linux-kernel - Re: [PATCH v3] sched: cpufreq: Rename map_util_perf to sugov_apply_dvfs

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Message-ID: <bfcedd2d-6459-4611-8c4a-08cfdbdfbc5c@arm.com>
Date: Fri, 16 Aug 2024 16:44:45 +0100
From: Christian Loehle <christian.loehle@....com>
To: Qais Yousef <qyousef@...alina.io>, Ingo Molnar <mingo@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, "Rafael J. Wysocki"
 <rafael@...nel.org>, Viresh Kumar <viresh.kumar@...aro.org>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>
Cc: linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH v3] sched: cpufreq: Rename map_util_perf to
 sugov_apply_dvfs_headroom

On 8/9/24 00:44, Qais Yousef wrote:
> We are providing headroom for the utilization to grow until the next
> decision point to pick the next frequency. Give the function a better
> name and give it some documentation. It is not really mapping anything.
> 
> Also move it to cpufreq_schedutil.c. This function relies on updating
> util signal appropriately to give a headroom to grow. This is tied to
> schedutil and scheduler and not something that can be shared with other
> governors.
> 
> Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
> Acked-by: Rafael J. Wysocki <rafael@...nel.org>
> Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
> Signed-off-by: Qais Yousef <qyousef@...alina.io>
> ---
> 
> Changes in v3:
> 
> 	1. Add Reviewed-by from Vincent
> 
> Changes in v2:
> 
> 	1. Add Acked-by from Viresh and Raphael (Thanks!)
> 	2. Move the function to cpufreq_schedutil.c instead of sched.h
> 	3. Name space the function with sugov_ to indicate it is special to
> 	   this governor only and not generic.
> 
>  include/linux/sched/cpufreq.h    |  5 -----
>  kernel/sched/cpufreq_schedutil.c | 20 +++++++++++++++++++-
>  2 files changed, 19 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/sched/cpufreq.h b/include/linux/sched/cpufreq.h
> index bdd31ab93bc5..d01755d3142f 100644
> --- a/include/linux/sched/cpufreq.h
> +++ b/include/linux/sched/cpufreq.h
> @@ -28,11 +28,6 @@ static inline unsigned long map_util_freq(unsigned long util,
>  {
>  	return freq * util / cap;
>  }
> -
> -static inline unsigned long map_util_perf(unsigned long util)
> -{
> -	return util + (util >> 2);
> -}
>  #endif /* CONFIG_CPU_FREQ */
>  
>  #endif /* _LINUX_SCHED_CPUFREQ_H */
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index eece6244f9d2..575df3599813 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -178,12 +178,30 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
>  	return cpufreq_driver_resolve_freq(policy, freq);
>  }
>  
> +/*
> + * DVFS decision are made at discrete points. If CPU stays busy, the util will
> + * continue to grow, which means it could need to run at a higher frequency
> + * before the next decision point was reached. IOW, we can't follow the util as
> + * it grows immediately, but there's a delay before we issue a request to go to
> + * higher frequency. The headroom caters for this delay so the system continues
> + * to run at adequate performance point.
> + *
> + * This function provides enough headroom to provide adequate performance
> + * assuming the CPU continues to be busy.
> + *
> + * At the moment it is a constant multiplication with 1.25.
> + */
> +static inline unsigned long sugov_apply_dvfs_headroom(unsigned long util)
> +{
> +	return util + (util >> 2);
> +}
> +
>  unsigned long sugov_effective_cpu_perf(int cpu, unsigned long actual,
>  				 unsigned long min,
>  				 unsigned long max)
>  {
>  	/* Add dvfs headroom to actual utilization */
> -	actual = map_util_perf(actual);
> +	actual = sugov_apply_dvfs_headroom(actual);

Maybe you can even get rid of the comment above now.
sugov_apply_dvfs_headroom(actual) is pretty self-explanatory.

Anyway
Reviewed-by: Christian Loehle <christian.loehle@....com>

>  	/* Actually we don't need to target the max performance */
>  	if (actual < max)
>  		max = actual;