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
 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(&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(&p->events);
 67	unsigned long flags;
 68
 69	/*
 70	 * Don't do anything if there are no events.
 71	 */
 72	if (events <= 1)
 73		return false;
 74	local_irq_save(flags);
 75	write_seqcount_begin(&p->sequence);
 76	if (periods < 64)
 77		events -= events >> periods;
 78	/* Use addition to avoid losing events happening between sum and set */
 79	percpu_counter_add(&p->events, -events);
 80	p->period += periods;
 81	write_seqcount_end(&p->sequence);
 82	local_irq_restore(flags);
 83
 84	return true;
 85}
 86
 87/*
 88 * ---- PERCPU ----
 89 */
 90#define PROP_BATCH (8*(1+ilog2(nr_cpu_ids)))
 91
 92int fprop_local_init_percpu(struct fprop_local_percpu *pl, gfp_t gfp)
 93{
 94	int err;
 95
 96	err = percpu_counter_init(&pl->events, 0, gfp);
 97	if (err)
 98		return err;
 99	pl->period = 0;
100	raw_spin_lock_init(&pl->lock);
101	return 0;
102}
103
104void fprop_local_destroy_percpu(struct fprop_local_percpu *pl)
105{
106	percpu_counter_destroy(&pl->events);
107}
108
109static void fprop_reflect_period_percpu(struct fprop_global *p,
110					struct fprop_local_percpu *pl)
111{
112	unsigned int period = p->period;
113	unsigned long flags;
114
115	/* Fast path - period didn't change */
116	if (pl->period == period)
117		return;
118	raw_spin_lock_irqsave(&pl->lock, flags);
119	/* Someone updated pl->period while we were spinning? */
120	if (pl->period >= period) {
121		raw_spin_unlock_irqrestore(&pl->lock, flags);
122		return;
123	}
124	/* Aging zeroed our fraction? */
125	if (period - pl->period < BITS_PER_LONG) {
126		s64 val = percpu_counter_read(&pl->events);
127
128		if (val < (nr_cpu_ids * PROP_BATCH))
129			val = percpu_counter_sum(&pl->events);
130
131		percpu_counter_add_batch(&pl->events,
132			-val + (val >> (period-pl->period)), PROP_BATCH);
133	} else
134		percpu_counter_set(&pl->events, 0);
135	pl->period = period;
136	raw_spin_unlock_irqrestore(&pl->lock, flags);
137}
138
139/* Event of type pl happened */
140void __fprop_add_percpu(struct fprop_global *p, struct fprop_local_percpu *pl,
141		long nr)
142{
143	fprop_reflect_period_percpu(p, pl);
144	percpu_counter_add_batch(&pl->events, nr, PROP_BATCH);
145	percpu_counter_add(&p->events, nr);
146}
147
148void fprop_fraction_percpu(struct fprop_global *p,
149			   struct fprop_local_percpu *pl,
150			   unsigned long *numerator, unsigned long *denominator)
151{
152	unsigned int seq;
153	s64 num, den;
154
155	do {
156		seq = read_seqcount_begin(&p->sequence);
157		fprop_reflect_period_percpu(p, pl);
158		num = percpu_counter_read_positive(&pl->events);
159		den = percpu_counter_read_positive(&p->events);
160	} while (read_seqcount_retry(&p->sequence, seq));
161
162	/*
163	 * Make fraction <= 1 and denominator > 0 even in presence of percpu
164	 * counter errors
165	 */
166	if (den <= num) {
167		if (num)
168			den = num;
169		else
170			den = 1;
171	}
172	*denominator = den;
173	*numerator = num;
174}
175
176/*
177 * Like __fprop_add_percpu() except that event is counted only if the given
178 * type has fraction smaller than @max_frac/FPROP_FRAC_BASE
179 */
180void __fprop_add_percpu_max(struct fprop_global *p,
181		struct fprop_local_percpu *pl, int max_frac, long nr)
182{
183	if (unlikely(max_frac < FPROP_FRAC_BASE)) {
184		unsigned long numerator, denominator;
185		s64 tmp;
186
187		fprop_fraction_percpu(p, pl, &numerator, &denominator);
188		/* Adding 'nr' to fraction exceeds max_frac/FPROP_FRAC_BASE? */
189		tmp = (u64)denominator * max_frac -
190					((u64)numerator << FPROP_FRAC_SHIFT);
191		if (tmp < 0) {
192			/* Maximum fraction already exceeded? */
193			return;
194		} else if (tmp < nr * (FPROP_FRAC_BASE - max_frac)) {
195			/* Add just enough for the fraction to saturate */
196			nr = div_u64(tmp + FPROP_FRAC_BASE - max_frac - 1,
197					FPROP_FRAC_BASE - max_frac);
198		}
199	}
200
201	__fprop_add_percpu(p, pl, nr);
202}