/*
* menu.c - the menu idle governor
*
* Copyright (C) 2006-2007 Adam Belay <abelay@novell.com>
* Copyright (C) 2009 Intel Corporation
* Author:
* Arjan van de Ven <arjan@linux.intel.com>
*
* This code is licenced under the GPL version 2 as described
* in the COPYING file that acompanies the Linux Kernel.
*/
#include <linux/kernel.h>
#include <linux/cpuidle.h>
#include <linux/pm_qos.h>
#include <linux/time.h>
#include <linux/ktime.h>
#include <linux/hrtimer.h>
#include <linux/tick.h>
#include <linux/sched.h>
#include <linux/math64.h>
#include <linux/module.h>
/*
* Please note when changing the tuning values:
* If (MAX_INTERESTING-1) * RESOLUTION > UINT_MAX, the result of
* a scaling operation multiplication may overflow on 32 bit platforms.
* In that case, #define RESOLUTION as ULL to get 64 bit result:
* #define RESOLUTION 1024ULL
*
* The default values do not overflow.
*/
#define BUCKETS 12
#define INTERVAL_SHIFT 3
#define INTERVALS (1UL << INTERVAL_SHIFT)
#define RESOLUTION 1024
#define DECAY 8
#define MAX_INTERESTING 50000
/*
* Concepts and ideas behind the menu governor
*
* For the menu governor, there are 3 decision factors for picking a C
* state:
* 1) Energy break even point
* 2) Performance impact
* 3) Latency tolerance (from pmqos infrastructure)
* These these three factors are treated independently.
*
* Energy break even point
* -----------------------
* C state entry and exit have an energy cost, and a certain amount of time in
* the C state is required to actually break even on this cost. CPUIDLE
* provides us this duration in the "target_residency" field. So all that we
* need is a good prediction of how long we'll be idle. Like the traditional
* menu governor, we start with the actual known "next timer event" time.
*
* Since there are other source of wakeups (interrupts for example) than
* the next timer event, this estimation is rather optimistic. To get a
* more realistic estimate, a correction factor is applied to the estimate,
* that is based on historic behavior. For example, if in the past the actual
* duration always was 50% of the next timer tick, the correction factor will
* be 0.5.
*
* menu uses a running average for this correction factor, however it uses a
* set of factors, not just a single factor. This stems from the realization
* that the ratio is dependent on the order of magnitude of the expected
* duration; if we expect 500 milliseconds of idle time the likelihood of
* getting an interrupt very early is much higher than if we expect 50 micro
* seconds of idle time. A second independent factor that has big impact on
* the actual factor is if there is (disk) IO outstanding or not.
* (as a special twist, we consider every sleep longer than 50 milliseconds
* as perfect; there are no power gains for sleeping longer than this)
*
* For these two reasons we keep an array of 12 independent factors, that gets
* indexed based on the magnitude of the expected duration as well as the
* "is IO outstanding" property.
*
* Repeatable-interval-detector
* ----------------------------
* There are some cases where "next timer" is a completely unusable predictor:
* Those cases where the interval is fixed, for example due to hardware
* interrupt mitigation, but also due to fixed transfer rate devices such as
* mice.
* For this, we use a different predictor: We track the duration of the last 8
* intervals and if the stand deviation of these 8 intervals is below a
* threshold value, we use the average of these intervals as prediction.
*
* Limiting Performance Impact
* ---------------------------
* C states, especially those with large exit latencies, can have a real
* noticeable impact on workloads, which is not acceptable for most sysadmins,
* and in addition, less performance has a power price of its own.
*
* As a general rule of thumb, menu assumes that the following heuristic
* holds:
* The busier the system, the less impact of C states is acceptable
*
* This rule-of-thumb is implemented using a performance-multiplier:
* If the exit latency times the performance multiplier is longer than
* the predicted duration, the C state is not considered a candidate
* for selection due to a too high performance impact. So the higher
* this multiplier is, the longer we need to be idle to pick a deep C
* state, and thus the less likely a busy CPU will hit such a deep
* C state.
*
* Two factors are used in determing this multiplier:
* a value of 10 is added for each point of "per cpu load average" we have.
* a value of 5 points is added for each process that is waiting for
* IO on this CPU.
* (these values are experimentally determined)
*
* The load average factor gives a longer term (few seconds) input to the
* decision, while the iowait value gives a cpu local instantanious input.
* The iowait factor may look low, but realize that this is also already
* represented in the system load average.
*
*/
struct menu_device {
int last_state_idx;
int needs_update;
unsigned int next_timer_us;
unsigned int predicted_us;
unsigned int bucket;
unsigned int correction_factor[BUCKETS];
unsigned int intervals[INTERVALS];
int interval_ptr;
};
#define LOAD_INT(x) ((x) >> FSHIFT)
#define LOAD_FRAC(x) LOAD_INT(((x) & (FIXED_1-1)) * 100)
static inline int get_loadavg(unsigned long load)
{
return LOAD_INT(load) * 10 + LOAD_FRAC(load) / 10;
}
static inline int which_bucket(unsigned int duration, unsigned long nr_iowaiters)
{
int bucket = 0;
/*
* We keep two groups of stats; one with no
* IO pending, one without.
