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linuxdebug/drivers/cpuidle/governors/teo.c

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// SPDX-License-Identifier: GPL-2.0
/*
* Timer events oriented CPU idle governor
*
* Copyright (C) 2018 - 2021 Intel Corporation
* Author: Rafael J. Wysocki <rafael.j.wysocki@intel.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.
*/
#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 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" mertics for all bins.
* @next_recent_idx: Index of the next @recent_idx entry to update.
* @recent_idx: Indices of bins corresponding to recent "intercepts".
*/
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];
};
static DEFINE_PER_CPU(struct teo_cpu, teo_cpus);
/**
* 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;
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++) {
s64 target_residency_ns = drv->states[i].target_residency_ns;
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;
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 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;
}
cpu_data->total += PULSE;
}
static bool teo_time_ok(u64 interval_ns)
{
return !tick_nohz_tick_stopped() || interval_ns >= TICK_NSEC;
}
static s64 teo_middle_of_bin(int idx, struct cpuidle_driver *drv)
{
return (drv->states[idx].target_residency_ns +
drv->states[idx+1].target_residency_ns) / 2;
}
/**
* 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.
*/
static int teo_find_shallower_state(struct cpuidle_driver *drv,
struct cpuidle_device *dev, int state_idx,
s64 duration_ns)
{
int i;
for (i = state_idx - 1; i >= 0; i--) {
if (dev->states_usage[i].disable)
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);
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;
ktime_t delta_tick;
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();
duration_ns = tick_nohz_get_sleep_length(&delta_tick);
cpu_data->sleep_length_ns = duration_ns;
/* Check if there is any choice in the first place. */
if (drv->state_count < 2) {
idx = 0;
goto end;
}
if (!dev->states_usage[0].disable) {
idx = 0;
if (drv->states[1].target_residency_ns > duration_ns)
goto end;
}
/*
* Find the deepest idle state whose target residency does not exceed
* the current sleep length and the deepest idle state not deeper than
* the former whose exit latency does not exceed the current latency
* constraint. 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) {
idx = i; /* first enabled state */
idx0 = i;
}
if (s->target_residency_ns > duration_ns)
break;
idx = i;
if (s->exit_latency_ns <= latency_req)
constraint_idx = i;
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 end;
} else if (idx == idx0) {
goto end;
}
/*
* 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, the CPU is likely to wake up early, so find an alternative
* idle state to select.
*/
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) {
s64 first_suitable_span_ns = duration_ns;
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 latency constraint and 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];
s64 span_ns;
intercept_sum += bin->intercepts;
recent_sum += bin->recent;
span_ns = teo_middle_of_bin(i, drv);
if ((!alt_recent || 2 * recent_sum > idx_recent_sum) &&
(!alt_intercepts ||
2 * intercept_sum > idx_intercept_sum)) {
if (teo_time_ok(span_ns) &&
!dev->states_usage[i].disable) {
idx = i;
duration_ns = span_ns;
} else {
/*
* The current state is too shallow or
* disabled, so take the first enabled
* deeper state with suitable time span.
*/
idx = first_suitable_idx;
duration_ns = first_suitable_span_ns;
}
break;
}
if (dev->states_usage[i].disable)
continue;
if (!teo_time_ok(span_ns)) {
/*
* 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_span_ns = span_ns;
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;
end:
/*
* 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_ns < TICK_NSEC) && !tick_nohz_tick_stopped()) {
*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 > idx0 &&
drv->states[idx].target_residency_ns > delta_tick)
idx = teo_find_shallower_state(drv, dev, idx, delta_tick);
}
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);
int i;
memset(cpu_data, 0, sizeof(*cpu_data));
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);
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