1/*
2 * Floating proportions
3 *
4 *  Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
5 *
6 * Description:
7 *
8 * The floating proportion is a time derivative with an exponentially decaying
9 * history:
10 *
11 *   p_{j} = \Sum_{i=0} (dx_{j}/dt_{-i}) / 2^(1+i)
12 *
13 * Where j is an element from {prop_local}, x_{j} is j's number of events,
14 * and i the time period over which the differential is taken. So d/dt_{-i} is
15 * the differential over the i-th last period.
16 *
17 * The decaying history gives smooth transitions. The time differential carries
18 * the notion of speed.
19 *
20 * The denominator is 2^(1+i) because we want the series to be normalised, ie.
21 *
22 *   \Sum_{i=0} 1/2^(1+i) = 1
23 *
24 * Further more, if we measure time (t) in the same events as x; so that:
25 *
26 *   t = \Sum_{j} x_{j}
27 *
28 * we get that:
29 *
30 *   \Sum_{j} p_{j} = 1
31 *
32 * Writing this in an iterative fashion we get (dropping the 'd's):
33 *
34 *   if (++x_{j}, ++t > period)
35 *     t /= 2;
36 *     for_each (j)
37 *       x_{j} /= 2;
38 *
39 * so that:
40 *
41 *   p_{j} = x_{j} / t;
42 *
43 * We optimize away the '/= 2' for the global time delta by noting that:
44 *
45 *   if (++t > period) t /= 2:
46 *
47 * Can be approximated by:
48 *
49 *   period/2 + (++t % period/2)
50 *
51 * [ Furthermore, when we choose period to be 2^n it can be written in terms of
52 *   binary operations and wraparound artefacts disappear. ]
53 *
54 * Also note that this yields a natural counter of the elapsed periods:
55 *
56 *   c = t / (period/2)
57 *
58 * [ Its monotonic increasing property can be applied to mitigate the wrap-
59 *   around issue. ]
60 *
61 * This allows us to do away with the loop over all prop_locals on each period
62 * expiration. By remembering the period count under which it was last accessed
63 * as c_{j}, we can obtain the number of 'missed' cycles from:
64 *
65 *   c - c_{j}
66 *
67 * We can then lazily catch up to the global period count every time we are
68 * going to use x_{j}, by doing:
69 *
70 *   x_{j} /= 2^(c - c_{j}), c_{j} = c
71 */
72
73#include <linux/proportions.h>
74#include <linux/rcupdate.h>
75
76/*
77 * Limit the time part in order to ensure there are some bits left for the
78 * cycle counter.
79 */
80#define PROP_MAX_SHIFT (3*BITS_PER_LONG/4)
81
82int prop_descriptor_init(struct prop_descriptor *pd, int shift)
83{
84        int err;
85
86        if (shift > PROP_MAX_SHIFT)
87                shift = PROP_MAX_SHIFT;
88
89        pd->index = 0;
90        pd->pg[0].shift = shift;
91        mutex_init(&pd->mutex);
92        err = percpu_counter_init_irq(&pd->pg[0].events, 0);
93        if (err)
94                goto out;
95
96        err = percpu_counter_init_irq(&pd->pg[1].events, 0);
97        if (err)
98                percpu_counter_destroy(&pd->pg[0].events);
99
100out:
101        return err;
102}
103
104/*
105 * We have two copies, and flip between them to make it seem like an atomic
106 * update. The update is not really atomic wrt the events counter, but
107 * it is internally consistent with the bit layout depending on shift.
108 *
109 * We copy the events count, move the bits around and flip the index.
110 */
111void prop_change_shift(struct prop_descriptor *pd, int shift)
112{
113        int index;
114        int offset;
115        u64 events;
116        unsigned long flags;
117
118        if (shift > PROP_MAX_SHIFT)
119                shift = PROP_MAX_SHIFT;
120
121        mutex_lock(&pd->mutex);
122
123        index = pd->index ^ 1;
124        offset = pd->pg[pd->index].shift - shift;
125        if (!offset)
126                goto out;
127
128        pd->pg[index].shift = shift;
129
130        local_irq_save(flags);
131        events = percpu_counter_sum(&pd->pg[pd->index].events);
132        if (offset < 0)
133                events <<= -offset;
134        else
135                events >>= offset;
136        percpu_counter_set(&pd->pg[index].events, events);
137
138        /*
139         * ensure the new pg is fully written before the switch
140         */
141        smp_wmb();
142        pd->index = index;
143        local_irq_restore(flags);
144
145        synchronize_rcu();
146
147out:
148        mutex_unlock(&pd->mutex);
149}
150
151/*
153 * this is used to track the active references.
