linux/lib/flex_proportions.c
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   1/*
   2 *  Floating proportions with flexible aging period
   3 *
   4 *   Copyright (C) 2011, SUSE, Jan Kara <jack@suse.cz>
   5 *
   6 * The goal of this code is: Given different types of event, measure proportion
   7 * of each type of event over time. The proportions are measured with
   8 * exponentially decaying history to give smooth transitions. A formula
   9 * expressing proportion of event of type 'j' is:
  10 *
  11 *   p_{j} = (\Sum_{i>=0} x_{i,j}/2^{i+1})/(\Sum_{i>=0} x_i/2^{i+1})
  12 *
  13 * Where x_{i,j} is j's number of events in i-th last time period and x_i is
  14 * total number of events in i-th last time period.
  15 *
  16 * Note that p_{j}'s are normalised, i.e.
  17 *
  18 *   \Sum_{j} p_{j} = 1,
  19 *
  20 * This formula can be straightforwardly computed by maintaing denominator
  21 * (let's call it 'd') and for each event type its numerator (let's call it
  22 * 'n_j'). When an event of type 'j' happens, we simply need to do:
  23 *   n_j++; d++;
  24 *
  25 * When a new period is declared, we could do:
  26 *   d /= 2
  27 *   for each j
  28 *     n_j /= 2
  29 *
  30 * To avoid iteration over all event types, we instead shift numerator of event
  31 * j lazily when someone asks for a proportion of event j or when event j
  32 * occurs. This can bit trivially implemented by remembering last period in
  33 * which something happened with proportion of type j.
  34 */
  35#include <linux/flex_proportions.h>
  36
  37int fprop_global_init(struct fprop_global *p)
  38{
  39        int err;
  40
  41        p->period = 0;
  42        /* Use 1 to avoid dealing with periods with 0 events... */
  43        err = percpu_counter_init(&p->events, 1);
  44        if (err)
  45                return err;
  46        seqcount_init(&p->sequence);
  47        return 0;
  48}
  49
  50void fprop_global_destroy(struct fprop_global *p)
  51{
  52        percpu_counter_destroy(&p->events);
  53}
  54
  55/*
  56 * Declare @periods new periods. It is upto the caller to make sure period
  57 * transitions cannot happen in parallel.
  58 *
  59 * The function returns true if the proportions are still defined and false
  60 * if aging zeroed out all events. This can be used to detect whether declaring
  61 * further periods has any effect.
  62 */
  63bool fprop_new_period(struct fprop_global *p, int periods)
  64{
  65        s64 events;
  66        unsigned long flags;
  67
  68        local_irq_save(flags);
  69        events = percpu_counter_sum(&p->events);
  70        /*
  71         * Don't do anything if there are no events.
  72         */
  73        if (events <= 1) {
  74                local_irq_restore(flags);
  75                return false;
  76        }
  77        write_seqcount_begin(&p->sequence);
  78        if (periods < 64)
  79                events -= events >> periods;
  80        /* Use addition to avoid losing events happening between sum and set */
  81        percpu_counter_add(&p->events, -events);
  82        p->period += periods;
  83        write_seqcount_end(&p->sequence);
  84        local_irq_restore(flags);
  85
  86        return true;
  87}
  88
  89/*
  90 * ---- SINGLE ----
  91 */
  92
  93int fprop_local_init_single(struct fprop_local_single *pl)
  94{
  95        pl->events = 0;
  96        pl->period = 0;
  97        raw_spin_lock_init(&pl->lock);
  98        return 0;
  99}
 100
 101void fprop_local_destroy_single(struct fprop_local_single *pl)
 102{
 103}
 104
 105static void fprop_reflect_period_single(struct fprop_global *p,
 106                                        struct fprop_local_single *pl)
 107{
 108        unsigned int period = p->period;
 109        unsigned long flags;
 110
 111        /* Fast path - period didn't change */
 112        if (pl->period == period)
 113                return;
 114        raw_spin_lock_irqsave(&pl->lock, flags);
 115        /* Someone updated pl->period while we were spinning? */
 116        if (pl->period >= period) {
 117                raw_spin_unlock_irqrestore(&pl->lock, flags);
 118                return;
 119        }
 120        /* Aging zeroed our fraction? */
 121        if (period - pl->period < BITS_PER_LONG)
 122                pl->events >>= period - pl->period;
 123        else
 124                pl->events = 0;
 125        pl->period = period;
 126        raw_spin_unlock_irqrestore(&pl->lock, flags);
 127}
 128
 129/* Event of type pl happened */
 130void __fprop_inc_single(struct fprop_global *p, struct fprop_local_single *pl)
 131{
 132        fprop_reflect_period_single(p, pl);
 133        pl->events++;
 134        percpu_counter_add(&p->events, 1);
 135}
 136
 137/* Return fraction of events of type pl */
 138void fprop_fraction_single(struct fprop_global *p,