* This allows us to calculate
* E(duration)|iowait
*/
if (nr_iowaiters)
bucket = BUCKETS/2;
if (duration < 10)
return bucket;
if (duration < 100)
return bucket + 1;
if (duration < 1000)
return bucket + 2;
if (duration < 10000)
return bucket + 3;
if (duration < 100000)
return bucket + 4;
return bucket + 5;
}
/*
* Return a multiplier for the exit latency that is intended
* to take performance requirements into account.
* The more performance critical we estimate the system
* to be, the higher this multiplier, and thus the higher
* the barrier to go to an expensive C state.
*/
static inline int performance_multiplier(unsigned long nr_iowaiters, unsigned long load)
{
int mult = 1;
/* for higher loadavg, we are more reluctant */
/*
* this doesn't work as intended - it is almost always 0, but can
* sometimes, depending on workload, spike very high into the hundreds
* even when the average cpu load is under 10%.
*/
/* mult += 2 * get_loadavg(); */
/* for IO wait tasks (per cpu!) we add 5x each */
mult += 10 * nr_iowaiters;
return mult;
}
static DEFINE_PER_CPU(struct menu_device, menu_devices);
static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev);
/*
* Try detecting repeating patterns by keeping track of the last 8
* intervals, and checking if the standard deviation of that set
* of points is below a threshold. If it is... then use the
* average of these 8 points as the estimated value.
*/
static void get_typical_interval(struct menu_device *data)
{
int i, divisor;
unsigned int max, thresh;
uint64_t avg, stddev;
thresh = UINT_MAX; /* Discard outliers above this value */
again:
/* First calculate the average of past intervals */
max = 0;
avg = 0;
divisor = 0;
for (i = 0; i < INTERVALS; i++) {
unsigned int value = data->intervals[i];
if (value <= thresh) {
avg += value;
divisor++;
if (value > max)
max = value;
}
}
if (divisor == INTERVALS)
avg >>= INTERVAL_SHIFT;
else
do_div(avg, divisor);
/* Then try to determine standard deviation */
stddev = 0;
for (i = 0; i < INTERVALS; i++) {
unsigned int value = data->intervals[i];
if (value <= thresh) {
int64_t diff = value - avg;
stddev += diff * diff;
}
}
if (divisor == INTERVALS)
stddev >>= INTERVAL_SHIFT;
else
do_div(stddev, divisor);
/*
* The typical interval is obtained when standard deviation is small
* or standard deviation is small compared to the average interval.
*
* int_sqrt() formal parameter type is unsigned long. When the
* greatest difference to an outlier exceeds ~65 ms * sqrt(divisor)
* the resulting squared standard deviation exceeds the input domain
* of int_sqrt on platforms where unsigned long is 32 bits in size.
* In such case reject the candidate average.
*
* Use this result only if there is no timer to wake us up sooner.
*/
if (likely(stddev <= ULONG_MAX)) {
stddev = int_sqrt(stddev);
if (((avg > stddev * 6) && (divisor * 4 >= INTERVALS * 3))
|| stddev <= 20) {
if (data->next_timer_us > avg)
data->predicted_us = avg;
return;
}
}
/*
* If we have outliers to the upside in our distribution, discard
* those by setting the threshold to exclude these outliers, then
* calculate the average and standard deviation again. Once we get
* down to the bottom 3/4 of our samples, stop excluding samples.
*
* This can deal with workloads that have long pauses interspersed
* with sporadic activity with a bunch of short pauses.
*/
if ((divisor * 4) <= INTERVALS * 3)
return;
thresh = max - 1;
goto again;
}
/**
* menu_select - selects the next idle state to enter
* @drv: cpuidle driver containing state data
* @dev: the CPU
*/
static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev)
{
struct menu_device *data = this_cpu_ptr(&menu_devices);
int latency_req = pm_qos_request(PM_QOS_CPU_DMA_LATENCY);
int i;
unsigned int interactivity_req;
unsigned long nr_iowaiters, cpu_load;
if (data->needs_update) {
menu_update(drv, dev);
data->needs_update = 0;
}
data->last_state_idx = CPUIDLE_DRIVER_STATE_START - 1;
/* Special case when user has set very strict latency requirement */
if (unlikely(latency_req == 0))
return 0;
/* determine the expected residency time, round up */
data->next_timer_us = ktime_to_us(tick_nohz_get_sleep_length());
get_iowait_load(&nr_iowaiters, &cpu_load);
data->bucket = which_bucket(data->next_timer_us, nr_iowaiters);
/*
* Force the result of multiplication to be 64 bits even if both
* operands are 32 bits.
