linux-kernel - Re: [RFT][PATCH v1 4/5] cpuidle: menu: Eliminate outliers on both ends of the sample set

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Message-ID: <3bbd1ada-0d26-4c91-b1f3-a947caf5abd0@arm.com>
Date: Mon, 17 Feb 2025 13:26:40 +0000
From: Christian Loehle <christian.loehle@....com>
To: "Rafael J. Wysocki" <rjw@...ysocki.net>,
 Linux PM <linux-pm@...r.kernel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
 Daniel Lezcano <daniel.lezcano@...aro.org>,
 Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
 Aboorva Devarajan <aboorvad@...ux.ibm.com>
Subject: Re: [RFT][PATCH v1 4/5] cpuidle: menu: Eliminate outliers on both
 ends of the sample set

On 2/6/25 14:26, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> Currently, get_typical_interval() attempts to eliminate outliers at the
> high end of the sample set only (probably in order to bias the prediction
> toward lower values), but this it problematic because if the outliers are
> present at the low end of the sample set, discarding the highest values
> will not help to reduce the variance.
> 
> Since the presence of outliers at the low end of the sample set is
> generally as likely as their presence at the high end of the sample
> set, modify get_typical_interval() to treat samples at the largest
> distances from the average (on both ends of the sample set) as outliers.
> 
> This should increase the likelihood of making a meaningful prediction
> in some cases.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>  drivers/cpuidle/governors/menu.c |   32 ++++++++++++++++++++++----------
>  1 file changed, 22 insertions(+), 10 deletions(-)
> 
> --- a/drivers/cpuidle/governors/menu.c
> +++ b/drivers/cpuidle/governors/menu.c
> @@ -116,30 +116,37 @@
>   */
>  static unsigned int get_typical_interval(struct menu_device *data)
>  {
> -	unsigned int max, divisor, thresh = UINT_MAX;
> +	s64 value, min_thresh = -1, max_thresh = UINT_MAX;
> +	unsigned int max, min, divisor;
>  	u64 avg, variance, avg_sq;
>  	int i;
>  
>  again:
>  	/* Compute the average and variance of past intervals. */
>  	max = 0;
> +	min = UINT_MAX;
>  	avg = 0;
>  	variance = 0;
>  	divisor = 0;
>  	for (i = 0; i < INTERVALS; i++) {
> -		unsigned int value = data->intervals[i];
> -
> -		/* Discard data points above or at the threshold. */
> -		if (value >= thresh)
> +		value = data->intervals[i];
> +		/*
> +		 * Discard the samples outside the interval between the min and
> +		 * max thresholds.
> +		 */
> +		if (value <= min_thresh || value >= max_thresh)
>  			continue;
>  
>  		divisor++;
>  
>  		avg += value;
> -		variance += (u64)value * value;
> +		variance += value * value;
>  
>  		if (value > max)
>  			max = value;
> +
> +		if (value < min)
> +			min = value;
>  	}
>  
>  	if (!max)
> @@ -175,10 +182,10 @@
>  	}
>  
>  	/*
> -	 * If we have outliers to the upside in our distribution, discard
> -	 * those by setting the threshold to exclude these outliers, then
> +	 * If there are outliers, discard them by setting thresholds to exclude
> +	 * data points at a large enough distance from the average, then
>  	 * calculate the average and standard deviation again. Once we get
> -	 * down to the bottom 3/4 of our samples, stop excluding samples.
> +	 * down to the last 3/4 of our samples, stop excluding samples.
>  	 *
>  	 * This can deal with workloads that have long pauses interspersed
>  	 * with sporadic activity with a bunch of short pauses.
> @@ -186,7 +193,12 @@
>  	if ((divisor * 4) <= INTERVALS * 3)
>  		return UINT_MAX;
>  
> -	thresh = max;
> +	/* Update the thresholds for the next round. */
> +	if (avg - min > max - avg)
> +		min_thresh = min;
> +	else
> +		max_thresh = max;
> +
>  	goto again;
>  }

Reviewed-by: Christian Loehle <christian.loehle@....com>