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;
  67         unsigned long flags;
  68
  69         local_irq_save(flags);
  70         events = percpu_counter_sum(&p->events);
  71         /*
  72          * Don't do anything if there are no events.
  73          */
  74         if (events <= 1) {
  75                 local_irq_restore(flags);
  76                 return false;
  77         }
  78         write_seqcount_begin(&p->sequence);
  79         if (periods < 64)
  80                 events -= events >> periods;
  81         /* Use addition to avoid losing events happening between sum and set */
  82         percpu_counter_add(&p->events, -events);
  83         p->period += periods;
  84         write_seqcount_end(&p->sequence);
  85         local_irq_restore(flags);
  86
  87         return true;
  88 }
  89
  90 /*
  91  * ---- SINGLE ----
  92  */
  93
  94 int fprop_local_init_single(struct fprop_local_single *pl)
  95 {
  96         pl->events = 0;
  97         pl->period = 0;
  98         raw_spin_lock_init(&pl->lock);
  99         return 0;
 100 }
 101
 102 void fprop_local_destroy_single(struct fprop_local_single *pl)
 103 {
 104 }
 105
 106 static void fprop_reflect_period_single(struct fprop_global *p,
 107                                         struct fprop_local_single *pl)
 108 {
 109         unsigned int period = p->period;
 110         unsigned long flags;
 111
 112         /* Fast path - period didn't change */
 113         if (pl->period == period)
 114                 return;
 115         raw_spin_lock_irqsave(&pl->lock, flags);
 116         /* Someone updated pl->period while we were spinning? */
 117         if (pl->period >= period) {
 118                 raw_spin_unlock_irqrestore(&pl->lock, flags);
 119                 return;
 120         }
 121         /* Aging zeroed our fraction? */
 122         if (period - pl->period < BITS_PER_LONG)
 123                 pl->events >>= period - pl->period;
 124         else
 125                 pl->events = 0;
 126         pl->period = period;
 127         raw_spin_unlock_irqrestore(&pl->lock, flags);
 128 }
 129
 130 /* Event of type pl happened */
 131 void __fprop_inc_single(struct fprop_global *p, struct fprop_local_single *pl)
 132 {
 133         fprop_reflect_period_single(p, pl);
 134         pl->events++;
 135         percpu_counter_add(&p->events, 1);
 136 }
 137
 138 /* Return fraction of events of type pl */
 139 void fprop_fraction_single(struct fprop_global *p,
 140                            struct fprop_local_single *pl,
 141                            unsigned long *numerator, unsigned long *denominator)
 142 {
 143         unsigned int seq;
 144         s64 num, den;
 145
 146         do {
 147                 seq = read_seqcount_begin(&p->sequence);
 148                 fprop_reflect_period_single(p, pl);
 149                 num = pl->events;
 150                 den = percpu_counter_read_positive(&p->events);
 151         } while (read_seqcount_retry(&p->sequence, seq));
 152
 153         /*
 154          * Make fraction <= 1 and denominator > 0 even in presence of percpu
 155          * counter errors
 156          */
 157         if (den <= num) {
 158                 if (num)
 159                         den = num;
 160                 else
 161                         den = 1;
 162         }
 163         *denominator = den;
 164         *numerator = num;
 165 }
 166
 167 /*
 168  * ---- PERCPU ----
 169  */
 170 #define PROP_BATCH (8*(1+ilog2(nr_cpu_ids)))
 171
 172 int fprop_local_init_percpu(struct fprop_local_percpu *pl, gfp_t gfp)
 173 {
 174         int err;
 175
 176         err = percpu_counter_init(&pl->events, 0, gfp);
 177         if (err)
 178                 return err;
 179         pl->period = 0;
 180         raw_spin_lock_init(&pl->lock);
 181         return 0;
 182 }
 183
 184 void fprop_local_destroy_percpu(struct fprop_local_percpu *pl)
 185 {
 186         percpu_counter_destroy(&pl->events);
 187 }
 188
 189 static void fprop_reflect_period_percpu(struct fprop_global *p,
 190                                         struct fprop_local_percpu *pl)
 191 {
 192         unsigned int period = p->period;
 193         unsigned long flags;
 194
 195         /* Fast path - period didn't change */
 196         if (pl->period == period)
 197                 return;
 198         raw_spin_lock_irqsave(&pl->lock, flags);
 199         /* Someone updated pl->period while we were spinning? */
 200         if (pl->period >= period) {
 201                 raw_spin_unlock_irqrestore(&pl->lock, flags);
 202                 return;
 203         }
 204         /* Aging zeroed our fraction? */
 205         if (period - pl->period < BITS_PER_LONG) {
 206                 s64 val = percpu_counter_read(&pl->events);
 207
 208                 if (val < (nr_cpu_ids * PROP_BATCH))
 209                         val = percpu_counter_sum(&pl->events);
 210
 211                 percpu_counter_add_batch(&pl->events,
 212                         -val + (val >> (period-pl->period)), PROP_BATCH);
 213         } else
 214                 percpu_counter_set(&pl->events, 0);
 215         pl->period = period;
 216         raw_spin_unlock_irqrestore(&pl->lock, flags);
 217 }
 218
 219 /* Event of type pl happened */
 220 void __fprop_inc_percpu(struct fprop_global *p, struct fprop_local_percpu *pl)
 221 {
 222         fprop_reflect_period_percpu(p, pl);
 223         percpu_counter_add_batch(&pl->events, 1, PROP_BATCH);
 224         percpu_counter_add(&p->events, 1);
 225 }
 226
 227 void fprop_fraction_percpu(struct fprop_global *p,
 228                            struct fprop_local_percpu *pl,
 229                            unsigned long *numerator, unsigned long *denominator)
 230 {
 231         unsigned int seq;
 232         s64 num, den;
 233
 234         do {
 235                 seq = read_seqcount_begin(&p->sequence);
 236                 fprop_reflect_period_percpu(p, pl);
 237                 num = percpu_counter_read_positive(&pl->events);
 238                 den = percpu_counter_read_positive(&p->events);
 239         } while (read_seqcount_retry(&p->sequence, seq));
 240
 241         /*
 242          * Make fraction <= 1 and denominator > 0 even in presence of percpu
 243          * counter errors
 244          */
 245         if (den <= num) {
 246                 if (num)
 247                         den = num;
 248                 else
 249                         den = 1;
 250         }
 251         *denominator = den;
 252         *numerator = num;
 253 }
 254
 255 /*
 256  * Like __fprop_inc_percpu() except that event is counted only if the given
 257  * type has fraction smaller than @max_frac/FPROP_FRAC_BASE
 258  */
 259 void __fprop_inc_percpu_max(struct fprop_global *p,
 260                             struct fprop_local_percpu *pl, int max_frac)
 261 {
 262         if (unlikely(max_frac < FPROP_FRAC_BASE)) {
 263                 unsigned long numerator, denominator;
 264
 265                 fprop_fraction_percpu(p, pl, &numerator, &denominator);
 266                 if (numerator >
 267                     (((u64)denominator) * max_frac) >> FPROP_FRAC_SHIFT)
 268                         return;
 269         }
 270
 271         __fprop_inc_percpu(p, pl);
 272 }