linux-kernel - Re: [PATCH v1] cpuidle: teo: Update idle duration estimate when choosing shallower state

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Date:   Thu, 27 Jul 2023 22:12:56 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Linux PM <linux-pm@...r.kernel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        Kajetan Puchalski <kajetan.puchalski@....com>
Subject: Re: [PATCH v1] cpuidle: teo: Update idle duration estimate when
 choosing shallower state

On Thu, Jul 27, 2023 at 10:05 PM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>
> The TEO governor takes CPU utilization into account by refining idle state
> selection when the utilization is above a certain threshold.  The idle state
> selection is then refined by choosing an idle state shallower than the
> previously selected one.
>
> However, when this is done, the idle duration estimate needs to be updated
> so as to prevent the scheduler tick from being stopped while the candidate
> idle state is shallow, which may lead to excessive energy usage if the CPU
> is not interrupted quickly enough going forward.  Moreover, in case the
> scheduler tick has been stopped already and the new idle duration estimate
> is too small, the replacement candidate state cannot be used.
>
> Modify the relevant code to take the above observations into account.
>
> Fixes: 9ce0f7c4bc64 ("cpuidle: teo: Introduce util-awareness")
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>
> @Peter: This doesn't attempt to fix the tick stopping problem, it just makes
> the current behavior consistent.
>
> @Anna-Maria: This is likely to basically prevent the tick from being stopped
> at all if the CPU utilization is above a certain threshold.  I'm wondering if
> your results will be affected by it and in what way.
>
> ---
>  drivers/cpuidle/governors/teo.c |   33 ++++++++++++++++++++++++++-------
>  1 file changed, 26 insertions(+), 7 deletions(-)
>
> Index: linux-pm/drivers/cpuidle/governors/teo.c
> ===================================================================
> --- linux-pm.orig/drivers/cpuidle/governors/teo.c
> +++ linux-pm/drivers/cpuidle/governors/teo.c
> @@ -397,13 +397,22 @@ static int teo_select(struct cpuidle_dri
>          * the shallowest non-polling state and exit.
>          */
>         if (drv->state_count < 3 && cpu_data->utilized) {
> -               for (i = 0; i < drv->state_count; ++i) {
> -                       if (!dev->states_usage[i].disable &&
> -                           !(drv->states[i].flags & CPUIDLE_FLAG_POLLING)) {
> -                               idx = i;
> +               /*
> +                * If state 0 is enabled and it is not a polling one, select it
> +                * right away and update the idle duration estimate accordingly,
> +                * unless the scheduler tick has been stopped.
> +                */
> +               if (!idx && !(drv->states[0].flags & CPUIDLE_FLAG_POLLING)) {
> +                       s64 span_ns = teo_middle_of_bin(0, drv);
> +
> +                       if (teo_time_ok(span_ns)) {
> +                               duration_ns = span_ns;
>                                 goto end;
>                         }
>                 }
> +               /* Assume that state 1 is not a polling one and select it. */

Well, I should also check if it is not disabled.  Will send a v2 tomorrow.

> +               idx = 1;
> +               goto end;
>         }
>
>         /*
> @@ -539,10 +548,20 @@ static int teo_select(struct cpuidle_dri
>
>         /*
>          * If the CPU is being utilized over the threshold, choose a shallower
> -        * non-polling state to improve latency
> +        * non-polling state to improve latency, unless the scheduler tick has
> +        * been stopped already and the shallower state's target residency is
> +        * not sufficiently large.
>          */
> -       if (cpu_data->utilized)
> -               idx = teo_find_shallower_state(drv, dev, idx, duration_ns, true);
> +       if (cpu_data->utilized) {
> +               s64 span_ns;
> +
> +               i = teo_find_shallower_state(drv, dev, idx, duration_ns, true);
> +               span_ns = teo_middle_of_bin(i, drv);
> +               if (teo_time_ok(span_ns)) {
> +                       idx = i;
> +                       duration_ns = span_ns;
> +               }
> +       }
>
>  end:
>         /*
>
>
>