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/*

 * Floating proportions

 *

 *  Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>

 *

 * Description:

 *

 * The floating proportion is a time derivative with an exponentially decaying

 * history:

 *

 *   p_{j} = \Sum_{i=0} (dx_{j}/dt_{-i}) / 2^(1+i)

 *

 * Where j is an element from {prop_local}, x_{j} is j's number of events,

 * and i the time period over which the differential is taken. So d/dt_{-i} is

 * the differential over the i-th last period.

 *

 * The decaying history gives smooth transitions. The time differential carries

 * the notion of speed.

 *

 * The denominator is 2^(1+i) because we want the series to be normalised, ie.

 *

 *   \Sum_{i=0} 1/2^(1+i) = 1

 *

 * Further more, if we measure time (t) in the same events as x; so that:

 *

 *   t = \Sum_{j} x_{j}

 *

 * we get that:

 *

 *   \Sum_{j} p_{j} = 1

 *

 * Writing this in an iterative fashion we get (dropping the 'd's):

 *

 *   if (++x_{j}, ++t > period)

 *     t /= 2;

 *     for_each (j)

 *       x_{j} /= 2;

 *

 * so that:

 *

 *   p_{j} = x_{j} / t;

 *

 * We optimize away the '/= 2' for the global time delta by noting that:

 *

 *   if (++t > period) t /= 2:

 *

 * Can be approximated by:

 *

 *   period/2 + (++t % period/2)

 *

 * [ Furthermore, when we choose period to be 2^n it can be written in terms of

 *   binary operations and wraparound artefacts disappear. ]

 *

 * Also note that this yields a natural counter of the elapsed periods:

 *

 *   c = t / (period/2)

 *

 * [ Its monotonic increasing property can be applied to mitigate the wrap-

 *   around issue. ]

 *

 * This allows us to do away with the loop over all prop_locals on each period

 * expiration. By remembering the period count under which it was last accessed

 * as c_{j}, we can obtain the number of 'missed' cycles from:

 *

 *   c - c_{j}

 *

 * We can then lazily catch up to the global period count every time we are

 * going to use x_{j}, by doing:

 *

 *   x_{j} /= 2^(c - c_{j}), c_{j} = c

 */

#include <linux/proportions.h>

#include <linux/rcupdate.h>

/*

 * Limit the time part in order to ensure there are some bits left for the

 * cycle counter.

 */

#define PROP_MAX_SHIFT (3*BITS_PER_LONG/4)

int prop_descriptor_init(struct prop_descriptor *pd, int shift)

{

        int err;

        if (shift > PROP_MAX_SHIFT)

                shift = PROP_MAX_SHIFT;

        pd->index = 0;

        pd->pg[0].shift = shift;

        mutex_init(&pd->mutex);

        err = percpu_counter_init_irq(&pd->pg[0].events, 0);

        if (err)

                goto out;

        err = percpu_counter_init_irq(&pd->pg[1].events, 0);

        if (err)

                percpu_counter_destroy(&pd->pg[0].events);

out:

        return err;

}

/*

 * We have two copies, and flip between them to make it seem like an atomic

 * update. The update is not really atomic wrt the events counter, but

 * it is internally consistent with the bit layout depending on shift.

 *

 * We copy the events count, move the bits around and flip the index.

 */

void prop_change_shift(struct prop_descriptor *pd, int shift)

{

        int index;

        int offset;

        u64 events;

        unsigned long flags;

        if (shift > PROP_MAX_SHIFT)

                shift = PROP_MAX_SHIFT;

        mutex_lock(&pd->mutex);

        index = pd->index ^ 1;

        offset = pd->pg[pd->index].shift - shift;

        if (!offset)

                goto out;

        pd->pg[index].shift = shift;

        local_irq_save(flags);

        events = percpu_counter_sum(&pd->pg[pd->index].events);

        if (offset < 0)

                events <<= -offset;

        else

                events >>= offset;

        percpu_counter_set(&pd->pg[index].events, events);

        /*

         * ensure the new pg is fully written before the switch

         */

        smp_wmb();

        pd->index = index;

        local_irq_restore(flags);

        synchronize_rcu();

out:

        mutex_unlock(&pd->mutex);

}

/*

 * wrap the access to the data in an rcu_read_lock() section;

 * this is used to track the active references.

 */

static struct prop_global *prop_get_global(struct prop_descriptor *pd)

{

        int index;

        rcu_read_lock();

        index = pd->index;

        /*

         * match the wmb from vcd_flip()

         */

        smp_rmb();

        return &pd->pg[index];

}

static void prop_put_global(struct prop_descriptor *pd, struct prop_global *pg)

{

        rcu_read_unlock();

}

static void

prop_adjust_shift(int *pl_shift, unsigned long *pl_period, int new_shift)

{

        int offset = *pl_shift - new_shift;

        if (!offset)

                return;

        if (offset < 0)

                *pl_period <<= -offset;

        else

                *pl_period >>= offset;

        *pl_shift = new_shift;

}

/*

 * PERCPU

 */

#define PROP_BATCH (8*(1+ilog2(nr_cpu_ids)))

int prop_local_init_percpu(struct prop_local_percpu *pl)

{

        spin_lock_init(&pl->lock);

        pl->shift = 0;

        pl->period = 0;

        return percpu_counter_init_irq(&pl->events, 0);

}

void prop_local_destroy_percpu(struct prop_local_percpu *pl)

{

        percpu_counter_destroy(&pl->events);

}

/*

 * Catch up with missed period expirations.

