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authorRasmus Villemoes <linux@rasmusvillemoes.dk>2016-02-16 20:19:18 +0100
committerRafael J. Wysocki <rafael.j.wysocki@intel.com>2016-02-17 00:27:16 +0100
commit7024b18ca461bab45e5fb329f6e3d904d5109401 (patch)
tree2dd0d4bc8479ccf843094e6c584fafb7dd3260ac /drivers/cpuidle/governors
parent18558cae0272f8fd9647e69d3fec1565a7949865 (diff)
cpuidle: menu: avoid expensive square root computation
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>
Diffstat (limited to 'drivers/cpuidle/governors')
-rw-r--r--drivers/cpuidle/governors/menu.c35
1 files changed, 17 insertions, 18 deletions
diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
index 0742b3296673..beef7ae123ba 100644
--- a/drivers/cpuidle/governors/menu.c
+++ b/drivers/cpuidle/governors/menu.c
@@ -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;