4 * Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
8 * The floating proportion is a time derivative with an exponentially decaying
11 * p_{j} = \Sum_{i=0} (dx_{j}/dt_{-i}) / 2^(1+i)
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.
17 * The decaying history gives smooth transitions. The time differential carries
18 * the notion of speed.
20 * The denominator is 2^(1+i) because we want the series to be normalised, ie.
22 * \Sum_{i=0} 1/2^(1+i) = 1
24 * Further more, if we measure time (t) in the same events as x; so that:
32 * Writing this in an iterative fashion we get (dropping the 'd's):
34 * if (++x_{j}, ++t > period)
43 * We optimize away the '/= 2' for the global time delta by noting that:
45 * if (++t > period) t /= 2:
47 * Can be approximated by:
49 * period/2 + (++t % period/2)
51 * [ Furthermore, when we choose period to be 2^n it can be written in terms of
52 * binary operations and wraparound artefacts disappear. ]
54 * Also note that this yields a natural counter of the elapsed periods:
58 * [ Its monotonic increasing property can be applied to mitigate the wrap-
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:
67 * We can then lazily catch up to the global period count every time we are
68 * going to use x_{j}, by doing:
70 * x_{j} /= 2^(c - c_{j}), c_{j} = c
73 #include <linux/proportions.h>
74 #include <linux/rcupdate.h>
76 int prop_descriptor_init(struct prop_descriptor
*pd
, int shift
)
80 if (shift
> PROP_MAX_SHIFT
)
81 shift
= PROP_MAX_SHIFT
;
84 pd
->pg
[0].shift
= shift
;
85 mutex_init(&pd
->mutex
);
86 err
= percpu_counter_init(&pd
->pg
[0].events
, 0);
90 err
= percpu_counter_init(&pd
->pg
[1].events
, 0);
92 percpu_counter_destroy(&pd
->pg
[0].events
);
99 * We have two copies, and flip between them to make it seem like an atomic
100 * update. The update is not really atomic wrt the events counter, but
101 * it is internally consistent with the bit layout depending on shift.
103 * We copy the events count, move the bits around and flip the index.
105 void prop_change_shift(struct prop_descriptor
*pd
, int shift
)
112 if (shift
> PROP_MAX_SHIFT
)
113 shift
= PROP_MAX_SHIFT
;
115 mutex_lock(&pd
->mutex
);
117 index
= pd
->index
^ 1;
118 offset
= pd
->pg
[pd
->index
].shift
- shift
;
122 pd
->pg
[index
].shift
= shift
;
124 local_irq_save(flags
);
125 events
= percpu_counter_sum(&pd
->pg
[pd
->index
].events
);
130 percpu_counter_set(&pd
->pg
[index
].events
, events
);
133 * ensure the new pg is fully written before the switch
137 local_irq_restore(flags
);
142 mutex_unlock(&pd
->mutex
);
146 * wrap the access to the data in an rcu_read_lock() section;
147 * this is used to track the active references.
149 static struct prop_global
*prop_get_global(struct prop_descriptor
*pd
)
157 * match the wmb from vcd_flip()
160 return &pd
->pg
[index
];
163 static void prop_put_global(struct prop_descriptor
*pd
, struct prop_global
*pg
)
170 prop_adjust_shift(int *pl_shift
, unsigned long *pl_period
, int new_shift
)
172 int offset
= *pl_shift
- new_shift
;
178 *pl_period
<<= -offset
;
180 *pl_period
>>= offset
;
182 *pl_shift
= new_shift
;
189 #define PROP_BATCH (8*(1+ilog2(nr_cpu_ids)))
191 int prop_local_init_percpu(struct prop_local_percpu
*pl
)
193 raw_spin_lock_init(&pl
->lock
);
196 return percpu_counter_init(&pl
->events
, 0);
199 void prop_local_destroy_percpu(struct prop_local_percpu
*pl
)
201 percpu_counter_destroy(&pl
->events
);
205 * Catch up with missed period expirations.
212 void prop_norm_percpu(struct prop_global
*pg
, struct prop_local_percpu
*pl
)
214 unsigned long period
= 1UL << (pg
->shift
- 1);
215 unsigned long period_mask
= ~(period
- 1);
216 unsigned long global_period
;
219 global_period
= percpu_counter_read(&pg
->events
);
220 global_period
&= period_mask
;
223 * Fast path - check if the local and global period count still match
224 * outside of the lock.
226 if (pl
->period
== global_period
)
229 raw_spin_lock_irqsave(&pl
->lock
, flags
);
230 prop_adjust_shift(&pl
->shift
, &pl
->period
, pg
->shift
);
233 * For each missed period, we half the local counter.
