Contributors: 3
Author Tokens Token Proportion Commits Commit Proportion
Rafael J. Wysocki 1637 91.20% 26 89.66%
Kajetan Puchalski 150 8.36% 2 6.90%
Marcelo Tosatti 8 0.45% 1 3.45%
Total 1795 29 


// SPDX-License-Identifier: GPL-2.0
/*
 * 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.
 *
 * 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.
 *
 * 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.
 *
 * 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
 * idle states provided by the %CPUIdle driver in the ascending order.  That is,
 * the first bin spans from 0 up to, but not including, the target residency of
 * the second idle state (idle state 1), the second bin spans from the target
 * residency of idle state 1 up to, but not including, the target residency of
 * idle state 2, the third bin spans from the target residency of idle state 2
 * up to, but not including, the target residency of idle state 3 and so on.
 * The last bin spans from the target residency of the deepest idle state
 * supplied by the driver to infinity.
 *
 * Two metrics called "hits" and "intercepts" are associated with each bin.
 * They are updated every time before selecting an idle state for the given CPU
 * in accordance with what happened last time.
 *
 * The "hits" metric reflects the relative frequency of situations in which the
 * 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).
 *
 * 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.
 *
 * 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).
 *
 *    - 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 numbers of recent intercepts for all of the idle states
 *      shallower than the candidate one.
 *
 * 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.
 *
 *    - Traverse the 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:
 *
 * 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.
 *
 * In this case, 'utilized' means that the average runqueue util of the CPU is
 * above a certain threshold.
 *
 * 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.
 */

#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.
 */
#define UTIL_THRESHOLD_SHIFT 6

/*
 * The PULSE value is added to metrics when they grow and the DECAY_SHIFT value
 * is used for decreasing metrics on a regular basis.
 */
#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
 */
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;
};

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.
 */
static void teo_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
{
	struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);
	int i, idx_timer = 0, idx_duration = 0;
	s64 target_residency_ns;
	u64 measured_ns;

	if (cpu_data->time_span_ns >= cpu_data->sleep_length_ns) {
		/*
		 * 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.
		 */
		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
		 * executed by the CPU is not likely to be worst-case every
		 * time, so take 1/2 of the exit latency as a very rough
		 * approximation of the average of it.
		 */
		if (measured_ns >= lat_ns)
			measured_ns -= lat_ns / 2;
		else
			measured_ns /= 2;
	}

	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
	 * fall into.
	 */
	for (i = 0; i < drv->state_count; i++) {
		struct teo_bin *bin = &cpu_data->state_bins[i];

		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) {
			idx_timer = i;
			if (target_residency_ns <= measured_ns)
				idx_duration = i;
		}
	}

	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;
			}
		}
	}

	/*
	 * 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.
	 * Otherwise, update the "intercepts" metric for the bin fallen into by
	 * the measured idle duration.
	 */
	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;
	}

end:
	cpu_data->total += PULSE;
}

static bool teo_state_ok(int i, struct cpuidle_driver *drv)
{
	return !tick_nohz_tick_stopped() ||
		drv->states[i].target_residency_ns >= TICK_NSEC;
}

/**
 * teo_find_shallower_state - Find shallower idle state matching given duration.
 * @drv: cpuidle driver containing state data.
 * @dev: Target CPU.
 * @state_idx: Index of the capping idle state.
 * @duration_ns: Idle duration value to match.
 * @no_poll: Don't consider polling states.
 */
static int teo_find_shallower_state(struct cpuidle_driver *drv,
				    struct cpuidle_device *dev, int state_idx,
				    s64 duration_ns, bool no_poll)
{
	int i;

	for (i = state_idx - 1; i >= 0; i--) {
		if (dev->states_usage[i].disable ||
				(no_poll && drv->states[i].flags & CPUIDLE_FLAG_POLLING))
			continue;

		state_idx = i;
		if (drv->states[i].target_residency_ns <= duration_ns)
			break;
	}
	return state_idx;
}

/**
 * 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);
	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;

	if (dev->last_state_idx >= 0) {
		teo_update(drv, 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.
	 */
	cpu_data->sleep_length_ns = KTIME_MAX;

	/* Check if there is any choice in the first place. */
	if (drv->state_count < 2) {
		idx = 0;
		goto out_tick;
	}

	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];
		struct cpuidle_state *s = &drv->states[i];

		/*
		 * Update the sums of idle state mertics for all of the states
		 * shallower than the current one.
		 */
		intercept_sum += prev_bin->intercepts;
		hit_sum += prev_bin->hits;
		recent_sum += prev_bin->recent;

		if (dev->states_usage[i].disable)
			continue;

		if (idx < 0)
			idx0 = i; /* first enabled state */

		idx = i;

		if (s->exit_latency_ns <= latency_req)
			constraint_idx = i;

		/* 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. */
	if (idx < 0) {
		idx = 0; /* No states enabled, must use 0. */
		goto out_tick;
	}

	if (idx == idx0) {
		/*
		 * Only one idle state is enabled, so use it, but do not
		 * allow the tick to be stopped it is shallow enough.
		 */
		duration_ns = drv->states[idx].target_residency_ns;
		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.
	 */
	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) {
		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.
		 *
		 * 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)) {
				/*
				 * 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)
					idx = i;
				else
					idx = first_suitable_idx;

				break;
			}

			if (dev->states_usage[i].disable)
				continue;

			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.
				 */
				if (first_suitable_idx != idx)
					continue;

				break;
			}

			first_suitable_idx = i;
		}
	}

	/*
	 * If there is a latency constraint, it may be necessary to select an
	 * idle state shallower than the current candidate one.
	 */
	if (idx > constraint_idx)
		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 (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)
		goto out_tick;

	duration_ns = tick_nohz_get_sleep_length(&delta_tick);
	cpu_data->sleep_length_ns = duration_ns;

	/*
	 * If the closest expected timer is before the terget residency of the
	 * candidate state, a shallower one needs to be found.
	 */
	if (drv->states[idx].target_residency_ns > duration_ns) {
		i = teo_find_shallower_state(drv, dev, idx, duration_ns, false);
		if (teo_state_ok(i, drv))
			idx = i;
	}

	/*
	 * If the selected state's target residency is below the tick length
	 * and intercepts occurring before the tick length are the majority of
	 * 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)
		duration_ns = TICK_NSEC / 2;

end:
	/*
	 * Allow the tick to be stopped unless the selected state is a polling
	 * one or the expected idle duration is shorter than the tick period
	 * length.
	 */
	if ((!(drv->states[idx].flags & CPUIDLE_FLAG_POLLING) &&
	    duration_ns >= TICK_NSEC) || tick_nohz_tick_stopped())
		return idx;

	/*
	 * The tick is not going to be stopped, so if the target residency of
	 * the state to be returned is not within the time till the closest
	 * timer including the tick, try to correct that.
	 */
	if (idx > idx0 &&
	    drv->states[idx].target_residency_ns > delta_tick)
		idx = teo_find_shallower_state(drv, dev, idx, delta_tick, false);

out_tick:
	*stop_tick = false;
	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);

	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)) {
		dev->poll_time_limit = false;
		cpu_data->time_span_ns = cpu_data->sleep_length_ns;
	} else {
		cpu_data->time_span_ns = local_clock() - cpu_data->time_span_ns;
	}
}

/**
 * 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);
	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;
}

static struct cpuidle_governor teo_governor = {
	.name =		"teo",
	.rating =	19,
	.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);