/*
 * 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>

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(&pd->pg[0].events, 0, GFP_KERNEL);
        if (err)
                goto out;

        err = percpu_counter_init(&pd->pg[1].events, 0, GFP_KERNEL);
        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)
__acquires(RCU)
{
        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)
__releases(RCU)
{
        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)
{
        raw_spin_lock_init(&pl->lock);
        pl->shift = 0;
        pl->period = 0;
        return percpu_counter_init(&pl->events, 0, GFP_KERNEL);
}

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;

        raw_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;
        raw_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);
}

/*
 * identical to __prop_inc_percpu, except that it limits this pl's fraction to
 * @frac/PROP_FRAC_BASE by ignoring events when this limit has been exceeded.
 */
void __prop_inc_percpu_max(struct prop_descriptor *pd,
                           struct prop_local_percpu *pl, long frac)
{
        struct prop_global *pg = prop_get_global(pd);

        prop_norm_percpu(pg, pl);

        if (unlikely(frac != PROP_FRAC_BASE)) {
                unsigned long period_2 = 1UL << (pg->shift - 1);
                unsigned long counter_mask = period_2 - 1;
                unsigned long global_count;
                long numerator, denominator;

                numerator = percpu_counter_read_positive(&pl->events);
                global_count = percpu_counter_read(&pg->events);
                denominator = period_2 + (global_count & counter_mask);

                if (numerator > ((denominator * frac) >> PROP_FRAC_SHIFT))
                        goto out_put;
        }

        percpu_counter_add(&pl->events, 1);
        percpu_counter_add(&pg->events, 1);

out_put:
        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)
{
        raw_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;

        raw_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;
        raw_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);
}