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cpuidle: menu: avoid expensive square root computation

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Computing the integer square root is a rather expensive operation, at
least compared to doing a 64x64 -> 64 multiply (avg*avg) and, on 64
bit platforms, doing an extra comparison to a constant (variance <=
U64_MAX/36).

On 64 bit platforms, this does mean that we add a restriction on the
range of the variance where we end up using the estimate (since
previously the stddev <= ULONG_MAX was a tautology), but on the other
hand, we extend the range quite substantially on 32 bit platforms - in
both cases, we now allow standard deviations up to 715 seconds, which
is for example guaranteed if all observations are less than 1430
seconds.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
This commit is contained in:
Rasmus Villemoes 2016-02-16 20:19:18 +01:00 committed by Rafael J. Wysocki
parent 18558cae02
commit 7024b18ca4
1 changed files with 17 additions and 18 deletions
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35
drivers/cpuidle/governors/menu.c
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@ -200,7 +200,7 @@ static void get_typical_interval(struct menu_device *data)
{
int i, divisor;
unsigned int max, thresh;
uint64_t avg, stddev;
uint64_t avg, variance;
thresh = UINT_MAX; /* Discard outliers above this value */
@ -224,36 +224,35 @@ again:
else
do_div(avg, divisor);
/* Then try to determine standard deviation */
stddev = 0;
/* Then try to determine variance */
variance = 0;
for (i = 0; i < INTERVALS; i++) {
unsigned int value = data->intervals[i];
if (value <= thresh) {
int64_t diff = value - avg;
stddev += diff * diff;
variance += diff * diff;
}
}
if (divisor == INTERVALS)
stddev >>= INTERVAL_SHIFT;
variance >>= INTERVAL_SHIFT;
else
do_div(stddev, divisor);
do_div(variance, divisor);
/*
* The typical interval is obtained when standard deviation is small
* or standard deviation is small compared to the average interval.
*
* int_sqrt() formal parameter type is unsigned long. When the
* greatest difference to an outlier exceeds ~65 ms * sqrt(divisor)
* the resulting squared standard deviation exceeds the input domain
* of int_sqrt on platforms where unsigned long is 32 bits in size.
* In such case reject the candidate average.
* The typical interval is obtained when standard deviation is
* small (stddev <= 20 us, variance <= 400 us^2) or standard
* deviation is small compared to the average interval (avg >
* 6*stddev, avg^2 > 36*variance). The average is smaller than
* UINT_MAX aka U32_MAX, so computing its square does not
* overflow a u64. We simply reject this candidate average if
* the standard deviation is greater than 715 s (which is
* rather unlikely).
*
* Use this result only if there is no timer to wake us up sooner.
*/
if (likely(stddev <= ULONG_MAX)) {
stddev = int_sqrt(stddev);
if (((avg > stddev * 6) && (divisor * 4 >= INTERVALS * 3))
|| stddev <= 20) {
if (likely(variance <= U64_MAX/36)) {
if (((avg*avg > variance*36) && (divisor * 4 >= INTERVALS * 3))
|| variance <= 400) {
if (data->next_timer_us > avg)
data->predicted_us = avg;
return;