linux-kernel - Re: [RFC/RFT][PATCH v6] cpuidle: New timer events oriented governor for tickless systems

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Date:   Fri, 30 Nov 2018 09:51:19 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Doug Smythies <dsmythies@...us.net>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Giovanni Gherdovich <ggherdovich@...e.cz>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [RFC/RFT][PATCH v6] cpuidle: New timer events oriented governor
 for tickless systems

Hi Doug,

On Fri, Nov 30, 2018 at 8:49 AM Doug Smythies <dsmythies@...us.net> wrote:
>
> Hi Rafael,
>
> On 2018.11.23 02:36 Rafael J. Wysocki wrote:
>
> ... [snip]...
>
> > +/**
> > + * teo_find_shallower_state - Find shallower idle state matching given duration.
> > + * @drv: cpuidle driver containing state data.
> > + * @dev: Target CPU.
> > + * @state_idx: Index of the capping idle state.
> > + * @duration_us: Idle duration value to match.
> > + */
> > +static int teo_find_shallower_state(struct cpuidle_driver *drv,
> > +                                 struct cpuidle_device *dev, int state_idx,
> > +                                 unsigned int duration_us)
> > +{
> > +     int i;
> > +
> > +     for (i = state_idx - 1; i > 0; i--) {
> > +             if (drv->states[i].disabled || dev->states_usage[i].disable)
> > +                     continue;
> > +
> > +             if (drv->states[i].target_residency <= duration_us)
> > +                     break;
> > +     }
> > +     return i;
> > +}
>
> I think this subroutine has a problem when idle state 0
> is disabled.

You are right, thanks!

> Perhaps something like this might help:
>
> diff --git a/drivers/cpuidle/governors/teo.c b/drivers/cpuidle/governors/teo.c
> index bc1c9a2..5b97639 100644
> --- a/drivers/cpuidle/governors/teo.c
> +++ b/drivers/cpuidle/governors/teo.c
> @@ -196,7 +196,8 @@ static void teo_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
>  }
>
>  /**
> - * teo_find_shallower_state - Find shallower idle state matching given duration.
> + * teo_find_shallower_state - Find shallower idle state matching given
> + * duration, if possible.
>   * @drv: cpuidle driver containing state data.
>   * @dev: Target CPU.
>   * @state_idx: Index of the capping idle state.
> @@ -208,13 +209,15 @@ static int teo_find_shallower_state(struct cpuidle_driver *drv,
>  {
>         int i;
>
> -       for (i = state_idx - 1; i > 0; i--) {
> +       for (i = state_idx - 1; i >= 0; i--) {
>                 if (drv->states[i].disabled || dev->states_usage[i].disable)
>                         continue;
>
>                 if (drv->states[i].target_residency <= duration_us)
>                         break;
>         }
> +       if (i < 0)
> +               i = state_idx;
>         return i;
>  }

I'll do something slightly similar, but equivalent.

>
> @@ -264,7 +267,6 @@ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
>                         if (max_early_idx >= 0 &&
>                             count < cpu_data->states[i].early_hits)
>                                 count = cpu_data->states[i].early_hits;
> -
>                         continue;
>                 }
>
> There is an additional issue where if idle state 0 is disabled (with the above suggested code patch),
> idle state usage seems to fall to deeper states than idle state 1.
> This is not the expected behaviour.

No, it isn't.

> Kernel 4.20-rc3 works as expected.
> I have not figured this issue out yet, in the code.
>
> Example (1 minute per sample. Number of entries/exits per state):
>     State 0     State 1     State 2     State 3     State 4    Watts
>    28235143,         83,         26,         17,        837,  64.900
>     5583238,     657079,    5884941,    8498552,   30986831,  62.433 << Transition sample, after idle state 0 disabled
>           0,     793517,    7186099,   10559878,   38485721,  61.900 << ?? should have all gone into Idle state 1
>           0,     795414,    7340703,   10553117,   38513456,  62.050
>           0,     807028,    7288195,   10574113,   38523524,  62.167
>           0,     814983,    7403534,   10575108,   38571228,  62.167
>           0,     838302,    7747127,   10552289,   38556054,  62.183
>     9664999,     544473,    4914512,    6942037,   25295361,  63.633 << Transition sample, after idle state 0 enabled
>    27893504,         96,         40,          9,        912,  66.500
>    26556343,         83,         29,          7,        814,  66.683
>    27929227,         64,         20,         10,        931,  66.683

I see.

OK, I'll look into this too, thanks!