235 * pl->events >> (global_period - pl->period);
237 period
= (global_period
- pl
->period
) >> (pg
->shift
- 1);
238 if (period
< BITS_PER_LONG
) {
239 s64 val
= percpu_counter_read(&pl
->events
);
241 if (val
< (nr_cpu_ids
* PROP_BATCH
))
242 val
= percpu_counter_sum(&pl
->events
);
244 __percpu_counter_add(&pl
->events
, -val
+ (val
>> period
),
247 percpu_counter_set(&pl
->events
, 0);
249 pl
->period
= global_period
;
250 raw_spin_unlock_irqrestore(&pl
->lock
, flags
);
256 void __prop_inc_percpu(struct prop_descriptor
*pd
, struct prop_local_percpu
*pl
)
258 struct prop_global
*pg
= prop_get_global(pd
);
260 prop_norm_percpu(pg
, pl
);
261 __percpu_counter_add(&pl
->events
, 1, PROP_BATCH
);
262 percpu_counter_add(&pg
->events
, 1);
263 prop_put_global(pd
, pg
);
267 * identical to __prop_inc_percpu, except that it limits this pl's fraction to
268 * @frac/PROP_FRAC_BASE by ignoring events when this limit has been exceeded.
270 void __prop_inc_percpu_max(struct prop_descriptor
*pd
,
271 struct prop_local_percpu
*pl
, long frac
)
273 struct prop_global
*pg
= prop_get_global(pd
);
275 prop_norm_percpu(pg
, pl
);
277 if (unlikely(frac
!= PROP_FRAC_BASE
)) {
278 unsigned long period_2
= 1UL << (pg
->shift
- 1);
279 unsigned long counter_mask
= period_2
- 1;
280 unsigned long global_count
;
281 long numerator
, denominator
;
283 numerator
= percpu_counter_read_positive(&pl
->events
);
284 global_count
= percpu_counter_read(&pg
->events
);
285 denominator
= period_2
+ (global_count
& counter_mask
);
287 if (numerator
> ((denominator
* frac
) >> PROP_FRAC_SHIFT
))
291 percpu_counter_add(&pl
->events
, 1);
292 percpu_counter_add(&pg
->events
, 1);
295 prop_put_global(pd
, pg
);
299 * Obtain a fraction of this proportion
301 * p_{j} = x_{j} / (period/2 + t % period/2)
303 void prop_fraction_percpu(struct prop_descriptor
*pd
,
304 struct prop_local_percpu
*pl
,
305 long *numerator
, long *denominator
)
307 struct prop_global
*pg
= prop_get_global(pd
);
308 unsigned long period_2
= 1UL << (pg
->shift
- 1);
309 unsigned long counter_mask
= period_2
- 1;
310 unsigned long global_count
;
312 prop_norm_percpu(pg
, pl
);
313 *numerator
= percpu_counter_read_positive(&pl
->events
);
315 global_count
= percpu_counter_read(&pg
->events
);
316 *denominator
= period_2
+ (global_count
& counter_mask
);
318 prop_put_global(pd
, pg
);
325 int prop_local_init_single(struct prop_local_single
*pl
)
327 raw_spin_lock_init(&pl
->lock
);
334 void prop_local_destroy_single(struct prop_local_single
*pl
)
339 * Catch up with missed period expirations.
342 void prop_norm_single(struct prop_global
*pg
, struct prop_local_single
*pl
)
344 unsigned long period
= 1UL << (pg
->shift
- 1);
345 unsigned long period_mask
= ~(period
- 1);
346 unsigned long global_period
;
349 global_period
= percpu_counter_read(&pg
->events
);
350 global_period
&= period_mask
;
353 * Fast path - check if the local and global period count still match
354 * outside of the lock.
356 if (pl
->period
== global_period
)
359 raw_spin_lock_irqsave(&pl
->lock
, flags
);
360 prop_adjust_shift(&pl
->shift
, &pl
->period
, pg
->shift
);
362 * For each missed period, we half the local counter.
364 period
= (global_period
- pl
->period
) >> (pg
->shift
- 1);
365 if (likely(period
< BITS_PER_LONG
))
366 pl
->events
>>= period
;
369 pl
->period
= global_period
;
370 raw_spin_unlock_irqrestore(&pl
->lock
, flags
);
376 void __prop_inc_single(struct prop_descriptor
*pd
, struct prop_local_single
*pl
)
378 struct prop_global
*pg
= prop_get_global(pd
);
380 prop_norm_single(pg
, pl
);
382 percpu_counter_add(&pg
->events
, 1);
383 prop_put_global(pd
, pg
);
387 * Obtain a fraction of this proportion
389 * p_{j} = x_{j} / (period/2 + t % period/2)
391 void prop_fraction_single(struct prop_descriptor
*pd
,
392 struct prop_local_single
*pl
,
393 long *numerator
, long *denominator
)
395 struct prop_global
*pg
= prop_get_global(pd
);
396 unsigned long period_2
= 1UL << (pg
->shift
- 1);
397 unsigned long counter_mask
= period_2
- 1;
398 unsigned long global_count
;
400 prop_norm_single(pg
, pl
);
401 *numerator
= pl
->events
;
403 global_count
= percpu_counter_read(&pg
->events
);
404 *denominator
= period_2
+ (global_count
& counter_mask
);
406 prop_put_global(pd
, pg
);
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