linux-kernel - Re: [PATCH 2/3] PM: EM: Make em_cpu_energy() able to return bigger values

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Message-ID: <266f4b52-62c5-48bc-2680-1f09b6eb90cc@arm.com>
Date:   Mon, 5 Jul 2021 14:44:52 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Lukasz Luba <lukasz.luba@....com>, linux-kernel@...r.kernel.org
Cc:     Chris.Redpath@....com, morten.rasmussen@....com,
        qperret@...gle.com, linux-pm@...r.kernel.org, peterz@...radead.org,
        rjw@...ysocki.net, viresh.kumar@...aro.org,
        vincent.guittot@...aro.org, mingo@...hat.com,
        juri.lelli@...hat.com, rostedt@...dmis.org, segall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, CCj.Yeh@...iatek.com
Subject: Re: [PATCH 2/3] PM: EM: Make em_cpu_energy() able to return bigger
 values

On 25/06/2021 17:26, Lukasz Luba wrote:
> The Energy Model (EM) em_cpu_energy() is responsible for providing good
> estimation regarding CPUs energy. It contains proper data structures which
> are then used during calculation. The values stored in there are in
> milli-Watts precision (or in abstract scale) smaller that 0xffff, which use

I guess you refer to 'if (... || power > EM_MAX_POWER)' check in
em_create_perf_table() [kernel/power/energy_model.c].

> sufficient unsigned long even on 32-bit machines. There are scenarios where
                                                              ^^^^^^^^^

Can you describe these scenarios better with one example (EAS placement
of an example task on a 2 PD system) which highlights the issue and how
it this patch-set solves it?

In this example you can list all the things which must be there to
create a situation in EAS in which the patch-set helps.

> we would like to provide calculated estimations in a better precision and
> the values might be 1000 times bigger. This patch makes possible to use

Where is this `1000` coming from?

> quite big values for also 32-bit machines.
> 
> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
> ---
>  include/linux/energy_model.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/energy_model.h b/include/linux/energy_model.h
> index 3f221dbf5f95..2016f5a706e0 100644
> --- a/include/linux/energy_model.h
> +++ b/include/linux/energy_model.h
> @@ -101,7 +101,7 @@ void em_dev_unregister_perf_domain(struct device *dev);
>   * Return: the sum of the energy consumed by the CPUs of the domain assuming
>   * a capacity state satisfying the max utilization of the domain.
>   */
> -static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
> +static inline u64 em_cpu_energy(struct em_perf_domain *pd,
>  				unsigned long max_util, unsigned long sum_util,
>  				unsigned long allowed_cpu_cap)
>  {
> @@ -180,7 +180,7 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
>  	 *   pd_nrg = ------------------------                       (4)
>  	 *                  scale_cpu
>  	 */
> -	return ps->cost * sum_util / scale_cpu;
> +	return div_u64((u64)ps->cost * sum_util, scale_cpu);
>  }
>  
>  /**
> @@ -217,7 +217,7 @@ static inline struct em_perf_domain *em_pd_get(struct device *dev)
>  {
>  	return NULL;
>  }
> -static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
> +static inline u64 em_cpu_energy(struct em_perf_domain *pd,
>  			unsigned long max_util, unsigned long sum_util,
>  			unsigned long allowed_cpu_cap)
>  {
>