Re: [PATCH v1 4/4] cpuidle: governors: teo: Decay metrics below DECAY_SHIFT threshold

[Christian Loehle <christian.loehle@arm.com>]

On 11/12/25 16:25, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> If a given governor metric falls below a certain value (8 for
> DECAY_SHIFT equal to 3), it will not decay any more due to the
> simplistic decay implementation.  This may in some cases lead to
> subtle inconsistencies in the governor behavior, so change the
> decay implementation to take it into account and set the metric
> at hand to 0 in that case.
> 
> Suggested-by: Christian Loehle <christian.loehle@arm.com>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
>  drivers/cpuidle/governors/teo.c |   20 +++++++++++++++-----
>  1 file changed, 15 insertions(+), 5 deletions(-)
> 
> --- a/drivers/cpuidle/governors/teo.c
> +++ b/drivers/cpuidle/governors/teo.c
> @@ -148,6 +148,16 @@ struct teo_cpu {
>  
>  static DEFINE_PER_CPU(struct teo_cpu, teo_cpus);
>  
> +static void teo_decay(unsigned int *metric)
> +{
> +	unsigned int delta = *metric >> DECAY_SHIFT;
> +
> +	if (delta)
> +		*metric -= delta;
> +	else
> +		*metric = 0;
> +}
> +
>  /**
>   * teo_update - Update CPU metrics after wakeup.
>   * @drv: cpuidle driver containing state data.
> @@ -159,7 +169,7 @@ static void teo_update(struct cpuidle_dr
>  	int i, idx_timer = 0, idx_duration = 0;
>  	s64 target_residency_ns, measured_ns;
>  
> -	cpu_data->short_idles -= cpu_data->short_idles >> DECAY_SHIFT;
> +	teo_decay(&cpu_data->short_idles);
>  
>  	if (cpu_data->artificial_wakeup) {
>  		/*
> @@ -195,8 +205,8 @@ static void teo_update(struct cpuidle_dr
>  	for (i = 0; i < drv->state_count; i++) {
>  		struct teo_bin *bin = &cpu_data->state_bins[i];
>  
> -		bin->hits -= bin->hits >> DECAY_SHIFT;
> -		bin->intercepts -= bin->intercepts >> DECAY_SHIFT;
> +		teo_decay(&bin->hits);
> +		teo_decay(&bin->intercepts);
>  
>  		target_residency_ns = drv->states[i].target_residency_ns;
>  
> @@ -207,7 +217,7 @@ static void teo_update(struct cpuidle_dr
>  		}
>  	}
>  
> -	cpu_data->tick_intercepts -= cpu_data->tick_intercepts >> DECAY_SHIFT;
> +	teo_decay(&cpu_data->tick_intercepts);
>  	/*
>  	 * If the measured idle duration falls into the same bin as the sleep
>  	 * length, this is a "hit", so update the "hits" metric for that bin.
> @@ -222,7 +232,7 @@ static void teo_update(struct cpuidle_dr
>  			cpu_data->tick_intercepts += PULSE;
>  	}
>  
> -	cpu_data->total -= cpu_data->total >> DECAY_SHIFT;
> +	teo_decay(&cpu_data->total);
>  	cpu_data->total += PULSE;

This will result in total no longer being a strict sum of the bins.
Any reason not to do something like:

-----8<-----

diff --git a/drivers/cpuidle/governors/teo.c b/drivers/cpuidle/governors/teo.c
index e5b795cf3155..ff58d70ee80d 100644
--- a/drivers/cpuidle/governors/teo.c
+++ b/drivers/cpuidle/governors/teo.c
@@ -148,14 +148,19 @@ struct teo_cpu {
 
 static DEFINE_PER_CPU(struct teo_cpu, teo_cpus);
 
-static void teo_decay(unsigned int *metric)
+static unsigned int teo_decay(unsigned int *metric)
 {
        unsigned int delta = *metric >> DECAY_SHIFT;
+       unsigned int decay;
 
-       if (delta)
+       if (delta) {
                *metric -= delta;
-       else
-               *metric = 0;
+               return delta;
+       }
+
+       decay = *metric;
+       *metric = 0;
+       return decay;
 }
 
 /**
@@ -168,6 +173,7 @@ static void teo_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
        struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);
        int i, idx_timer = 0, idx_duration = 0;
        s64 target_residency_ns, measured_ns;
+       unsigned int total_decay = 0;
 
        teo_decay(&cpu_data->short_idles);
 
@@ -205,8 +211,8 @@ static void teo_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
        for (i = 0; i < drv->state_count; i++) {
                struct teo_bin *bin = &cpu_data->state_bins[i];
 
-               teo_decay(&bin->hits);
-               teo_decay(&bin->intercepts);
+               total_decay += teo_decay(&bin->hits);
+               total_decay += teo_decay(&bin->intercepts);
 
                target_residency_ns = drv->states[i].target_residency_ns;
 
@@ -232,7 +238,7 @@ static void teo_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
                        cpu_data->tick_intercepts += PULSE;
        }
 
-       teo_decay(&cpu_data->total);
+       cpu_data->total -= total_decay;
        cpu_data->total += PULSE;
 }