/*
* 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_params.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>
#define BUCKETS 12
#define RESOLUTION 1024
#define DECAY 4
#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.
*
* Limiting Performance Impact
* ---------------------------
* C states, especially those with large exit latencies, can have a real
* noticable 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 expected_us;
u64 predicted_us;
unsigned int measured_us;
unsigned int exit_us;
unsigned int bucket;
u64 correction_factor[BUCKETS];
};
#define LOAD_INT(x) ((x) >> FSHIFT)
#define LOAD_FRAC(x) LOAD_INT(((x) & (FIXED_1-1)) * 100)
static int get_loadavg(void)
{
unsigned long this = this_cpu_load();
return LOAD_INT(this) * 10 + LOAD_FRAC(this) / 10;
}
static inline int which_bucket(unsigned int duration)
{
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_iowait_cpu())
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(void)
{
int mult = 1;
/* for higher loadavg, we are more reluctant */
mult += 2 * get_loadavg();
/* for IO wait tasks (per cpu!) we add 5x each */
mult += 10 * nr_iowait_cpu();
return mult;
}
static DEFINE_PER_CPU(struct menu_device, menu_devices);
static void menu_update(struct cpuidle_device *dev);
/* This implements DIV_ROUND_CLOSEST but avoids 64 bit division */
static u64 div_round64(u64 dividend, u32 divisor)
{
return div_u64(dividend + (divisor / 2), divisor);
}
/**
* menu_select - selects the next idle state to enter
* @dev: the CPU
*/
static int menu_select(struct cpuidle_device *dev)
{
struct menu_device *data = &__get_cpu_var(menu_devices);
int latency_req = pm_qos_requirement(PM_QOS_CPU_DMA_LATENCY);
int i;
int multiplier;
data->last_state_idx = 0;
data->exit_us = 0;
if (data->needs_update) {
menu_update(dev);
data->needs_update = 0;
}
/* 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->expected_us =
DIV_ROUND_UP((u32)ktime_to_ns(tick_nohz_get_sleep_length()), 1000);
data->bucket = which_bucket(data->expected_us);
multiplier = performance_multiplier();
/*
* if the correction factor is 0 (eg first time init or cpu hotplug
* etc), we actually want to start out with a unity factor.
*/
if (data->correction_factor[data->bucket] == 0)
data->correction_factor[data->bucket] = RESOLUTION * DECAY;
/* Make sure to round up for half microseconds */
data->predicted_us = div_round64(data->expected_us * data->correction_factor[data->bucket],
RESOLUTION * DECAY);
/*
* We want to default to C1 (hlt), not to busy polling
* unless the timer is happening really really soon.
*/
if (data->expected_us > 5)
data->last_state_idx = CPUIDLE_DRIVER_STATE_START;
/* find the deepest idle state that satisfies our constraints */
for (i = CPUIDLE_DRIVER_STATE_START; i < dev->state_count; i++) {
struct cpuidle_state *s = &dev->states[i];
if (s->target_residency > data->predicted_us)
break;
if (s->exit_latency > latency_req)
break;
if (s->exit_latency * multiplier > data->predicted_us)
break;
data->exit_us = s->exit_latency;
data->last_state_idx = i;
}
return data->last_state_idx;
}
/**
* menu_reflect - records that data structures need update
* @dev: the CPU
*
* 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)
{
struct menu_device *data = &__get_cpu_var(menu_devices);
data->needs_update = 1;
}
/**
* menu_update - attempts to guess what happened after entry
* @dev: the CPU
*/
static void menu_update(struct cpuidle_device *dev)
{
struct menu_device *data = &__get_cpu_var(menu_devices);
int last_idx = data->last_state_idx;
unsigned int last_idle_us = cpuidle_get_last_residency(dev);
struct cpuidle_state *target = &dev->states[last_idx];
unsigned int measured_us;
u64 new_factor;
/*
* Ugh, this idle state doesn't support residency measurements, so we
* are basically lost in the dark. As a compromise, assume we slept
* for the whole expected time.
*/
if (unlikely(!(target->flags & CPUIDLE_FLAG_TIME_VALID)))
last_idle_us = data->expected_us;
measured_us = last_idle_us;
/*
* We correct for the exit latency; we are assuming here that the
* exit latency happens after the event that we're interested in.
*/
if (measured_us > data->exit_us)
measured_us -= data->exit_us;
/* update our correction ratio */
new_factor = data->correction_factor[data->bucket]
* (DECAY - 1) / DECAY;
if (data->expected_us > 0 && data->measured_us < MAX_INTERESTING)
new_factor += RESOLUTION * measured_us / data->expected_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.
*/
if (new_factor == 0)
new_factor = 1;
data->correction_factor[data->bucket] = new_factor;
}
/**
* menu_enable_device - scans a CPU's states and does setup
* @dev: the CPU
*/
static int menu_enable_device(struct cpuidle_device *dev)
{
struct menu_device *data = &per_cpu(menu_devices, dev->cpu);
memset(data, 0, sizeof(struct menu_device));
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);
}
/**
* exit_menu - exits the governor
*/
static void __exit exit_menu(void)
{
cpuidle_unregister_governor(&menu_governor);
}
MODULE_LICENSE("GPL");
module_init(init_menu);
module_exit(exit_menu);
