xref: /openbmc/linux/lib/flex_proportions.c (revision ac73d4bf)
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  * ---- SINGLE ----
88  */
89 
90 int fprop_local_init_single(struct fprop_local_single *pl)
91 {
92 	pl->events = 0;
93 	pl->period = 0;
94 	raw_spin_lock_init(&pl->lock);
95 	return 0;
96 }
97 
98 void fprop_local_destroy_single(struct fprop_local_single *pl)
99 {
100 }
101 
102 static void fprop_reflect_period_single(struct fprop_global *p,
103 					struct fprop_local_single *pl)
104 {
105 	unsigned int period = p->period;
106 	unsigned long flags;
107 
108 	/* Fast path - period didn't change */
109 	if (pl->period == period)
110 		return;
111 	raw_spin_lock_irqsave(&pl->lock, flags);
112 	/* Someone updated pl->period while we were spinning? */
113 	if (pl->period >= period) {
114 		raw_spin_unlock_irqrestore(&pl->lock, flags);
115 		return;
116 	}
117 	/* Aging zeroed our fraction? */
118 	if (period - pl->period < BITS_PER_LONG)
119 		pl->events >>= period - pl->period;
120 	else
121 		pl->events = 0;
122 	pl->period = period;
123 	raw_spin_unlock_irqrestore(&pl->lock, flags);
124 }
125 
126 /* Event of type pl happened */
127 void __fprop_inc_single(struct fprop_global *p, struct fprop_local_single *pl)
128 {
129 	fprop_reflect_period_single(p, pl);
130 	pl->events++;
131 	percpu_counter_add(&p->events, 1);
132 }
133 
134 /* Return fraction of events of type pl */
135 void fprop_fraction_single(struct fprop_global *p,
136 			   struct fprop_local_single *pl,
137 			   unsigned long *numerator, unsigned long *denominator)
138 {
139 	unsigned int seq;
140 	s64 num, den;
141 
142 	do {
143 		seq = read_seqcount_begin(&p->sequence);
144 		fprop_reflect_period_single(p, pl);
145 		num = pl->events;
146 		den = percpu_counter_read_positive(&p->events);
147 	} while (read_seqcount_retry(&p->sequence, seq));
148 
149 	/*
150 	 * Make fraction <= 1 and denominator > 0 even in presence of percpu
151 	 * counter errors
152 	 */
153 	if (den <= num) {
154 		if (num)
155 			den = num;
156 		else
157 			den = 1;
158 	}
159 	*denominator = den;
160 	*numerator = num;
161 }
162 
163 /*
164  * ---- PERCPU ----
165  */
166 #define PROP_BATCH (8*(1+ilog2(nr_cpu_ids)))
167 
168 int fprop_local_init_percpu(struct fprop_local_percpu *pl, gfp_t gfp)
169 {
170 	int err;
171 
172 	err = percpu_counter_init(&pl->events, 0, gfp);
173 	if (err)
174 		return err;
175 	pl->period = 0;
176 	raw_spin_lock_init(&pl->lock);
177 	return 0;
178 }
179 
180 void fprop_local_destroy_percpu(struct fprop_local_percpu *pl)
181 {
182 	percpu_counter_destroy(&pl->events);
183 }
184 
185 static void fprop_reflect_period_percpu(struct fprop_global *p,
186 					struct fprop_local_percpu *pl)
187 {
188 	unsigned int period = p->period;
189 	unsigned long flags;
190 
191 	/* Fast path - period didn't change */
192 	if (pl->period == period)
193 		return;
194 	raw_spin_lock_irqsave(&pl->lock, flags);
195 	/* Someone updated pl->period while we were spinning? */
196 	if (pl->period >= period) {
197 		raw_spin_unlock_irqrestore(&pl->lock, flags);
198 		return;
199 	}
200 	/* Aging zeroed our fraction? */
201 	if (period - pl->period < BITS_PER_LONG) {
202 		s64 val = percpu_counter_read(&pl->events);
203 
204 		if (val < (nr_cpu_ids * PROP_BATCH))
205 			val = percpu_counter_sum(&pl->events);
206 
207 		percpu_counter_add_batch(&pl->events,
208 			-val + (val >> (period-pl->period)), PROP_BATCH);
209 	} else
210 		percpu_counter_set(&pl->events, 0);
211 	pl->period = period;
212 	raw_spin_unlock_irqrestore(&pl->lock, flags);
213 }
214 
215 /* Event of type pl happened */
216 void __fprop_add_percpu(struct fprop_global *p, struct fprop_local_percpu *pl,
217 		long nr)
218 {
219 	fprop_reflect_period_percpu(p, pl);
220 	percpu_counter_add_batch(&pl->events, nr, PROP_BATCH);
221 	percpu_counter_add(&p->events, nr);
222 }
223 
224 void fprop_fraction_percpu(struct fprop_global *p,
225 			   struct fprop_local_percpu *pl,
226 			   unsigned long *numerator, unsigned long *denominator)
227 {
228 	unsigned int seq;
229 	s64 num, den;
230 
231 	do {
232 		seq = read_seqcount_begin(&p->sequence);
233 		fprop_reflect_period_percpu(p, pl);
234 		num = percpu_counter_read_positive(&pl->events);
235 		den = percpu_counter_read_positive(&p->events);
236 	} while (read_seqcount_retry(&p->sequence, seq));
237 
238 	/*
239 	 * Make fraction <= 1 and denominator > 0 even in presence of percpu
240 	 * counter errors
241 	 */
242 	if (den <= num) {
243 		if (num)
244 			den = num;
245 		else
246 			den = 1;
247 	}
248 	*denominator = den;
249 	*numerator = num;
250 }
251 
252 /*
253  * Like __fprop_add_percpu() except that event is counted only if the given
254  * type has fraction smaller than @max_frac/FPROP_FRAC_BASE
255  */
256 void __fprop_add_percpu_max(struct fprop_global *p,
257 		struct fprop_local_percpu *pl, int max_frac, long nr)
258 {
259 	if (unlikely(max_frac < FPROP_FRAC_BASE)) {
260 		unsigned long numerator, denominator;
261 		s64 tmp;
262 
263 		fprop_fraction_percpu(p, pl, &numerator, &denominator);
264 		/* Adding 'nr' to fraction exceeds max_frac/FPROP_FRAC_BASE? */
265 		tmp = (u64)denominator * max_frac -
266 					((u64)numerator << FPROP_FRAC_SHIFT);
267 		if (tmp < 0) {
268 			/* Maximum fraction already exceeded? */
269 			return;
270 		} else if (tmp < nr * (FPROP_FRAC_BASE - max_frac)) {
271 			/* Add just enough for the fraction to saturate */
272 			nr = div_u64(tmp + FPROP_FRAC_BASE - max_frac - 1,
273 					FPROP_FRAC_BASE - max_frac);
274 		}
275 	}
276 
277 	__fprop_add_percpu(p, pl, nr);
278 }
279