linux-kernel - Re: [PATCH v3 1/5] powercap/drivers/dtpm: Encapsulate even more the code

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Date:   Fri, 12 Mar 2021 10:11:52 +0000
From:   Lukasz Luba <lukasz.luba@....com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     rafael@...nel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v3 1/5] powercap/drivers/dtpm: Encapsulate even more the
 code



On 3/11/21 10:57 AM, Daniel Lezcano wrote:
> On 11/03/2021 11:15, Lukasz Luba wrote:
>> Hi Daniel,
>>
>> On 3/10/21 11:02 AM, Daniel Lezcano wrote:
>>> In order to increase the self-encapsulation of the dtpm generic code,
>>> the following changes are adding a power update ops to the dtpm
>>> ops. That allows the generic code to call directly the dtpm backend
>>> function to update the power values.
>>>
>>> The power update function does compute the power characteristics when
>>> the function is invoked. In the case of the CPUs, the power
>>> consumption depends on the number of online CPUs. The online CPUs mask
>>> is not up to date at CPUHP_AP_ONLINE_DYN state in the tear down
>>> callback. That is the reason why the online / offline are at separate
>>> state. As there is already an existing state for DTPM, this one is
>>> only moved to the DEAD state, so there is no addition of new state
>>> with these changes. The dtpm node is not removed when the cpu is
>>> unplugged.
>>>
>>> That simplifies the code for the next changes and results in a more
>>> self-encapsulated code.
>>>
>>> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
>>> ---
>>> V2:
>>>    - Updated the changelog with the CPU node not being removed
>>>    - Commented the cpu hotplug callbacks to explain why there are two
>>> callbacks
>>>    - Changed 'upt_power_uw' to 'update_power_uw'
>>>    - Removed unused cpumask variable
>>> ---
>>>    drivers/powercap/dtpm.c     |  54 ++++++-------
>>>    drivers/powercap/dtpm_cpu.c | 148 ++++++++++++++++--------------------
>>>    include/linux/cpuhotplug.h  |   2 +-
>>>    include/linux/dtpm.h        |   3 +-
>>>    4 files changed, 97 insertions(+), 110 deletions(-)
>>>
>>
>> [snip]
>>
>>> @@ -210,27 +175,20 @@ static int cpuhp_dtpm_cpu_online(unsigned int cpu)
>>>        for_each_cpu(cpu, policy->related_cpus)
>>>            per_cpu(dtpm_per_cpu, cpu) = dtpm;
>>>    -    sprintf(name, "cpu%d", dtpm_cpu->cpu);
>>> +    sprintf(name, "cpu%d-cpufreq", dtpm_cpu->cpu);
>>
>> We should be safe in normal platforms, since there is less than
>> < 300 cores. although, I would use 2x CPUFREQ_NAME_LEN array.
> 
> I'm counting 9999 cores.
> 
> We have:
> #define CPUFREQ_NAME_LEN 16
> 
> "cpu-cpufreq\0" counts 12 characters.
> 
> There are 4 characters left, 9999 max then, no ?

Yes, my '< 300' was referring to some server platforms,
which IIRC had 2 sockets, each with 128 cores. I don't
know about future, though.

> 
> The code is designed for cpufreq with the energy model which targets
> ARM64 architecture and AFAICT we are far away of having so many cores on
> our phones.

True, otherwise with such big number of cores in mobile, we would face
probably more hard to investigate issues, than this simple one ;)

> 
> Except I'm wrong, I would prefer to keep the current CPUFREQ_NAME_LEN to
> not introduce subtle bugs (like stack overflow) if the length is
> increased in cpufreq.h.
> 
> What do you think ?

Agree, please skip my former comment and just take the reviewed tag.

> 
>> Reviewed-by: Lukasz Luba <lukasz.luba@....com>
>>
>> Regards,
>> Lukasz
>>
> 
>