linux-kernel - Re: [PATCH] cpufreq: Rebuild sched-domains when removing cpufreq driver

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Message-ID: <0b3c7c1b-9905-cded-dc86-17296a10152a@linux.vnet.ibm.com>
Date:   Wed, 20 Sep 2023 10:59:13 +0530
From:   Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
To:     Pierre Gondois <pierre.gondois@....com>
Cc:     vschneid@...hat.com, "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH] cpufreq: Rebuild sched-domains when removing cpufreq
 driver



On 9/19/23 1:19 PM, Pierre Gondois wrote:
> 
> 
> On 9/19/23 01:03, Dietmar Eggemann wrote:
>> On 18/09/2023 13:29, Pierre Gondois wrote:
>>> The Energy Aware Scheduler (EAS) relies on the schedutil governor.
>>> When moving to/from the schedutil governor, sched domains must be
>>> rebuilt to allow re-evaluating the enablement conditions of EAS.
>>> This is done through sched_cpufreq_governor_change().
>>>

Hi Pierre. It looks correct to update when removing the 
cpufreq governer. 


>>> Having a cpufreq governor assumes having a cpufreq driver running.
>>> Inserting/removing a cpufreq driver should trigger a re-evaluation
>>> of EAS enablement conditions, avoiding to see EAS enabled when
>>> removing a running cpufreq driver.
>>>
>>> Add a sched_cpufreq_governor_change() call in cpufreq driver removal
>>> path.
>>
>> Rebuilding SDs when inserting the driver is already covered by
>>
>>    cpufreq_online()
>>      cpufreq_set_policy()
>>        sched_cpufreq_governor_change()
>>          if (old or new gov eq. schedutil)
>>            schedule_work(&rebuild_sd_work)
>>
>> So what's missing is only a sched_cpufreq_governor_change() call when
>> removing the driver, right?
> 
> Yes exact, removing a cpufreq driver (e.g. `rmmod cppc_cpufreq.ko`) goes
> through:
> cpufreq_remove_dev()
> \-__cpufreq_offline()
> 
> so the path you mentioned is not used in this case.
> 

One Doubt, while looking through code. Not well versed with this area. 

cpuhp_cpufreq_offline is being registered with CPU hotplug. That ends up 
calling cpufreq_offline. This may cause non desired issues.
1. rebuild of sched domains twice instead, once by CPU hotplug and once by this.
2. offline/online of CPU (non-SMT) may not disabling EAS. 




> Regards,
> Pierre