linux-kernel - Re: [PATCH v2 1/6] cpufreq/sched: Fix the usage of CPUFREQ_NEED_UPDATE

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Message-ID: <aARJdWE8VtWZPpK7@sultan-box.localdomain>
Date: Sun, 20 Apr 2025 11:10:13 +1000
From: Sultan Alsawaf <sultan@...neltoast.com>
To: "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc: Linux PM <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Mario Limonciello <mario.limonciello@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Christian Loehle <christian.loehle@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Valentin Schneider <vschneid@...hat.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2 1/6] cpufreq/sched: Fix the usage of
 CPUFREQ_NEED_UPDATE_LIMITS

On Tue, Apr 15, 2025 at 11:58:08AM +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> Commit 8e461a1cb43d ("cpufreq: schedutil: Fix superfluous updates caused
> by need_freq_update") modified sugov_should_update_freq() to set the
> need_freq_update flag only for drivers with CPUFREQ_NEED_UPDATE_LIMITS
> set, but that flag generally needs to be set when the policy limits
> change because the driver callback may need to be invoked for the new
> limits to take effect.
> 
> However, if the return value of cpufreq_driver_resolve_freq() after
> applying the new limits is still equal to the previously selected
> frequency, the driver callback needs to be invoked only in the case
> when CPUFREQ_NEED_UPDATE_LIMITS is set (which means that the driver
> specifically wants its callback to be invoked every time the policy
> limits change).
> 
> Update the code accordingly to avoid missing policy limits changes for
> drivers without CPUFREQ_NEED_UPDATE_LIMITS.
> 
> Fixes: 8e461a1cb43d ("cpufreq: schedutil: Fix superfluous updates caused by need_freq_update")
> Closes: https://lore.kernel.org/lkml/Z_Tlc6Qs-tYpxWYb@linaro.org/
> Reported-by: Stephan Gerhold <stephan.gerhold@...aro.org>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

Thanks for fixing this.

Reviewed-by: Sultan Alsawaf <sultan@...neltoast.com>

> ---
> 
> v1 -> v2:
>    * Always set need_freq_update when limits_changed is set.
>    * Take CPUFREQ_NEED_UPDATE_LIMITS into account in sugov_update_next_freq().
> 
> ---
>  kernel/sched/cpufreq_schedutil.c |   18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
> 
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -83,7 +83,7 @@
>  
>  	if (unlikely(sg_policy->limits_changed)) {
>  		sg_policy->limits_changed = false;
> -		sg_policy->need_freq_update = cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS);
> +		sg_policy->need_freq_update = true;
>  		return true;
>  	}
>  
> @@ -95,10 +95,22 @@
>  static bool sugov_update_next_freq(struct sugov_policy *sg_policy, u64 time,
>  				   unsigned int next_freq)
>  {
> -	if (sg_policy->need_freq_update)
> +	if (sg_policy->need_freq_update) {
>  		sg_policy->need_freq_update = false;
> -	else if (sg_policy->next_freq == next_freq)
> +		/*
> +		 * The policy limits have changed, but if the return value of
> +		 * cpufreq_driver_resolve_freq() after applying the new limits
> +		 * is still equal to the previously selected frequency, the
> +		 * driver callback need not be invoked unless the driver
> +		 * specifically wants that to happen on every update of the
> +		 * policy limits.
> +		 */
> +		if (sg_policy->next_freq == next_freq &&
> +		    !cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS))
> +			return false;
> +	} else if (sg_policy->next_freq == next_freq) {
>  		return false;
> +	}
>  
>  	sg_policy->next_freq = next_freq;
>  	sg_policy->last_freq_update_time = time;
> 
> 
>