 139                           struct fprop_local_single *pl,
 140                           unsigned long *numerator, unsigned long *denominator)
 141{
 142        unsigned int seq;
 143        s64 num, den;
 144
 145        do {
 146                seq = read_seqcount_begin(&p->sequence);
 147                fprop_reflect_period_single(p, pl);
 148                num = pl->events;
 149                den = percpu_counter_read_positive(&p->events);
 150        } while (read_seqcount_retry(&p->sequence, seq));
 151
 152        /*
 153         * Make fraction <= 1 and denominator > 0 even in presence of percpu
 154         * counter errors
 155         */
 156        if (den <= num) {
 157                if (num)
 158                        den = num;
 159                else
 160                        den = 1;
 161        }
 162        *denominator = den;
 163        *numerator = num;
 164}
 165
 166/*
 167 * ---- PERCPU ----
 168 */
 169#define PROP_BATCH (8*(1+ilog2(nr_cpu_ids)))
 170
 171int fprop_local_init_percpu(struct fprop_local_percpu *pl)
 172{
 173        int err;
 174
 175        err = percpu_counter_init(&pl->events, 0);
 176        if (err)
 177                return err;
 178        pl->period = 0;
 179        raw_spin_lock_init(&pl->lock);
 180        return 0;
 181}
 182
 183void fprop_local_destroy_percpu(struct fprop_local_percpu *pl)
 184{
 185        percpu_counter_destroy(&pl->events);
 186}
 187
 188static void fprop_reflect_period_percpu(struct fprop_global *p,
 189                                        struct fprop_local_percpu *pl)
 190{
 191        unsigned int period = p->period;
 192        unsigned long flags;
 193
 194        /* Fast path - period didn't change */
 195        if (pl->period == period)
 196                return;
 197        raw_spin_lock_irqsave(&pl->lock, flags);
 198        /* Someone updated pl->period while we were spinning? */
 199        if (pl->period >= period) {
 200                raw_spin_unlock_irqrestore(&pl->lock, flags);
 201                return;
 202        }
 203        /* Aging zeroed our fraction? */
 204        if (period - pl->period < BITS_PER_LONG) {
 205                s64 val = percpu_counter_read(&pl->events);
 206
 207                if (val < (nr_cpu_ids * PROP_BATCH))
 208                        val = percpu_counter_sum(&pl->events);
 209
 210                __percpu_counter_add(&pl->events,
 211                        -val + (val >> (period-pl->period)), PROP_BATCH);
 212        } else
 213                percpu_counter_set(&pl->events, 0);
 214        pl->period = period;
 215        raw_spin_unlock_irqrestore(&pl->lock, flags);
 216}
 217
 218/* Event of type pl happened */
 219void __fprop_inc_percpu(struct fprop_global *p, struct fprop_local_percpu *pl)
 220{
 221        fprop_reflect_period_percpu(p, pl);
 222        __percpu_counter_add(&pl->events, 1, PROP_BATCH);
 223        percpu_counter_add(&p->events, 1);
 224}
 225
 226void fprop_fraction_percpu(struct fprop_global *p,
 227                           struct fprop_local_percpu *pl,
 228                           unsigned long *numerator, unsigned long *denominator)
 229{
 230        unsigned int seq;
 231        s64 num, den;
 232
 233        do {
 234                seq = read_seqcount_begin(&p->sequence);
 235                fprop_reflect_period_percpu(p, pl);
 236                num = percpu_counter_read_positive(&pl->events);
 237                den = percpu_counter_read_positive(&p->events);
 238        } while (read_seqcount_retry(&p->sequence, seq));
 239
 240        /*
 241         * Make fraction <= 1 and denominator > 0 even in presence of percpu
 242         * counter errors
 243         */
 244        if (den <= num) {
 245                if (num)
 246                        den = num;
 247                else
 248                        den = 1;
 249        }
 250        *denominator = den;
 251        *numerator = num;
 252}
 253
 254/*
 255 * Like __fprop_inc_percpu() except that event is counted only if the given
 256 * type has fraction smaller than @max_frac/FPROP_FRAC_BASE
 257 */
 258void __fprop_inc_percpu_max(struct fprop_global *p,
 259                            struct fprop_local_percpu *pl, int max_frac)
 260{
 261        if (unlikely(max_frac < FPROP_FRAC_BASE)) {
 262                unsigned long numerator, denominator;
 263
 264                fprop_fraction_percpu(p, pl, &numerator, &denominator);
 265                if (numerator >
 266                    (((u64)denominator) * max_frac) >> FPROP_FRAC_SHIFT)
 267                        return;
 268        } else
 269                fprop_reflect_period_percpu(p, pl);
 270        __percpu_counter_add(&pl->events, 1, PROP_BATCH);
 271        percpu_counter_add(&p->events, 1);
 272}
 273
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