* Make sure to round up for half microseconds.
*/
data->predicted_us = DIV_ROUND_CLOSEST_ULL((uint64_t)data->next_timer_us *
data->correction_factor[data->bucket],
RESOLUTION * DECAY);
get_typical_interval(data);
/*
* Performance multiplier defines a minimum predicted idle
* duration / latency ratio. Adjust the latency limit if
* necessary.
*/
interactivity_req = data->predicted_us / performance_multiplier(nr_iowaiters, cpu_load);
if (latency_req > interactivity_req)
latency_req = interactivity_req;
/*
* We want to default to C1 (hlt), not to busy polling
* unless the timer is happening really really soon.
*/
if (data->next_timer_us > 5 &&
!drv->states[CPUIDLE_DRIVER_STATE_START].disabled &&
dev->states_usage[CPUIDLE_DRIVER_STATE_START].disable == 0)
data->last_state_idx = CPUIDLE_DRIVER_STATE_START;
/*
* Find the idle state with the lowest power while satisfying
* our constraints.
*/
for (i = CPUIDLE_DRIVER_STATE_START; 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)
continue;
if (s->target_residency > data->predicted_us)
continue;
if (s->exit_latency > latency_req)
continue;
data->last_state_idx = i;
}
return data->last_state_idx;
}
/**
* menu_reflect - records that data structures need update
* @dev: the CPU
* @index: the index of actual entered state
*
* NOTE: it's important to be fast here because this operation will add to
* the overall exit latency.
*/
static void menu_reflect(struct cpuidle_device *dev, int index)
{
struct menu_device *data = this_cpu_ptr(&menu_devices);
data->last_state_idx = index;
if (index >= 0)
data->needs_update = 1;
}
/**
* menu_update - attempts to guess what happened after entry
* @drv: cpuidle driver containing state data
* @dev: the CPU
*/
static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
{
struct menu_device *data = this_cpu_ptr(&menu_devices);
int last_idx = data->last_state_idx;
struct cpuidle_state *target = &drv->states[last_idx];
unsigned int measured_us;
unsigned int new_factor;
/*
* Try to figure out how much time passed between entry to low
* power state and occurrence of the wakeup event.
*
* If the entered idle state didn't support residency measurements,
* we use them anyway if they are short, and if long,
* truncate to the whole expected time.
*
* Any measured amount of time will include the exit latency.
* Since we are interested in when the wakeup begun, not when it
* was completed, we must subtract the exit latency. However, if
* the measured amount of time is less than the exit latency,
* assume the state was never reached and the exit latency is 0.
*/
/* measured value */
measured_us = cpuidle_get_last_residency(dev);
/* Deduct exit latency */
if (measured_us > target->exit_latency)
measured_us -= target->exit_latency;
/* Make sure our coefficients do not exceed unity */
if (measured_us > data->next_timer_us)
measured_us = data->next_timer_us;
/* Update our correction ratio */
new_factor = data->correction_factor[data->bucket];
new_factor -= new_factor / DECAY;
if (data->next_timer_us > 0 && measured_us < MAX_INTERESTING)
new_factor += RESOLUTION * measured_us / data->next_timer_us;
else
/*
* we were idle so long that we count it as a perfect
* prediction
*/
new_factor += RESOLUTION;
/*
* We don't want 0 as factor; we always want at least
* a tiny bit of estimated time. Fortunately, due to rounding,
* new_factor will stay nonzero regardless of measured_us values
* and the compiler can eliminate this test as long as DECAY > 1.
*/
if (DECAY == 1 && unlikely(new_factor == 0))
new_factor = 1;
data->correction_factor[data->bucket] = new_factor;
/* update the repeating-pattern data */
data->intervals[data->interval_ptr++] = measured_us;
if (data->interval_ptr >= INTERVALS)
data->interval_ptr = 0;
}
/**
* menu_enable_device - scans a CPU's states and does setup
* @drv: cpuidle driver
* @dev: the CPU
*/
static int menu_enable_device(struct cpuidle_driver *drv,
struct cpuidle_device *dev)
{
struct menu_device *data = &per_cpu(menu_devices, dev->cpu);
int i;
memset(data, 0, sizeof(struct menu_device));
/*
* if the correction factor is 0 (eg first time init or cpu hotplug
* etc), we actually want to start out with a unity factor.
*/
for(i = 0; i < BUCKETS; i++)
data->correction_factor[i] = RESOLUTION * DECAY;
return 0;
}
static struct cpuidle_governor menu_governor = {
.name = "menu",
.rating = 20,
.enable = menu_enable_device,
.select = menu_select,
.reflect = menu_reflect,
.owner = THIS_MODULE,
};
/**
* init_menu - initializes the governor
*/
static int __init init_menu(void)
{
return cpuidle_register_governor(&menu_governor);
}
postcore_initcall(init_menu);