linux-kernel - Re: [PATCH v1 4/4] cpuidle: governors: teo: Decay metrics below DECAY

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Message-ID: <CAJZ5v0jcGsFh1ATM-Aw1oxZy-zazm+GaMUC4gwEaCskn9V-amg@mail.gmail.com>
Date: Wed, 12 Nov 2025 18:51:53 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Christian Loehle <christian.loehle@....com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Linux PM <linux-pm@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, Reka Norman <rekanorman@...omium.org>
Subject: Re: [PATCH v1 4/4] cpuidle: governors: teo: Decay metrics below
 DECAY_SHIFT threshold

On Wed, Nov 12, 2025 at 6:29 PM Christian Loehle
<christian.loehle@....com> wrote:
>
> On 11/12/25 16:25, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.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@....com>
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.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.

Ah, good point.

> Any reason not to do something like:

Well, it would be more straightforward to just compute "total" from
scratch instead of using total_decay (it would be the same amount of
computation minus the teo_decay() changes AFAICS).

I'll send an update of this patch.