Root/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
37int 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
50void 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 */
63bool fprop_new_period(struct fprop_global *p, int periods)
64{
65    s64 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
93int 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
101void fprop_local_destroy_single(struct fprop_local_single *pl)
102{
103}
104
105static 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 */
130void __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 */
138void 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
171int 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
183void fprop_local_destroy_percpu(struct fprop_local_percpu *pl)
184{
185    percpu_counter_destroy(&pl->events);
186}
187
188static 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 */
219void __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
226void 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 */
258void __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}
273

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