linux-kernel - Re: [PATCH v5 11/23] PM: EM: Add API for updating the runtime modifiable EM

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Message-ID: <8ae21f77-994a-42d3-9851-dfda661cf018@arm.com>
Date: Thu, 4 Jan 2024 16:55:21 +0000
From: Lukasz Luba <lukasz.luba@....com>
To: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: rui.zhang@...el.com, amit.kucheria@...durent.com,
 linux-kernel@...r.kernel.org, amit.kachhap@...il.com,
 daniel.lezcano@...aro.org, viresh.kumar@...aro.org, len.brown@...el.com,
 pavel@....cz, mhiramat@...nel.org, qyousef@...alina.io, wvw@...gle.com,
 linux-pm@...r.kernel.org, rafael@...nel.org
Subject: Re: [PATCH v5 11/23] PM: EM: Add API for updating the runtime
 modifiable EM



On 1/4/24 15:45, Dietmar Eggemann wrote:
> On 20/12/2023 09:06, Lukasz Luba wrote:
>>
>>
>> On 12/12/23 18:50, Dietmar Eggemann wrote:
>>> On 29/11/2023 12:08, Lukasz Luba wrote:
> 
> [...]
> 
>>>> +int em_dev_update_perf_domain(struct device *dev,
>>>> +                  struct em_perf_table __rcu *new_table)
>>>> +{
>>>> +    struct em_perf_table __rcu *old_table;
>>>> +    struct em_perf_domain *pd;
>>>> +
>>>> +    /*
>>>> +     * The lock serializes update and unregister code paths. When the
>>>> +     * EM has been unregistered in the meantime, we should capture that
>>>> +     * when entering this critical section. It also makes sure that
>>>
>>> What do you want to capture here? You want to block in this moment,
>>> right? Don't understand the 2. sentence here.
>>>
>>> [...]
>>
>> There is general issue with module... they can reload. A driver which
>> registered EM can than later disappear. I had similar issues for the
>> devfreq cooling. It can happen at any time. In this scenario let's
>> consider scenario w/ 2 kernel drivers:
>> 1. Main driver which registered EM, e.g. GPU driver
>> 2. Thermal driver which updates that EM
>> When 1. starts unload process, it has to make sure that it will
>> not free the main EM 'pd', because the 2. might try to use e.g.
>> 'pd->nr_perf_states' while doing update at the moment.
>> Thus, this 'pd' has local mutex, to avoid issues of
>> module unload vs. EM update. The EM unregister will block on
>> that mutex and let the background update finish it's critical
>> section.
> 
> All true but wouldn't
> 
>      /* Serialize update/unregister or concurrent updates */
> 
> be sufficient as a comment here?
> 

Sounds good, I'll change that.