Re: [PATCH] cpufreq: Don't unregister cpufreq cooling on CPU hotplug

[Mukesh Ojha <quic_mojha@quicinc.com>]

On 2/29/2024 1:42 PM, Viresh Kumar wrote:
> Offlining a CPU and bringing it back online is a common operation and it
> happens frequently during system suspend/resume, where the non-boot CPUs
> are hotplugged out during suspend and brought back at resume.
> 
> The cpufreq core already tries to make this path as fast as possible as
> the changes are only temporary in nature and full cleanup of resources
> isn't required in this case. For example the drivers can implement
> online()/offline() callbacks to avoid a lot of tear down of resources.
> 
> On similar lines, there is no need to unregister the cpufreq cooling
> device during suspend / resume, but only while the policy is getting
> removed.
> 
> Moreover, unregistering the cpufreq cooling device is resulting in an
> unwanted outcome, where the system suspend is eventually aborted in the
> process.  Currently, during system suspend the cpufreq core unregisters
> the cooling device, which in turn removes a kobject using device_del()
> and that generates a notification to the userspace via uevent broadcast.
> This causes system suspend to abort in some setups.
> 
> This was also earlier reported (indirectly) by Roman [1]. Maybe there is
> another way around to fixing that problem properly, but this change
> makes sense anyways.
> 
> Move the registering and unregistering of the cooling device to policy
> creation and removal times onlyy.
> 
> Reported-by: Manaf Meethalavalappu Pallikunhi <quic_manafm@quicinc.com>
> Reported-by: Roman Stratiienko <r.stratiienko@gmail.com>
> Link: https://patchwork.kernel.org/project/linux-pm/patch/20220710164026.541466-1-r.stratiienko@gmail.com/ [1]
> Tested-by: Manaf Meethalavalappu Pallikunhi <quic_manafm@quicinc.com>
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
>   drivers/cpufreq/cpufreq.c | 17 +++++++++++------
>   1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 44db4f59c4cc..4133c606dacb 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -1571,7 +1571,8 @@ static int cpufreq_online(unsigned int cpu)
>   	if (cpufreq_driver->ready)
>   		cpufreq_driver->ready(policy);
>   
> -	if (cpufreq_thermal_control_enabled(cpufreq_driver))
> +	/* Register cpufreq cooling only for a new policy */
> +	if (new_policy && cpufreq_thermal_control_enabled(cpufreq_driver))
>   		policy->cdev = of_cpufreq_cooling_register(policy);
>   
>   	pr_debug("initialization complete\n");
> @@ -1655,11 +1656,6 @@ static void __cpufreq_offline(unsigned int cpu, struct cpufreq_policy *policy)
>   	else
>   		policy->last_policy = policy->policy;
>   
> -	if (cpufreq_thermal_control_enabled(cpufreq_driver)) {
> -		cpufreq_cooling_unregister(policy->cdev);
> -		policy->cdev = NULL;
> -	}
> -
>   	if (has_target())
>   		cpufreq_exit_governor(policy);
>   
> @@ -1720,6 +1716,15 @@ static void cpufreq_remove_dev(struct device *dev, struct subsys_interface *sif)
>   		return;
>   	}
>   
> +	/*
> +	 * Unregister cpufreq cooling once all the CPUs of the policy are
> +	 * removed.
> +	 */
> +	if (cpufreq_thermal_control_enabled(cpufreq_driver)) {
> +		cpufreq_cooling_unregister(policy->cdev);
> +		policy->cdev = NULL;
> +	}
> +

Looks fine than other solution..

-Mukesh
>   	/* We did light-weight exit earlier, do full tear down now */
>   	if (cpufreq_driver->offline)
>   		cpufreq_driver->exit(policy);