+				break;
+
+			/*
+			 * It is more likely that one of the shallower states
+			 * will match the idle duration measured after wakeup,
+			 * so take the one with the maximum "early hits" metric,
+			 * but if that cannot be determined, just use the state
+			 * selected so far.
+			 */
+			if (max_early_idx >= 0) {
+				idx = max_early_idx;
+				duration_us = drv->states[idx].target_residency;
+			}
+			break;
+		}
+		if (s->exit_latency > latency_req) {
+			/*
+			 * If we break out of the loop for latency reasons, use
+			 * the target residency of the selected state as the
+			 * expected idle duration to avoid stopping the tick
+			 * as long as that target residency is low enough.
+			 */
+			duration_us = drv->states[idx].target_residency;
+			break;
+		}
+
+		idx = i;
+
+		if (max_early_count < cpu_data->states[i].early_hits) {
+			max_early_count = cpu_data->states[i].early_hits;
+			max_early_idx = i;
+		}
+	}
+
+	if (idx < 0)
+		idx = 0; /* No states enabled. Must use 0. */
+
+	/*
+	 * Don't stop the tick if the selected state is a polling one or if the
+	 * expected idle duration is shorter than the tick period length.
+	 */
+	if (((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) ||
+	    duration_us < TICK_USEC) && !tick_nohz_tick_stopped()) {
+		unsigned int delta_tick_us = ktime_to_us(delta_tick);
+
+		*stop_tick = false;
+
+		if (idx > 0 && drv->states[idx].target_residency > delta_tick_us) {
+			/*
+			 * The tick is not going to be stopped and the target
+			 * residency of the state to be returned is not within
+			 * the time until the closest timer event including the
+			 * tick, so try to correct that.
+			 */
+			for (i = idx - 1; i > 0; i--) {
+				if (drv->states[i].disabled ||
+				    dev->states_usage[i].disable)
+					continue;
+
+				if (drv->states[i].target_residency <= delta_tick_us)
+					break;
+			}
+			idx = i;
+		}
+	}
+
+	return idx;
+}
+
+/**
+ * teo_reflect - Note that governor data for the CPU need to be updated.
+ * @dev: Target CPU.
+ * @state: Entered state.
+ */
+static void teo_reflect(struct cpuidle_device *dev, int state)
+{
+	struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);
+
+	cpu_data->last_state = state;
+	cpu_data->time_span_ns = local_clock() - cpu_data->time_span_ns;
+	/*
+	 * 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.
+	 */
+	cpu_data->max_duration = (tick_nohz_idle_got_tick() &&
+				  cpu_data->sleep_length_ns > TICK_NSEC) ||
+				 dev->poll_time_limit;
+}
+
+/**
+ * teo_enable_device - Initialize the governor's data for the target CPU.
+ * @drv: cpuidle driver (not used).
+ * @dev: Target CPU.
+ */
+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);
+	int i;
+
+	memset(cpu_data, 0, sizeof(*cpu_data));
+
+	for (i = 0; i < INTERVALS; i++)
+		cpu_data->intervals[i] = UINT_MAX;
+
+	return 0;
+}
+
+static struct cpuidle_governor teo_governor = {
+	.name =		"teo",
+	.rating =	22,
+	.enable =	teo_enable_device,
+	.select =	teo_select,
+	.reflect =	teo_reflect,
+};
+
+static int __init teo_governor_init(void)
+{
+	return cpuidle_register_governor(&teo_governor);
+}
+
+postcore_initcall(teo_governor_init);
Index: linux-pm/drivers/cpuidle/Kconfig
===================================================================
--- linux-pm.orig/drivers/cpuidle/Kconfig
+++ linux-pm/drivers/cpuidle/Kconfig
@@ -23,6 +23,17 @@ config CPU_IDLE_GOV_LADDER
 config CPU_IDLE_GOV_MENU
 	bool "Menu governor (for tickless system)"
 
+config CPU_IDLE_GOV_TEO
+	bool "Timer events oriented governor (for tickless systems)"
+	help
+	  Menu governor derivative that uses a simplified idle state
+	  selection method focused on timer events and does not do any
+	  interactivity boosting.
+
+	  Some workloads benefit from using this governor and it generally
+	  should be safe to use.  Say Y here if you are not happy with the
+	  alternatives.
+
 config DT_IDLE_STATES
 	bool
 
Index: linux-pm/drivers/cpuidle/governors/Makefile
===================================================================
--- linux-pm.orig/drivers/cpuidle/governors/Makefile
+++ linux-pm/drivers/cpuidle/governors/Makefile
@@ -4,3 +4,4 @@
 
 obj-$(CONFIG_CPU_IDLE_GOV_LADDER) += ladder.o
 obj-$(CONFIG_CPU_IDLE_GOV_MENU) += menu.o
+obj-$(CONFIG_CPU_IDLE_GOV_TEO) += teo.o