// SPDX-License-Identifier: GPL-2.0
/*
* Timer events oriented CPU idle governor
*
* Copyright (C) 2018 Intel Corporation
* Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
*
* 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 taking a few most recent idle time intervals of the
* CPU into account. However, even in that case it is not necessary to consider
* idle duration values greater than the time till the closest timer, 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:
*
* - Find an idle 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 equal
* to the sleep length.
*
* o Select it if it matched both the sleep length and the observed idle
* duration in the past more often than it matched the sleep length alone
* (i.e. the observed idle duration was significantly shorter than the sleep
* length matched by it).
*
* o Otherwise, select the shallower state with the greatest matched "early"
* wakeups metric.
*
* - If the majority of the most recent idle duration values are below the
* target residency of the idle state selected so far, use those values to
* compute the new expected idle duration and find an idle state matching it
* (which has to be shallower than the one selected so far).
*/
#include <linux/cpuidle.h>
#include <linux/jiffies.h>
#include <linux/kernel.h>
#include <linux/sched/clock.h>
#include <linux/tick.h>
/*
* 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 wakeup patterns.
*/
#define INTERVALS 8
/**
* struct teo_idle_state - Idle state data used by the TEO cpuidle governor.
* @early_hits: "Early" CPU wakeups "matching" this state.
* @hits: "On time" CPU wakeups "matching" this state.
* @misses: CPU wakeups "missing" 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
* residency 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 perspective, 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.
*/
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];
};
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->time_span_ns >= cpu_data->sleep_length_ns) {
/*
* One of the safety nets has triggered or this was a timer
* wakeup (or equivalent).
*/
measured_us = sleep_length_us;
} else {
unsigned int lat = drv->states[cpu_data->last_state].exit_latency;
measured_us = ktime_to_us(cpu_data->time_span_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_us >= lat)
measured_us -= lat / 2;
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 <= sleep_length_us) {
idx_timer = i;
if (drv->states[i].target_residency <= measured_us)
idx_hit = 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 += PULSE;
if (idx_hit >= 0)
cpu_data->states[idx_hit].early_hits += PULSE;
} else {
hits += PULSE;
}
cpu_data->states[idx_timer].misses = misses;
cpu_data->states[idx_timer].hits = hits;
}
/*
* 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;
/*
* Save idle duration values corresponding to non-timer wakeups for
* pattern detection.
*/
cpu_data->intervals[cpu_data->interval_idx++] = measured_us;
if (cpu_data->interval_idx > INTERVALS)
cpu_data->interval_idx = 0;
}
/**
* 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_us: Idle duration value to match.
*/
static int teo_find_shallower_state(struct cpuidle_driver *drv,
struct cpuidle_device *dev, int state_idx,
unsigned int duration_us)
{
int i;
for (i = state_idx - 1; i >= 0; i--) {
if (drv->states[i].disabled || dev->states_usage[i].disable)
continue;
state_idx = i;
if (drv->states[i].target_residency <= duration_us)
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);
int latency_req = cpuidle_governor_latency_req(dev->cpu);
unsigned int duration_us, 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);
duration_us = ktime_to_us(cpu_data->sleep_length_ns);
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 &&
count < cpu_data->states[i].early_hits)
count = cpu_data->states[i].early_hits;
continue;
}
if (idx < 0)
idx = i; /* first enabled state */
if (s->target_residency > duration_us)
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;
goto refine;
}
idx = i;
if (count < cpu_data->states[i].early_hits &&
!(tick_nohz_tick_stopped() &&
drv->states[i].target_residency < TICK_USEC)) {
count = cpu_data->states[i].early_hits;
max_early_idx = i;
}
}
/*
* If the "hits" metric of the idle state matching the sleep length is
* greater than its "misses" metric, that is the one to use. Otherwise,
* it is more likely that one of the shallower states will match the
* idle duration observed 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 (cpu_data->states[idx].hits <= cpu_data->states[idx].misses &&
max_early_idx >= 0) {
idx = max_early_idx;
duration_us = drv->states[idx].target_residency;
}
refine:
if (idx < 0) {
idx = 0; /* No states enabled. Must use 0. */
} else if (idx > 0) {
u64 sum = 0;
count = 0;
/*
* Count and sum the most recent idle duration values less than
* the target residency of the state selected so far, find the
* max.
*/
for (i = 0; i < INTERVALS; i++) {
unsigned int val = cpu_data->intervals[i];
if (val >= drv->states[idx].target_residency)
continue;
count++;
sum += val;
}
/*
* Give up unless the majority of the most recent idle duration
* values are in the interesting range.
*/
if (count > INTERVALS / 2) {
unsigned int avg_us = div64_u64(sum, count);
/*
* Avoid spending too much time in an idle state that
* would be too shallow.
*/
if (!(tick_nohz_tick_stopped() && avg_us < TICK_USEC)) {
idx = teo_find_shallower_state(drv, dev, idx, avg_us);
duration_us = avg_us;
}
}
}
/*
* 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;
/*
* 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 > 0 && drv->states[idx].target_residency > delta_tick_us)
idx = teo_find_shallower_state(drv, dev, idx, delta_tick_us);
}
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;
/*
* 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);
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 = 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);
