Commit 7024b18c authored by Rasmus Villemoes's avatar Rasmus Villemoes Committed by Rafael J. Wysocki
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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: default avatarRasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
parent 18558cae

drivers/cpuidle/governors/menu.c

+17 −18
Original line number Diff line number Diff line
@@ -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;

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