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average: change to declare precision, not factor

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Declaring the factor is counter-intuitive, and people are prone
to using small(-ish) values even when that makes no sense.

Change the DECLARE_EWMA() macro to take the fractional precision,
in bits, rather than a factor, and update all users.

While at it, add some more documentation.

Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
This commit is contained in:
Johannes Berg 2017-02-15 09:49:26 +01:00
parent 48cac18ecf
commit eb1e011a14
7 changed files with 47 additions and 26 deletions
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61
include/linux/average.h
View file

@ -1,45 +1,66 @@
#ifndef _LINUX_AVERAGE_H
#define _LINUX_AVERAGE_H
/* Exponentially weighted moving average (EWMA) */
/*
* Exponentially weighted moving average (EWMA)
*
* This implements a fixed-precision EWMA algorithm, with both the
* precision and fall-off coefficient determined at compile-time
* and built into the generated helper funtions.
*
* The first argument to the macro is the name that will be used
* for the struct and helper functions.
*
* The second argument, the precision, expresses how many bits are
* used for the fractional part of the fixed-precision values.
*
* The third argument, the weight reciprocal, determines how the
* new values will be weighed vs. the old state, new values will
* get weight 1/weight_rcp and old values 1-1/weight_rcp. Note
* that this parameter must be a power of two for efficiency.
*/
#define DECLARE_EWMA(name, _factor, _weight) \
#define DECLARE_EWMA(name, _precision, _weight_rcp) \
struct ewma_##name { \
unsigned long internal; \
}; \
static inline void ewma_##name##_init(struct ewma_##name *e) \
{ \
BUILD_BUG_ON(!__builtin_constant_p(_factor)); \
BUILD_BUG_ON(!__builtin_constant_p(_weight)); \
BUILD_BUG_ON_NOT_POWER_OF_2(_factor); \
BUILD_BUG_ON_NOT_POWER_OF_2(_weight); \
BUILD_BUG_ON(!__builtin_constant_p(_precision)); \
BUILD_BUG_ON(!__builtin_constant_p(_weight_rcp)); \
/* \
* Even if you want to feed it just 0/1 you should have \
* some bits for the non-fractional part... \
*/ \
BUILD_BUG_ON((_precision) > 30); \
BUILD_BUG_ON_NOT_POWER_OF_2(_weight_rcp); \
e->internal = 0; \
} \
static inline unsigned long \
ewma_##name##_read(struct ewma_##name *e) \
{ \
BUILD_BUG_ON(!__builtin_constant_p(_factor)); \
BUILD_BUG_ON(!__builtin_constant_p(_weight)); \
BUILD_BUG_ON_NOT_POWER_OF_2(_factor); \
BUILD_BUG_ON_NOT_POWER_OF_2(_weight); \
return e->internal >> ilog2(_factor); \
BUILD_BUG_ON(!__builtin_constant_p(_precision)); \
BUILD_BUG_ON(!__builtin_constant_p(_weight_rcp)); \
BUILD_BUG_ON((_precision) > 30); \
BUILD_BUG_ON_NOT_POWER_OF_2(_weight_rcp); \
return e->internal >> (_precision); \
} \
static inline void ewma_##name##_add(struct ewma_##name *e, \
unsigned long val) \
{ \
unsigned long internal = ACCESS_ONCE(e->internal); \
unsigned long weight = ilog2(_weight); \
unsigned long factor = ilog2(_factor); \
unsigned long weight_rcp = ilog2(_weight_rcp); \
unsigned long precision = _precision; \
\
BUILD_BUG_ON(!__builtin_constant_p(_factor)); \
BUILD_BUG_ON(!__builtin_constant_p(_weight)); \
BUILD_BUG_ON_NOT_POWER_OF_2(_factor); \
BUILD_BUG_ON_NOT_POWER_OF_2(_weight); \
BUILD_BUG_ON(!__builtin_constant_p(_precision)); \
BUILD_BUG_ON(!__builtin_constant_p(_weight_rcp)); \
BUILD_BUG_ON((_precision) > 30); \
BUILD_BUG_ON_NOT_POWER_OF_2(_weight_rcp); \
\
ACCESS_ONCE(e->internal) = internal ? \
(((internal << weight) - internal) + \
(val << factor)) >> weight : \
(val << factor); \
(((internal << weight_rcp) - internal) + \
(val << precision)) >> weight_rcp : \
(val << precision); \
}
#endif /* _LINUX_AVERAGE_H */