lib/flex_proportions.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  *  Floating proportions with flexible aging period
   4  *
   5  *   Copyright (C) 2011, SUSE, Jan Kara <jack@suse.cz>
   6  *
   7  * The goal of this code is: Given different types of event, measure proportion
   8  * of each type of event over time. The proportions are measured with
   9  * exponentially decaying history to give smooth transitions. A formula
  10  * expressing proportion of event of type 'j' is:
  11  *
  12  *   p_{j} = (\Sum_{i>=0} x_{i,j}/2^{i+1})/(\Sum_{i>=0} x_i/2^{i+1})
  13  *
  14  * Where x_{i,j} is j's number of events in i-th last time period and x_i is
  15  * total number of events in i-th last time period.
  16  *
  17  * Note that p_{j}'s are normalised, i.e.
  18  *
  19  *   \Sum_{j} p_{j} = 1,
  20  *
  21  * This formula can be straightforwardly computed by maintaining denominator
  22  * (let's call it 'd') and for each event type its numerator (let's call it
  23  * 'n_j'). When an event of type 'j' happens, we simply need to do:
  24  *   n_j++; d++;
  25  *
  26  * When a new period is declared, we could do:
  27  *   d /= 2
  28  *   for each j
  29  *     n_j /= 2
  30  *
  31  * To avoid iteration over all event types, we instead shift numerator of event
  32  * j lazily when someone asks for a proportion of event j or when event j
  33  * occurs. This can bit trivially implemented by remembering last period in
  34  * which something happened with proportion of type j.
  35  */
  36 #include <linux/flex_proportions.h>
  37
  38 int fprop_global_init(struct fprop_global *p, gfp_t gfp)
  39 {
  40         int err;
  41
  42         p->period = 0;
  43         /* Use 1 to avoid dealing with periods with 0 events... */
  44         err = percpu_counter_init(&p->events, 1, gfp);
  45         if (err)
  46                 return err;
  47         seqcount_init(&p->sequence);
  48         return 0;
  49 }
  50
  51 void fprop_global_destroy(struct fprop_global *p)
  52 {
  53         percpu_counter_destroy(&p->events);
  54 }
  55
  56 /*
  57  * Declare @periods new periods. It is upto the caller to make sure period
  58  * transitions cannot happen in parallel.
  59  *
  60  * The function returns true if the proportions are still defined and false
  61  * if aging zeroed out all events. This can be used to detect whether declaring
  62  * further periods has any effect.
  63  */
  64 bool fprop_new_period(struct fprop_global *p, int periods)
  65 {
  66         s64 events = percpu_counter_sum(&p->events);
  67
  68         /*
  69          * Don't do anything if there are no events.
  70          */
  71         if (events <= 1)
  72                 return false;
  73         write_seqcount_begin(&p->sequence);
  74         if (periods < 64)
  75                 events -= events >> periods;
  76         /* Use addition to avoid losing events happening between sum and set */
  77         percpu_counter_add(&p->events, -events);
  78         p->period += periods;
  79         write_seqcount_end(&p->sequence);
  80
  81         return true;
  82 }
  83
  84 /*
  85  * ---- SINGLE ----
  86  */
  87
  88 int fprop_local_init_single(struct fprop_local_single *pl)
  89 {
  90         pl->events = 0;
  91         pl->period = 0;
  92         raw_spin_lock_init(&pl->lock);
  93         return 0;
  94 }
  95
  96 void fprop_local_destroy_single(struct fprop_local_single *pl)
  97 {
  98 }
  99
 100 static void fprop_reflect_period_single(struct fprop_global *p,
 101                                         struct fprop_local_single *pl)
 102 {
 103         unsigned int period = p->period;
 104         unsigned long flags;
 105
 106         /* Fast path - period didn't change */
 107         if (pl->period == period)
 108                 return;
 109         raw_spin_lock_irqsave(&pl->lock, flags);
 110         /* Someone updated pl->period while we were spinning? */
 111         if (pl->period >= period) {
 112                 raw_spin_unlock_irqrestore(&pl->lock, flags);
 113                 return;
 114         }
 115         /* Aging zeroed our fraction? */
 116         if (period - pl->period < BITS_PER_LONG)
 117                 pl->events >>= period - pl->period;
 118         else
 119                 pl->events = 0;
 120         pl->period = period;
 121         raw_spin_unlock_irqrestore(&pl->lock, flags);
 122 }
 123
 124 /* Event of type pl happened */
 125 void __fprop_inc_single(struct fprop_global *p, struct fprop_local_single *pl)
 126 {
 127         fprop_reflect_period_single(p, pl);
 128         pl->events++;
 129         percpu_counter_add(&p->events, 1);
 130 }
 131
 132 /* Return fraction of events of type pl */
 133 void fprop_fraction_single(struct fprop_global *p,
 134                            struct fprop_local_single *pl,
 135                            unsigned long *numerator, unsigned long *denominator)
 136 {
 137         unsigned int seq;
 138         s64 num, den;
 139
 140         do {
 141                 seq = read_seqcount_begin(&p->sequence);
