03144b5869
To ensure ewma_read() without a lock returns a valid but possibly out of date average, modify ewma_add() by using ACCESS_ONCE to prevent intermediate wrong values from being written to avg->internal. Suggested-by: Eric Dumazet <eric.dumazet@gmail.com> Acked-by: Michael S. Tsirkin <mst@redhat.com> Acked-by: Eric Dumazet <edumazet@google.com> Signed-off-by: Michael Dalton <mwdalton@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
64 lines
1.9 KiB
C
64 lines
1.9 KiB
C
/*
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* lib/average.c
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*
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* This source code is licensed under the GNU General Public License,
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* Version 2. See the file COPYING for more details.
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*/
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#include <linux/export.h>
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#include <linux/average.h>
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#include <linux/kernel.h>
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#include <linux/bug.h>
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#include <linux/log2.h>
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/**
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* DOC: Exponentially Weighted Moving Average (EWMA)
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*
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* These are generic functions for calculating Exponentially Weighted Moving
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* Averages (EWMA). We keep a structure with the EWMA parameters and a scaled
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* up internal representation of the average value to prevent rounding errors.
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* The factor for scaling up and the exponential weight (or decay rate) have to
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* be specified thru the init fuction. The structure should not be accessed
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* directly but only thru the helper functions.
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*/
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/**
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* ewma_init() - Initialize EWMA parameters
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* @avg: Average structure
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* @factor: Factor to use for the scaled up internal value. The maximum value
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* of averages can be ULONG_MAX/(factor*weight). For performance reasons
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* factor has to be a power of 2.
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* @weight: Exponential weight, or decay rate. This defines how fast the
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* influence of older values decreases. For performance reasons weight has
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* to be a power of 2.
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*
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* Initialize the EWMA parameters for a given struct ewma @avg.
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*/
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void ewma_init(struct ewma *avg, unsigned long factor, unsigned long weight)
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{
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WARN_ON(!is_power_of_2(weight) || !is_power_of_2(factor));
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avg->weight = ilog2(weight);
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avg->factor = ilog2(factor);
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avg->internal = 0;
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}
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EXPORT_SYMBOL(ewma_init);
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/**
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* ewma_add() - Exponentially weighted moving average (EWMA)
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* @avg: Average structure
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* @val: Current value
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*
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* Add a sample to the average.
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*/
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struct ewma *ewma_add(struct ewma *avg, unsigned long val)
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{
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unsigned long internal = ACCESS_ONCE(avg->internal);
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ACCESS_ONCE(avg->internal) = internal ?
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(((internal << avg->weight) - internal) +
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(val << avg->factor)) >> avg->weight :
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(val << avg->factor);
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return avg;
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}
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EXPORT_SYMBOL(ewma_add);
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