lib/flex_proportions.c

   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
  37 int 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
  50 void 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  */
  63 bool fprop_new_period(struct fprop_global *p, int periods)
  64 {
  65         u64 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
  93 int 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
 101 void fprop_local_destroy_single(struct fprop_local_single *pl)
 102 {
 103 }
 104
 105 static 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 */
 130 void __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 */
 138 void 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
 171 int 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
 183 void fprop_local_destroy_percpu(struct fprop_local_percpu *pl)
 184 {
 185         percpu_counter_destroy(&pl->events);
 186 }
 187
 188 static 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 */
 219 void __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
 226 void 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  */
 258 void __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 }