flex_proportions.c

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/*
 *  Floating proportions with flexible aging period
 *
 *   Copyright (C) 2011, SUSE, Jan Kara <[email protected]>
 *
 * The goal of this code is: Given different types of event, measure proportion
 * of each type of event over time. The proportions are measured with
 * exponentially decaying history to give smooth transitions. A formula
 * expressing proportion of event of type 'j' is:
 *
 *   p_{j} = (\Sum_{i>=0} x_{i,j}/2^{i+1})/(\Sum_{i>=0} x_i/2^{i+1})
 *
 * Where x_{i,j} is j's number of events in i-th last time period and x_i is
 * total number of events in i-th last time period.
 *
 * Note that p_{j}'s are normalised, i.e.
 *
 *   \Sum_{j} p_{j} = 1,
 *
 * This formula can be straightforwardly computed by maintaing denominator
 * (let's call it 'd') and for each event type its numerator (let's call it
 * 'n_j'). When an event of type 'j' happens, we simply need to do:
 *   n_j++; d++;
 *
 * When a new period is declared, we could do:
 *   d /= 2
 *   for each j
 *     n_j /= 2
 *
 * To avoid iteration over all event types, we instead shift numerator of event
 * j lazily when someone asks for a proportion of event j or when event j
 * occurs. This can bit trivially implemented by remembering last period in
 * which something happened with proportion of type j.
 */
#include <linux/flex_proportions.h>

int fprop_global_init(struct fprop_global *p)
{
    int err;

    p->period = 0;
    /* Use 1 to avoid dealing with periods with 0 events... */
    err = percpu_counter_init(&p->events, 1);
    if (err)
        return err;
    seqcount_init(&p->sequence);
    return 0;
}

void fprop_global_destroy(struct fprop_global *p)
{
    percpu_counter_destroy(&p->events);
}

/*
 * Declare @periods new periods. It is upto the caller to make sure period
 * transitions cannot happen in parallel.
 *
 * The function returns true if the proportions are still defined and false
 * if aging zeroed out all events. This can be used to detect whether declaring
 * further periods has any effect.
 */
bool fprop_new_period(struct fprop_global *p, int periods)
{
    s64 events;
    unsigned long flags;

    local_irq_save(flags);
    events = percpu_counter_sum(&p->events);
    /*
     * Don't do anything if there are no events.
     */
    if (events <= 1) {
        local_irq_restore(flags);
        return false;
    }
    write_seqcount_begin(&p->sequence);
    if (periods < 64)
        events -= events >> periods;
    /* Use addition to avoid losing events happening between sum and set */
    percpu_counter_add(&p->events, -events);
    p->period += periods;
    write_seqcount_end(&p->sequence);
    local_irq_restore(flags);

    return true;
}

/*
 * ---- SINGLE ----
 */

int fprop_local_init_single(struct fprop_local_single *pl)
{
    pl->events = 0;
    pl->period = 0;
    raw_spin_lock_init(&pl->lock);
    return 0;
}

void fprop_local_destroy_single(struct fprop_local_single *pl)
{
}

static void fprop_reflect_period_single(struct fprop_global *p,
                    struct fprop_local_single *pl)
{
    unsigned int period = p->period;
    unsigned long flags;

    /* Fast path - period didn't change */
    if (pl->period == period)
        return;
    raw_spin_lock_irqsave(&pl->lock, flags);
    /* Someone updated pl->period while we were spinning? */
    if (pl->period >= period) {
        raw_spin_unlock_irqrestore(&pl->lock, flags);
        return;
    }
    /* Aging zeroed our fraction? */
    if (period - pl->period < BITS_PER_LONG)
        pl->events >>= period - pl->period;
    else
        pl->events = 0;
    pl->period = period;
    raw_spin_unlock_irqrestore(&pl->lock, flags);
}

/* Event of type pl happened */
void __fprop_inc_single(struct fprop_global *p, struct fprop_local_single *pl)
{
    fprop_reflect_period_single(p, pl);
    pl->events++;
    percpu_counter_add(&p->events, 1);
}

