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
38int 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
51void fprop_global_destroy(struct fprop_global *p)
52{
53 percpu_counter_destroy(fbc: &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 */
64bool fprop_new_period(struct fprop_global *p, int periods)
65{
66 s64 events = percpu_counter_sum(fbc: &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(fbc: &p->events, amount: -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
91int 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
103void fprop_local_destroy_percpu(struct fprop_local_percpu *pl)
104{
105 percpu_counter_destroy(fbc: &pl->events);
106}
107
108static 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(fbc: &pl->events);
126
127 if (val < (nr_cpu_ids * PROP_BATCH))
128 val = percpu_counter_sum(fbc: &pl->events);
129
130 percpu_counter_add_batch(fbc: &pl->events,
131 amount: -val + (val >> (period-pl->period)), PROP_BATCH);
132 } else
133 percpu_counter_set(fbc: &pl->events, amount: 0);
134 pl->period = period;
135 raw_spin_unlock_irqrestore(&pl->lock, flags);
136}
137
138/* Event of type pl happened */
139void __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(fbc: &pl->events, amount: nr, PROP_BATCH);
144 percpu_counter_add(fbc: &p->events, amount: nr);
145}
146
147void 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(fbc: &pl->events);
158 den = percpu_counter_read_positive(fbc: &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 */
179void __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: &numerator, denominator: &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(dividend: tmp + FPROP_FRAC_BASE - max_frac - 1,
196 FPROP_FRAC_BASE - max_frac);
197 }
198 }
199
200 __fprop_add_percpu(p, pl, nr);
201}
202