linux-kernel - Re: [PATCH] cpufreq: fail to start a governor if limits weren't updated correctly

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Date: Mon, 20 May 2024 12:41:55 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: Fares Mehanna <faresx@...zon.de>
Cc: rkagan@...zon.de, "Rafael J. Wysocki" <rafael@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Valentin Schneider <vschneid@...hat.com>, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpufreq: fail to start a governor if limits weren't
 updated correctly

On 30-04-24, 14:39, Fares Mehanna wrote:
> Current cpufreq governors are using `__cpufreq_driver_target()` in their
> `.limits()` functions to update the frequency. `__cpufreq_driver_target()`
> will eventually call `.target()` or `.target_index()` in the cpufreq driver
> to update the frequency.
> 
> `.target()`, `.target_index()` and `__cpufreq_driver_target()` may fail and
> all do return an error code, this error code is dropped by the governor and
> not propagated to the core.
> 
> This have the downside of accepting a new CPU governor even if it fails to
> set the wanted limits. This is misleading to the sysfs user, as setting the
> governor will be accepted but the governor itself is not functioning as
> expected. Especially with `performance` and `powersave` where they only
> target specific frequency during starting of the governor and stays the
> same during their lifetime.
> 
> This change will cause a failure to start the new governor if `.limits()`
> failed, propagating back to userspace if the change is driven by sysfs.
> 
> Signed-off-by: Fares Mehanna <faresx@...zon.de>
> ---
>  drivers/cpufreq/cpufreq.c             |  7 +++++--
>  drivers/cpufreq/cpufreq_governor.c    |  6 ++++--
>  drivers/cpufreq/cpufreq_governor.h    |  2 +-
>  drivers/cpufreq/cpufreq_performance.c |  4 ++--
>  drivers/cpufreq/cpufreq_powersave.c   |  4 ++--
>  drivers/cpufreq/cpufreq_userspace.c   | 16 +++++++++-------
>  include/linux/cpufreq.h               | 13 +++++++------
>  kernel/sched/cpufreq_schedutil.c      |  6 ++++--
>  8 files changed, 34 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 66e10a19d76a..5ac44a44d319 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -2474,8 +2474,11 @@ int cpufreq_start_governor(struct cpufreq_policy *policy)
>  			return ret;
>  	}
>  
> -	if (policy->governor->limits)
> -		policy->governor->limits(policy);
> +	if (policy->governor->limits) {
> +		ret = policy->governor->limits(policy);
> +		if (ret)
> +			return ret;

You need to stop the governor here on failure as this function started it
successfully.

-- 
viresh