/* Return fraction of events of type pl */
void fprop_fraction_single(struct fprop_global *p,
               struct fprop_local_single *pl,
               unsigned long *numerator, unsigned long *denominator)
{
    unsigned int seq;
    s64 num, den;

    do {
        seq = read_seqcount_begin(&p->sequence);
        fprop_reflect_period_single(p, pl);
        num = pl->events;
        den = percpu_counter_read_positive(&p->events);
    } while (read_seqcount_retry(&p->sequence, seq));

    /*
     * Make fraction <= 1 and denominator > 0 even in presence of percpu
     * counter errors
     */
    if (den <= num) {
        if (num)
            den = num;
        else
            den = 1;
    }
    *denominator = den;
    *numerator = num;
}

/*
 * ---- PERCPU ----
 */
#define PROP_BATCH (8*(1+ilog2(nr_cpu_ids)))

int fprop_local_init_percpu(struct fprop_local_percpu *pl)
{
    int err;

    err = percpu_counter_init(&pl->events, 0);
    if (err)
        return err;
    pl->period = 0;
    raw_spin_lock_init(&pl->lock);
    return 0;
}

void fprop_local_destroy_percpu(struct fprop_local_percpu *pl)
{
    percpu_counter_destroy(&pl->events);
}

static void fprop_reflect_period_percpu(struct fprop_global *p,
                    struct fprop_local_percpu *pl)
{
    unsigned int period = p->period;
    unsigned long flags;

    /* Fast path - period didn't change */
    if (pl->period == period)
        return;
    raw_spin_lock_irqsave(&pl->lock, flags);
    /* Someone updated pl->period while we were spinning? */
    if (pl->period >= period) {
        raw_spin_unlock_irqrestore(&pl->lock, flags);
        return;
    }
    /* Aging zeroed our fraction? */
    if (period - pl->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-pl->period)), PROP_BATCH);
    } else
        percpu_counter_set(&pl->events, 0);
    pl->period = period;
    raw_spin_unlock_irqrestore(&pl->lock, flags);
}

/* Event of type pl happened */
void __fprop_inc_percpu(struct fprop_global *p, struct fprop_local_percpu *pl)
{
    fprop_reflect_period_percpu(p, pl);
    __percpu_counter_add(&pl->events, 1, PROP_BATCH);
    percpu_counter_add(&p->events, 1);
}

void fprop_fraction_percpu(struct fprop_global *p,
               struct fprop_local_percpu *pl,
               unsigned long *numerator, unsigned long *denominator)
{
    unsigned int seq;
    s64 num, den;

    do {
        seq = read_seqcount_begin(&p->sequence);
        fprop_reflect_period_percpu(p, pl);
        num = percpu_counter_read_positive(&pl->events);
        den = percpu_counter_read_positive(&p->events);
    } while (read_seqcount_retry(&p->sequence, seq));

    /*
     * Make fraction <= 1 and denominator > 0 even in presence of percpu
     * counter errors
     */
    if (den <= num) {
        if (num)
            den = num;
        else
            den = 1;
    }
    *denominator = den;
    *numerator = num;
}

/*
 * Like __fprop_inc_percpu() except that event is counted only if the given
 * type has fraction smaller than @max_frac/FPROP_FRAC_BASE
 */
void __fprop_inc_percpu_max(struct fprop_global *p,
                struct fprop_local_percpu *pl, int max_frac)
{
    if (unlikely(max_frac < FPROP_FRAC_BASE)) {
        unsigned long numerator, denominator;

        fprop_fraction_percpu(p, pl, &numerator, &denominator);
        if (numerator >
            (((u64)denominator) * max_frac) >> FPROP_FRAC_SHIFT)
            return;
    } else
        fprop_reflect_period_percpu(p, pl);
    __percpu_counter_add(&pl->events, 1, PROP_BATCH);
    percpu_counter_add(&p->events, 1);
}