linux-kernel - Re: [PATCH 1/2] cpufreq_schedutil: Refactor sugov_cpu_is

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Date: Wed, 19 Jun 2024 15:07:32 +0100
From: Christian Loehle <christian.loehle@....com>
To: Tejun Heo <tj@...nel.org>, rafael@...nel.org, viresh.kumar@...aro.org
Cc: linux-pm@...r.kernel.org, void@...ifault.com,
 linux-kernel@...r.kernel.org, kernel-team@...a.com, mingo@...hat.com,
 peterz@...radead.org, David Vernet <dvernet@...a.com>,
 "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH 1/2] cpufreq_schedutil: Refactor sugov_cpu_is_busy()

On 6/19/24 04:12, Tejun Heo wrote:
> sugov_cpu_is_busy() is used to avoid decreasing performance level while the
> CPU is busy and called by sugov_update_single_freq() and
> sugov_update_single_perf(). Both callers repeat the same pattern to first
> test for uclamp and then the business. Let's refactor so that the tests
> aren't repeated.
> 
> The new helper is named sugov_hold_freq() and tests both the uclamp
> exception and CPU business. No functional changes. This will make adding
> more exception conditions easier.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Reviewed-by: David Vernet <dvernet@...a.com>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Cc: Viresh Kumar <viresh.kumar@...aro.org>
> ---
>  kernel/sched/cpufreq_schedutil.c | 38 +++++++++++++++-----------------
>  1 file changed, 18 insertions(+), 20 deletions(-)
> 
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index eece6244f9d2..972b7dd65af2 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -325,16 +325,27 @@ static unsigned long sugov_iowait_apply(struct sugov_cpu *sg_cpu, u64 time,
>  }
>  
>  #ifdef CONFIG_NO_HZ_COMMON
> -static bool sugov_cpu_is_busy(struct sugov_cpu *sg_cpu)
> +static bool sugov_hold_freq(struct sugov_cpu *sg_cpu)
>  {
> -	unsigned long idle_calls = tick_nohz_get_idle_calls_cpu(sg_cpu->cpu);
> -	bool ret = idle_calls == sg_cpu->saved_idle_calls;
> +	unsigned long idle_calls;
> +	bool ret;
> +
> +	/* if capped by uclamp_max, always update to be in compliance */
> +	if (uclamp_rq_is_capped(cpu_rq(sg_cpu->cpu)))
> +		return false;
> +
> +	/*
> +	 * Maintain the frequency if the CPU has not been idle recently, as
> +	 * reduction is likely to be premature.
> +	 */
> +	idle_calls = tick_nohz_get_idle_calls_cpu(sg_cpu->cpu);
> +	ret = idle_calls == sg_cpu->saved_idle_calls;
>  
>  	sg_cpu->saved_idle_calls = idle_calls;
>  	return ret;
>  }
>  #else
> -static inline bool sugov_cpu_is_busy(struct sugov_cpu *sg_cpu) { return false; }
> +static inline bool sugov_hold_freq(struct sugov_cpu *sg_cpu) { return false; }
>  #endif /* CONFIG_NO_HZ_COMMON */
>  
>  /*
> @@ -382,14 +393,8 @@ static void sugov_update_single_freq(struct update_util_data *hook, u64 time,
>  		return;
>  
>  	next_f = get_next_freq(sg_policy, sg_cpu->util, max_cap);
> -	/*
> -	 * Do not reduce the frequency if the CPU has not been idle
> -	 * recently, as the reduction is likely to be premature then.
> -	 *
> -	 * Except when the rq is capped by uclamp_max.
> -	 */
> -	if (!uclamp_rq_is_capped(cpu_rq(sg_cpu->cpu)) &&
> -	    sugov_cpu_is_busy(sg_cpu) && next_f < sg_policy->next_freq &&
> +
> +	if (sugov_hold_freq(sg_cpu) && next_f < sg_policy->next_freq &&
>  	    !sg_policy->need_freq_update) {
>  		next_f = sg_policy->next_freq;
>  

Not necessarily related to your changes, but in case you're touching this
again, maybe sugov_hold_freq() could be the last condition?
And do we want something like
#ifdef CONFIG_NO_HZ_COMMON                                                      
else
	sg_cpu->saved_idle_calls = tick_nohz_get_idle_calls_cpu(sg_cpu->cpu);
#endif
here?

> @@ -436,14 +441,7 @@ static void sugov_update_single_perf(struct update_util_data *hook, u64 time,
>  	if (!sugov_update_single_common(sg_cpu, time, max_cap, flags))
>  		return;
>  
> -	/*
> -	 * Do not reduce the target performance level if the CPU has not been
> -	 * idle recently, as the reduction is likely to be premature then.
> -	 *
> -	 * Except when the rq is capped by uclamp_max.
> -	 */
> -	if (!uclamp_rq_is_capped(cpu_rq(sg_cpu->cpu)) &&
> -	    sugov_cpu_is_busy(sg_cpu) && sg_cpu->util < prev_util)
> +	if (sugov_hold_freq(sg_cpu) && sg_cpu->util < prev_util)
>  		sg_cpu->util = prev_util;
>  
>  	cpufreq_driver_adjust_perf(sg_cpu->cpu, sg_cpu->bw_min,

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