cpuidle: fix the menu governor to boost IO performance

 * 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.

 * 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/ktime.h>

#include <linux/hrtimer.h>

#include <linux/tick.h>

#include <linux/sched.h>

#define BREAK_FUZZ	4	/* 4 us */

#define PRED_HISTORY_PCT	50

#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;

	unsigned int	expected_us;

	unsigned int	predicted_us;

	unsigned int    current_predicted_us;

	unsigned int	last_measured_us;

	unsigned int	elapsed_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);

/**

	struct menu_device *data = &__get_cpu_var(menu_devices);

	int latency_req = pm_qos_requirement(PM_QOS_CPU_DMA_LATENCY);

	int i;

	int multiplier;

	/* Special case when user has set very strict latency requirement */

	if (unlikely(latency_req == 0)) {

	data->last_state_idx = 0;

	data->exit_us = 0;

	/* Special case when user has set very strict latency requirement */

	if (unlikely(latency_req == 0))

		return 0;

	}

	/* determine the expected residency time */

	/* determine the expected residency time, round up */

	data->expected_us =

		(u32) ktime_to_ns(tick_nohz_get_sleep_length()) / 1000;

	    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_ROUND_CLOSEST(

		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;

	/* Recalculate predicted_us based on prediction_history_pct */

	data->predicted_us *= PRED_HISTORY_PCT;

	data->predicted_us += (100 - PRED_HISTORY_PCT) *

				data->current_predicted_us;

	data->predicted_us /= 100;

	/* find the deepest idle state that satisfies our constraints */

	for (i = CPUIDLE_DRIVER_STATE_START + 1; i < dev->state_count; i++) {

	for (i = CPUIDLE_DRIVER_STATE_START; i < dev->state_count; i++) {

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

		if (s->target_residency > data->expected_us)

			break;

		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;

	}

	data->last_state_idx = i - 1;

	return i - 1;

	return 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 one full standard timer tick.  However, be aware that this

	 * could potentially result in a suboptimal state transition.

	 * for the whole expected time.

	 */

	if (unlikely(!(target->flags & CPUIDLE_FLAG_TIME_VALID)))

		last_idle_us = USEC_PER_SEC / HZ;

		last_idle_us = data->expected_us;

	measured_us = last_idle_us;

	/*

	 * measured_us and elapsed_us are the cumulative idle time, since the

	 * last time we were woken out of idle by an interrupt.

	 * We correct for the exit latency; we are assuming here that the

	 * exit latency happens after the event that we're interested in.

	 */

	if (data->elapsed_us <= data->elapsed_us + last_idle_us)

		measured_us = data->elapsed_us + last_idle_us;

	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

		measured_us = -1;

		/*

		 * we were idle so long that we count it as a perfect

		 * prediction

		 */

		new_factor += RESOLUTION;

	/* Predict time until next break event */

	data->current_predicted_us = max(measured_us, data->last_measured_us);

	/*

	 * 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;

	if (last_idle_us + BREAK_FUZZ <

	    data->expected_us - target->exit_latency) {

		data->last_measured_us = measured_us;

		data->elapsed_us = 0;

	} else {

		data->elapsed_us = measured_us;

	}

	data->correction_factor[data->bucket] = new_factor;

}

/**

extern unsigned long nr_running(void);

extern unsigned long nr_uninterruptible(void);

extern unsigned long nr_iowait(void);

extern unsigned long nr_iowait_cpu(void);

extern unsigned long this_cpu_load(void);

extern void calc_global_load(void);

extern u64 cpu_nr_migrations(int cpu);

	return sum;

}

unsigned long nr_iowait_cpu(void)

{

	struct rq *this = this_rq();

	return atomic_read(&this->nr_iowait);

}

unsigned long this_cpu_load(void)

{

	struct rq *this = this_rq();

	return this->cpu_load[0];

}

/* Variables and functions for calc_load */

static atomic_long_t calc_load_tasks;

static unsigned long calc_load_update;