154 */
155static struct prop_global *prop_get_global(struct prop_descriptor *pd)
156{
157        int index;
158
160        index = pd->index;
161        /*
162         * match the wmb from vcd_flip()
163         */
164        smp_rmb();
165        return &pd->pg[index];
166}
167
168static void prop_put_global(struct prop_descriptor *pd, struct prop_global *pg)
169{
171}
172
173static void
174prop_adjust_shift(int *pl_shift, unsigned long *pl_period, int new_shift)
175{
176        int offset = *pl_shift - new_shift;
177
178        if (!offset)
179                return;
180
181        if (offset < 0)
182                *pl_period <<= -offset;
183        else
184                *pl_period >>= offset;
185
186        *pl_shift = new_shift;
187}
188
189/*
190 * PERCPU
191 */
192
193#define PROP_BATCH (8*(1+ilog2(nr_cpu_ids)))
194
195int prop_local_init_percpu(struct prop_local_percpu *pl)
196{
197        spin_lock_init(&pl->lock);
198        pl->shift = 0;
199        pl->period = 0;
200        return percpu_counter_init_irq(&pl->events, 0);
201}
202
203void prop_local_destroy_percpu(struct prop_local_percpu *pl)
204{
205        percpu_counter_destroy(&pl->events);
206}
207
208/*
209 * Catch up with missed period expirations.
210 *
211 *   until (c_{j} == c)
212 *     x_{j} -= x_{j}/2;
213 *     c_{j}++;
214 */
215static
216void prop_norm_percpu(struct prop_global *pg, struct prop_local_percpu *pl)
217{
218        unsigned long period = 1UL << (pg->shift - 1);
219        unsigned long period_mask = ~(period - 1);
220        unsigned long global_period;
221        unsigned long flags;
222
225
226        /*
227         * Fast path - check if the local and global period count still match
228         * outside of the lock.
229         */
230        if (pl->period == global_period)
231                return;
232
233        spin_lock_irqsave(&pl->lock, flags);
235
236        /*
237         * For each missed period, we half the local counter.
238         * basically:
239         *   pl->events >> (global_period - pl->period);
240         */
241        period = (global_period - pl->period) >> (pg->shift - 1);
242        if (period < BITS_PER_LONG) {
244
245                if (val < (nr_cpu_ids * PROP_BATCH))
246                        val = percpu_counter_sum(&pl->events);
247
248                __percpu_counter_add(&pl->events, -val + (val >> period),
249                                        PROP_BATCH);
250        } else
251                percpu_counter_set(&pl->events, 0);
252
253        pl->period = global_period;
254        spin_unlock_irqrestore(&pl->lock, flags);
255}
256
257/*
258 *   ++x_{j}, ++t
259 */
260void __prop_inc_percpu(struct prop_descriptor *pd, struct prop_local_percpu *pl)
261{
262        struct prop_global *pg = prop_get_global(pd);
263
264        prop_norm_percpu(pg, pl);
267        prop_put_global(pd, pg);
268}
269
270/*
271 * Obtain a fraction of this proportion
272 *
273 *   p_{j} = x_{j} / (period/2 + t % period/2)
274 */
275void prop_fraction_percpu(struct prop_descriptor *pd,
276                struct prop_local_percpu *pl,
277                long *numerator, long *denominator)
278{
279        struct prop_global *pg = prop_get_global(pd);
280        unsigned long period_2 = 1UL << (pg->shift - 1);
281        unsigned long counter_mask = period_2 - 1;
282        unsigned long global_count;
283
284        prop_norm_percpu(pg, pl);
286
288        *denominator = period_2 + (global_count & counter_mask);
289
290        prop_put_global(pd, pg);
291}
292
293/*
294 * SINGLE
295 */
296
297int prop_local_init_single(struct prop_local_single *pl)
298{
299        spin_lock_init(&pl->lock);
300        pl->shift = 0;
301        pl->period = 0;
302        pl->events = 0;
303        return 0;
304}
305
306void prop_local_destroy_single(struct prop_local_single *pl)
307{
308}
309
310/*
311 * Catch up with missed period expirations.
312 */
313static
314void prop_norm_single(struct prop_global *pg, struct prop_local_single *pl)
315{
316        unsigned long period = 1UL << (pg->shift - 1);
317        unsigned long period_mask = ~(period - 1);
318        unsigned long global_period;
319        unsigned long flags;
320
323
324        /*
325         * Fast path - check if the local and global period count still match
326         * outside of the lock.
327         */
328        if (pl->period == global_period)
329                return;
330
331        spin_lock_irqsave(&pl->lock, flags);
333        /*
334         * For each missed period, we half the local counter.
335         */
336        period = (global_period - pl->period) >> (pg->shift - 1);
337        if (likely(period < BITS_PER_LONG))
338                pl->events >>= period;
339        else
340                pl->events = 0;
341        pl->period = global_period;
342        spin_unlock_irqrestore(&pl->lock, flags);
343}
344
345/*
346 *   ++x_{j}, ++t
347 */
348void __prop_inc_single(struct prop_descriptor *pd, struct prop_local_single *pl)
349{
350        struct prop_global *pg = prop_get_global(pd);
351
352        prop_norm_single(pg, pl);
353        pl->events++;
355        prop_put_global(pd, pg);
356}
357
358/*
359 * Obtain a fraction of this proportion
360 *
361 *   p_{j} = x_{j} / (period/2 + t % period/2)
362 */
363void prop_fraction_single(struct prop_descriptor *pd,
364                struct prop_local_single *pl,
365                long *numerator, long *denominator)
366{
367        struct prop_global *pg = prop_get_global(pd);
368        unsigned long period_2 = 1UL << (pg->shift - 1);
369        unsigned long counter_mask = period_2 - 1;
370        unsigned long global_count;
371
372        prop_norm_single(pg, pl);
373        *numerator = pl->events;
374