/*

 * cpuidle_menu - menu governor for cpu idle, main idea come from Linux.

 *            drivers/cpuidle/governors/menu.c 

 *

 *  Copyright (C) 2006-2007 Adam Belay <abelay@novell.com>

 *  Copyright (C) 2007, 2008 Intel Corporation

 *

 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 *

 *  This program is free software; you can redistribute it and/or modify

 *  it under the terms of the GNU General Public License as published by

 *  the Free Software Foundation; either version 2 of the License, or (at

 *  your option) any later version.

 *

 *  This program is distributed in the hope that it will be useful, but

 *  WITHOUT ANY WARRANTY; without even the implied warranty of

 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

 *  General Public License for more details.

 *

 *  You should have received a copy of the GNU General Public License along

 *  with this program; If not, see <http://www.gnu.org/licenses/>.

 *

 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 */

#include <xen/errno.h>

#include <xen/lib.h>

#include <xen/types.h>

#include <xen/acpi.h>

#include <xen/timer.h>

#include <xen/cpuidle.h>

#include <asm/irq.h>

#define BUCKETS 6

#define RESOLUTION 1024

#define DECAY 4

#define MAX_INTERESTING 50000

#define LATENCY_MULTIPLIER 10

/*

 * 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 (TBD: from guest virtual C state)

 * 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.

 * For this reason we keep an array of 6 independent factors, that gets

 * indexed based on the magnitude of the expected duration

 *

 * 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 average interrupt interval:

 * If the exit latency times multiplier is longer than the average

 * interrupt interval, the C state is not considered a candidate

 * for selection due to a too high performance impact. So the smaller

 * the average interrupt interval is, the smaller C state latency should be

 * and thus the less likely a busy CPU will hit such a deep C state.

 *

 * As an additional rule to reduce the performance impact, menu tries to

 * limit the exit latency duration to be no more than 10% of the decaying

 * measured idle time.

 */

struct perf_factor{

    s_time_t    time_stamp;

    s_time_t    duration;

    unsigned int irq_count_stamp;

    unsigned int irq_sum;

};

struct menu_device

{

    int             last_state_idx;

    unsigned int    expected_us;

    u64             predicted_us;

    u64             latency_factor;

    unsigned int    measured_us;

    unsigned int    exit_us;

    unsigned int    bucket;

    u64             correction_factor[BUCKETS];

    struct perf_factor pf;

};

static DEFINE_PER_CPU(struct menu_device, menu_devices);

static inline int which_bucket(unsigned int duration)

{

   int bucket = 0;

   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 the average interrupt interval to take I/O performance

 * requirements into account. The smaller the average interrupt

 * interval to be, the more busy I/O activity, and thus the higher

 * the barrier to go to an expensive C state.

 */

/* 5 milisec sampling period */

#define SAMPLING_PERIOD     5000000

/* for I/O interrupt, we give 8x multiplier compared to C state latency*/

#define IO_MULTIPLIER       8

static inline s_time_t avg_intr_interval_us(void)

{

    struct menu_device *data = &__get_cpu_var(menu_devices);

    s_time_t    duration, now;

    s_time_t    avg_interval;

    unsigned int irq_sum;

    now = NOW();

    duration = (data->pf.duration + (now - data->pf.time_stamp)

            * (DECAY - 1)) / DECAY;

    irq_sum = (data->pf.irq_sum + (this_cpu(irq_count) - data->pf.irq_count_stamp)

            * (DECAY - 1)) / DECAY;

    if (irq_sum == 0)

        /* no irq recently, so return a big enough interval: 1 sec */

        avg_interval = 1000000;

    else

        avg_interval = duration / irq_sum / 1000; /* in us */

    if ( duration >= SAMPLING_PERIOD){

        data->pf.time_stamp = now;

        data->pf.duration = duration;

        data->pf.irq_count_stamp= this_cpu(irq_count);

        data->pf.irq_sum = irq_sum;

    }

    return avg_interval;

}

static unsigned int get_sleep_length_us(void)

{

    s_time_t us = (this_cpu(timer_deadline) - NOW()) / 1000;

    /*

     * while us < 0 or us > (u32)-1, return a large u32,

     * choose (unsigned int)-2000 to avoid wrapping while added with exit

     * latency because the latency should not larger than 2ms

     */

    return (us >> 32) ? (unsigned int)-2000 : (unsigned int)us;

}

static int menu_select(struct acpi_processor_power *power)

{

    struct menu_device *data = &__get_cpu_var(menu_devices);

    int i;

    s_time_t    io_interval;

    /*  TBD: Change to 0 if C0(polling mode) support is added later*/

    data->last_state_idx = CPUIDLE_DRIVER_STATE_START;

    data->exit_us = 0;

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

    data->expected_us = get_sleep_length_us();

    data->bucket = which_bucket(data->expected_us);

    io_interval = avg_intr_interval_us();

    data->latency_factor = DIV_ROUND(

            data->latency_factor * (DECAY - 1) + data->measured_us,

            DECAY);

    /*

     * 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(

            data->expected_us * data->correction_factor[data->bucket],

            RESOLUTION * DECAY);

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

    for ( i = CPUIDLE_DRIVER_STATE_START + 1; i < power->count; i++ )

    {

        struct acpi_processor_cx *s = &power->states[i];

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

            break;

        if (s->latency * IO_MULTIPLIER > io_interval)

            break;

        if (s->latency * LATENCY_MULTIPLIER > data->latency_factor)

            break;

        /* TBD: we need to check the QoS requirment in future */

        data->exit_us = s->latency;

        data->last_state_idx = i;

    }

    return data->last_state_idx;

}

static void menu_reflect(struct acpi_processor_power *power)

{

    struct menu_device *data = &__get_cpu_var(menu_devices);

    u64 new_factor;

    data->measured_us = power->last_residency;

    /*

     * 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->measured_us > data->exit_us)

        data->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 * data->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;

}

static int menu_enable_device(struct acpi_processor_power *power)

{

    if (!cpu_online(power->cpu))

        return -1;

    memset(&per_cpu(menu_devices, power->cpu), 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,

};

struct cpuidle_governor *cpuidle_current_governor = &menu_governor;

void menu_get_trace_data(u32 *expected, u32 *pred)

{

    struct menu_device *data = &__get_cpu_var(menu_devices);

    *expected = data->expected_us;

    *pred = data->predicted_us;

}