linux-kernel - Re: [PATCH 2/2] cpufreq: schedutil: consolidate capacity margin calculation

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Date:   Mon, 25 Sep 2017 14:55:07 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Chris Redpath <Chris.Redpath@....com>,
        Joel Fernandes <joelaf@...gle.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH 2/2] cpufreq: schedutil: consolidate capacity margin
 calculation

On 23-Sep 07:04, Leo Yan wrote:
> Scheduler CFS class has variable 'capacity_margin' to calculate the
> capacity margin, and schedutil governor also needs to compensate the
> same margin for frequency tipping point. Below are formulas used in
> CFS class and schedutil governor separately:
> 
> CFS:       U` = U * capacity_margin / 1024 = U * 1.25
> Schedutil: U` = U + U >> 2 = U + U * 0.25  = U * 1.25
> 
> This patch consolidates the capacity margin calculation so let
> schedutil to use same formula with CFS class. As result this can avoid
> the mismatch issue between schedutil and CFS class after change
> 'capacity_margin' to other values.
> 
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Morten Rasmussen <morten.rasmussen@....com>
> Cc: Chris Redpath <Chris.Redpath@....com>
> Cc: Joel Fernandes <joelaf@...gle.com>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Signed-off-by: Leo Yan <leo.yan@...aro.org>
> ---
>  kernel/sched/cpufreq_schedutil.c | 5 +++--
>  kernel/sched/sched.h             | 1 +
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 9209d83..067abbe 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -155,7 +155,8 @@ static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
>   *
>   * next_freq = C * curr_freq * util_raw / max
>   *
> - * Take C = 1.25 for the frequency tipping point at (util / max) = 0.8.
> + * Take C = capacity_margin / 1024 = 1.25, so it's for the frequency tipping
> + * point at (util / max) = 0.8.
>   *
>   * The lowest driver-supported frequency which is equal or greater than the raw
>   * next_freq (as calculated above) is returned, subject to policy min/max and
> @@ -168,7 +169,7 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
>  	unsigned int freq = arch_scale_freq_invariant() ?
>  				policy->cpuinfo.max_freq : policy->cur;
>  
> -	freq = (freq + (freq >> 2)) * util / max;
> +	freq = (freq * capacity_margin / 1024) * util / max;

The compiler should be smart enough but perhaps you can better use:

        freq *= (capacity_margin >> SCHED_CAPACITY_SHIFT));
        freq *= util / max;

>  	if (freq == sg_policy->cached_raw_freq && sg_policy->next_freq != UINT_MAX)
>  		return sg_policy->next_freq;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 14db76c..cf75bdc 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -52,6 +52,7 @@ struct cpuidle_state;
>  #define TASK_ON_RQ_MIGRATING	2
>  
>  extern __read_mostly int scheduler_running;
> +extern unsigned int capacity_margin __read_mostly;
>  
>  extern unsigned long calc_load_update;
>  extern atomic_long_t calc_load_tasks;
> -- 
> 2.7.4
> 

-- 
#include <best/regards.h>

Patrick Bellasi