Linux Kernel  3.7.1
flex_proportions.c
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1 /*
2  * Floating proportions with flexible aging period
3  *
4  * Copyright (C) 2011, SUSE, Jan Kara <[email protected]>
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
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;
47  return 0;
48 }
49
51 {
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  */
63 bool 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 */
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
94 {
95  pl->events = 0;
96  pl->period = 0;
98  return 0;
99 }
100
102 {
103 }
104
105 static 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 */
131 {
132  fprop_reflect_period_single(p, pl);
133  pl->events++;
135 }
136
137 /* Return fraction of events of type pl */
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 {
147  fprop_reflect_period_single(p, pl);
148  num = pl->events;
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
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
184 {
186 }
187
188 static 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) {
206
207  if (val < (nr_cpu_ids * PROP_BATCH))
208  val = percpu_counter_sum(&pl->events);
209
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 */
220 {
221  fprop_reflect_period_percpu(p, pl);
224 }
225
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 {
235  fprop_reflect_period_percpu(p, pl);
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  */
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);