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         preempt_disable_nested();
  74         write_seqcount_begin(&p->sequence);
  75         if (periods < 64)
  76                 events -= events >> periods;
  77         /* Use addition to avoid losing events happening between sum and set */
  78         percpu_counter_add(&p->events, -events);
  79         p->period += periods;
  80         write_seqcount_end(&p->sequence);
  81         preempt_enable_nested();
  82
  83         return true;
  84 }
  85
  86 /*
  87  * ---- PERCPU ----
  88  */
  89 #define PROP_BATCH (8*(1+ilog2(nr_cpu_ids)))
  90
  91 int fprop_local_init_percpu(struct fprop_local_percpu *pl, gfp_t gfp)
  92 {
  93         int err;
  94
  95         err = percpu_counter_init(&pl->events, 0, gfp);
  96         if (err)
  97                 return err;
  98         pl->period = 0;
  99         raw_spin_lock_init(&pl->lock);
 100         return 0;
 101 }
 102
 103 void fprop_local_destroy_percpu(struct fprop_local_percpu *pl)
 104 {
 105         percpu_counter_destroy(&pl->events);
 106 }
 107
 108 static void fprop_reflect_period_percpu(struct fprop_global *p,
 109                                         struct fprop_local_percpu *pl)
 110 {
 111         unsigned int period = p->period;
 112         unsigned long flags;
 113
 114         /* Fast path - period didn't change */
 115         if (pl->period == period)
 116                 return;
 117         raw_spin_lock_irqsave(&pl->lock, flags);
 118         /* Someone updated pl->period while we were spinning? */
 119         if (pl->period >= period) {
 120                 raw_spin_unlock_irqrestore(&pl->lock, flags);
 121                 return;
 122         }
 123         /* Aging zeroed our fraction? */
 124         if (period - pl->period < BITS_PER_LONG) {
 125                 s64 val = percpu_counter_read(&pl->events);
 126
 127                 if (val < (nr_cpu_ids * PROP_BATCH))
 128                         val = percpu_counter_sum(&pl->events);
 129
 130                 percpu_counter_add_batch(&pl->events,
 131                         -val + (val >> (period-pl->period)), PROP_BATCH);
 132         } else
 133                 percpu_counter_set(&pl->events, 0);
 134         pl->period = period;
 135         raw_spin_unlock_irqrestore(&pl->lock, flags);
 136 }
 137
 138 /* Event of type pl happened */
 139 void __fprop_add_percpu(struct fprop_global *p, struct fprop_local_percpu *pl,
 140                 long nr)
 141 {
 142         fprop_reflect_period_percpu(p, pl);
 143         percpu_counter_add_batch(&pl->events, nr, PROP_BATCH);
 144         percpu_counter_add(&p->events, nr);
 145 }
 146
 147 void fprop_fraction_percpu(struct fprop_global *p,
 148                            struct fprop_local_percpu *pl,
 149                            unsigned long *numerator, unsigned long *denominator)
 150 {
 151         unsigned int seq;
 152         s64 num, den;
 153
 154         do {
 155                 seq = read_seqcount_begin(&p->sequence);
 156                 fprop_reflect_period_percpu(p, pl);
 157                 num = percpu_counter_read_positive(&pl->events);
 158                 den = percpu_counter_read_positive(&p->events);
 159         } while (read_seqcount_retry(&p->sequence, seq));
 160
 161         /*
 162          * Make fraction <= 1 and denominator > 0 even in presence of percpu
 163          * counter errors
 164          */
 165         if (den <= num) {
 166                 if (num)
 167                         den = num;
 168                 else
 169                         den = 1;
 170         }
 171         *denominator = den;
 172         *numerator = num;
 173 }
 174
 175 /*
 176  * Like __fprop_add_percpu() except that event is counted only if the given
 177  * type has fraction smaller than @max_frac/FPROP_FRAC_BASE
 178  */
 179 void __fprop_add_percpu_max(struct fprop_global *p,
 180                 struct fprop_local_percpu *pl, int max_frac, long nr)
 181 {
 182         if (unlikely(max_frac < FPROP_FRAC_BASE)) {
 183                 unsigned long numerator, denominator;
 184                 s64 tmp;
 185
 186                 fprop_fraction_percpu(p, pl, &numerator, &denominator);
 187                 /* Adding 'nr' to fraction exceeds max_frac/FPROP_FRAC_BASE? */
 188                 tmp = (u64)denominator * max_frac -
 189                                         ((u64)numerator << FPROP_FRAC_SHIFT);
 190                 if (tmp < 0) {
 191                         /* Maximum fraction already exceeded? */
 192                         return;
 193                 } else if (tmp < nr * (FPROP_FRAC_BASE - max_frac)) {
 194                         /* Add just enough for the fraction to saturate */
 195                         nr = div_u64(tmp + FPROP_FRAC_BASE - max_frac - 1,
 196                                         FPROP_FRAC_BASE - max_frac);
 197                 }
 198         }
 199
 200         __fprop_add_percpu(p, pl, nr);
 201 }