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 
fprop_global_init(struct fprop_global * p,gfp_t gfp)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 
fprop_global_destroy(struct fprop_global * p)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  */
fprop_new_period(struct fprop_global * p,int periods)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 
fprop_local_init_percpu(struct fprop_local_percpu * pl,gfp_t gfp)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 
fprop_local_destroy_percpu(struct fprop_local_percpu * pl)103 void fprop_local_destroy_percpu(struct fprop_local_percpu *pl)
104 {
105 	percpu_counter_destroy(&pl->events);
106 }
107 
fprop_reflect_period_percpu(struct fprop_global * p,struct fprop_local_percpu * pl)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 */
__fprop_add_percpu(struct fprop_global * p,struct fprop_local_percpu * pl,long nr)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 
fprop_fraction_percpu(struct fprop_global * p,struct fprop_local_percpu * pl,unsigned long * numerator,unsigned long * denominator)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  */
__fprop_add_percpu_max(struct fprop_global * p,struct fprop_local_percpu * pl,int max_frac,long nr)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 }
202