 142                 fprop_reflect_period_single(p, pl);
 143                 num = pl->events;
 144                 den = percpu_counter_read_positive(&p->events);
 145         } while (read_seqcount_retry(&p->sequence, seq));
 146
 147         /*
 148          * Make fraction <= 1 and denominator > 0 even in presence of percpu
 149          * counter errors
 150          */
 151         if (den <= num) {
 152                 if (num)
 153                         den = num;
 154                 else
 155                         den = 1;
 156         }
 157         *denominator = den;
 158         *numerator = num;
 159 }
 160
 161 /*
 162  * ---- PERCPU ----
 163  */
 164 #define PROP_BATCH (8*(1+ilog2(nr_cpu_ids)))
 165
 166 int fprop_local_init_percpu(struct fprop_local_percpu *pl, gfp_t gfp)
 167 {
 168         int err;
 169
 170         err = percpu_counter_init(&pl->events, 0, gfp);
 171         if (err)
 172                 return err;
 173         pl->period = 0;
 174         raw_spin_lock_init(&pl->lock);
 175         return 0;
 176 }
 177
 178 void fprop_local_destroy_percpu(struct fprop_local_percpu *pl)
 179 {
 180         percpu_counter_destroy(&pl->events);
 181 }
 182
 183 static void fprop_reflect_period_percpu(struct fprop_global *p,
 184                                         struct fprop_local_percpu *pl)
 185 {
 186         unsigned int period = p->period;
 187         unsigned long flags;
 188
 189         /* Fast path - period didn't change */
 190         if (pl->period == period)
 191                 return;
 192         raw_spin_lock_irqsave(&pl->lock, flags);
 193         /* Someone updated pl->period while we were spinning? */
 194         if (pl->period >= period) {
 195                 raw_spin_unlock_irqrestore(&pl->lock, flags);
 196                 return;
 197         }
 198         /* Aging zeroed our fraction? */
 199         if (period - pl->period < BITS_PER_LONG) {
 200                 s64 val = percpu_counter_read(&pl->events);
 201
 202                 if (val < (nr_cpu_ids * PROP_BATCH))
 203                         val = percpu_counter_sum(&pl->events);
 204
 205                 percpu_counter_add_batch(&pl->events,
 206                         -val + (val >> (period-pl->period)), PROP_BATCH);
 207         } else
 208                 percpu_counter_set(&pl->events, 0);
 209         pl->period = period;
 210         raw_spin_unlock_irqrestore(&pl->lock, flags);
 211 }
 212
 213 /* Event of type pl happened */
 214 void __fprop_add_percpu(struct fprop_global *p, struct fprop_local_percpu *pl,
 215                 long nr)
 216 {
 217         fprop_reflect_period_percpu(p, pl);
 218         percpu_counter_add_batch(&pl->events, nr, PROP_BATCH);
 219         percpu_counter_add(&p->events, nr);
 220 }
 221
 222 void fprop_fraction_percpu(struct fprop_global *p,
 223                            struct fprop_local_percpu *pl,
 224                            unsigned long *numerator, unsigned long *denominator)
 225 {
 226         unsigned int seq;
 227         s64 num, den;
 228
 229         do {
 230                 seq = read_seqcount_begin(&p->sequence);
 231                 fprop_reflect_period_percpu(p, pl);
 232                 num = percpu_counter_read_positive(&pl->events);
 233                 den = percpu_counter_read_positive(&p->events);
 234         } while (read_seqcount_retry(&p->sequence, seq));
 235
 236         /*
 237          * Make fraction <= 1 and denominator > 0 even in presence of percpu
 238          * counter errors
 239          */
 240         if (den <= num) {
 241                 if (num)
 242                         den = num;
 243                 else
 244                         den = 1;
 245         }
 246         *denominator = den;
 247         *numerator = num;
 248 }
 249
 250 /*
 251  * Like __fprop_add_percpu() except that event is counted only if the given
 252  * type has fraction smaller than @max_frac/FPROP_FRAC_BASE
 253  */
 254 void __fprop_add_percpu_max(struct fprop_global *p,
 255                 struct fprop_local_percpu *pl, int max_frac, long nr)
 256 {
 257         if (unlikely(max_frac < FPROP_FRAC_BASE)) {
 258                 unsigned long numerator, denominator;
 259                 s64 tmp;
 260
 261                 fprop_fraction_percpu(p, pl, &numerator, &denominator);
 262                 /* Adding 'nr' to fraction exceeds max_frac/FPROP_FRAC_BASE? */
 263                 tmp = (u64)denominator * max_frac -
 264                                         ((u64)numerator << FPROP_FRAC_SHIFT);
 265                 if (tmp < 0) {
 266                         /* Maximum fraction already exceeded? */
 267                         return;
 268                 } else if (tmp < nr * (FPROP_FRAC_BASE - max_frac)) {
 269                         /* Add just enough for the fraction to saturate */
 270                         nr = div_u64(tmp + FPROP_FRAC_BASE - max_frac - 1,
 271                                         FPROP_FRAC_BASE - max_frac);
 272                 }
 273         }
 274
 275         __fprop_add_percpu(p, pl, nr);
 276 }