 *

 *   until (c_{j} == c)

 *     x_{j} -= x_{j}/2;

 *     c_{j}++;

 */

static

void prop_norm_percpu(struct prop_global *pg, struct prop_local_percpu *pl)

{

        unsigned long period = 1UL << (pg->shift - 1);

        unsigned long period_mask = ~(period - 1);

        unsigned long global_period;

        unsigned long flags;

        global_period = percpu_counter_read(&pg->events);

        global_period &= period_mask;

        /*

         * Fast path - check if the local and global period count still match

         * outside of the lock.

         */

        if (pl->period == global_period)

                return;

        spin_lock_irqsave(&pl->lock, flags);

        prop_adjust_shift(&pl->shift, &pl->period, pg->shift);

        /*

         * For each missed period, we half the local counter.

         * basically:

         *   pl->events >> (global_period - pl->period);

         */

        period = (global_period - pl->period) >> (pg->shift - 1);

        if (period < BITS_PER_LONG) {

                s64 val = percpu_counter_read(&pl->events);

                if (val < (nr_cpu_ids * PROP_BATCH))

                        val = percpu_counter_sum(&pl->events);

                __percpu_counter_add(&pl->events, -val + (val >> period),

                                        PROP_BATCH);

        } else

                percpu_counter_set(&pl->events, 0);

        pl->period = global_period;

        spin_unlock_irqrestore(&pl->lock, flags);

}

/*

 *   ++x_{j}, ++t

 */

void __prop_inc_percpu(struct prop_descriptor *pd, struct prop_local_percpu *pl)

{

        struct prop_global *pg = prop_get_global(pd);

        prop_norm_percpu(pg, pl);

        __percpu_counter_add(&pl->events, 1, PROP_BATCH);

        percpu_counter_add(&pg->events, 1);

        prop_put_global(pd, pg);

}

/*

 * Obtain a fraction of this proportion

 *

 *   p_{j} = x_{j} / (period/2 + t % period/2)

 */

void prop_fraction_percpu(struct prop_descriptor *pd,

                struct prop_local_percpu *pl,

                long *numerator, long *denominator)

{

        struct prop_global *pg = prop_get_global(pd);

        unsigned long period_2 = 1UL << (pg->shift - 1);

        unsigned long counter_mask = period_2 - 1;

        unsigned long global_count;

        prop_norm_percpu(pg, pl);

        *numerator = percpu_counter_read_positive(&pl->events);

        global_count = percpu_counter_read(&pg->events);

        *denominator = period_2 + (global_count & counter_mask);

        prop_put_global(pd, pg);

}

/*

 * SINGLE

 */

int prop_local_init_single(struct prop_local_single *pl)

{

        spin_lock_init(&pl->lock);

        pl->shift = 0;

        pl->period = 0;

        pl->events = 0;

        return 0;

}

void prop_local_destroy_single(struct prop_local_single *pl)

{

}

/*

 * Catch up with missed period expirations.

 */

static

void prop_norm_single(struct prop_global *pg, struct prop_local_single *pl)

{

        unsigned long period = 1UL << (pg->shift - 1);

        unsigned long period_mask = ~(period - 1);

        unsigned long global_period;

        unsigned long flags;

        global_period = percpu_counter_read(&pg->events);

        global_period &= period_mask;

        /*

         * Fast path - check if the local and global period count still match

         * outside of the lock.

         */

        if (pl->period == global_period)

                return;

        spin_lock_irqsave(&pl->lock, flags);

        prop_adjust_shift(&pl->shift, &pl->period, pg->shift);

        /*

         * For each missed period, we half the local counter.

         */

        period = (global_period - pl->period) >> (pg->shift - 1);

        if (likely(period < BITS_PER_LONG))

                pl->events >>= period;

        else

                pl->events = 0;

        pl->period = global_period;

        spin_unlock_irqrestore(&pl->lock, flags);

}

/*

 *   ++x_{j}, ++t

 */

void __prop_inc_single(struct prop_descriptor *pd, struct prop_local_single *pl)

{

        struct prop_global *pg = prop_get_global(pd);

        prop_norm_single(pg, pl);

        pl->events++;

        percpu_counter_add(&pg->events, 1);

        prop_put_global(pd, pg);

}

/*

 * Obtain a fraction of this proportion

 *

 *   p_{j} = x_{j} / (period/2 + t % period/2)

 */

void prop_fraction_single(struct prop_descriptor *pd,

                struct prop_local_single *pl,

                long *numerator, long *denominator)

{

        struct prop_global *pg = prop_get_global(pd);

        unsigned long period_2 = 1UL << (pg->shift - 1);

        unsigned long counter_mask = period_2 - 1;

        unsigned long global_count;

        prop_norm_single(pg, pl);

        *numerator = pl->events;

        global_count = percpu_counter_read(&pg->events);

        *denominator = period_2 + (global_count & counter_mask);

        prop_put_global(pd, pg);

}