linux-kernel - Re: [PATCH 5/8] thermal/drivers/cpu_cooling: Introduce the cpu idle cooling driver

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Date:   Wed, 31 Jan 2018 10:56:31 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     Eduardo Valentin <edubezval@...il.com>,
        Kevin Wangtao <kevin.wangtao@...aro.org>,
        Leo Yan <leo.yan@...aro.org>,
        Amit Kachhap <amit.kachhap@...il.com>,
        viresh kumar <viresh.kumar@...aro.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Zhang Rui <rui.zhang@...el.com>,
        Javi Merino <javi.merino@...nel.org>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH 5/8] thermal/drivers/cpu_cooling: Introduce the cpu idle
 cooling driver

On 31 January 2018 at 10:50, Daniel Lezcano <daniel.lezcano@...aro.org> wrote:
> On 31/01/2018 10:46, Vincent Guittot wrote:
>> On 31 January 2018 at 10:33, Daniel Lezcano <daniel.lezcano@...aro.org> wrote:
>>> On 31/01/2018 10:01, Vincent Guittot wrote:
>>>> Hi Daniel,
>>>>
>>>> On 23 January 2018 at 16:34, Daniel Lezcano <daniel.lezcano@...aro.org> wrote:
>>>
>>> [ ... ] (please trim :)
>>>
>>>>> +               /*
>>>>> +                * Each cooling device is per package. Each package
>>>>> +                * has a set of cpus where the physical number is
>>>>> +                * duplicate in the kernel namespace. We need a way to
>>>>> +                * address the waitq[] and tsk[] arrays with index
>>>>> +                * which are not Linux cpu numbered.
>>>>> +                *
>>>>> +                * One solution is to use the
>>>>> +                * topology_core_id(cpu). Other solution is to use the
>>>>> +                * modulo.
>>>>> +                *
>>>>> +                * eg. 2 x cluster - 4 cores.
>>>>> +                *
>>>>> +                * Physical numbering -> Linux numbering -> % nr_cpus
>>>>> +                *
>>>>> +                * Pkg0 - Cpu0 -> 0 -> 0
>>>>> +                * Pkg0 - Cpu1 -> 1 -> 1
>>>>> +                * Pkg0 - Cpu2 -> 2 -> 2
>>>>> +                * Pkg0 - Cpu3 -> 3 -> 3
>>>>> +                *
>>>>> +                * Pkg1 - Cpu0 -> 4 -> 0
>>>>> +                * Pkg1 - Cpu1 -> 5 -> 1
>>>>> +                * Pkg1 - Cpu2 -> 6 -> 2
>>>>> +                * Pkg1 - Cpu3 -> 7 -> 3
>>>>
>>>>
>>>> I'm not sure that the assumption above for the CPU numbering is safe.
>>>> Can't you use a per cpu structure to point to resources that are per
>>>> cpu instead ? so you will not have to rely on CPU ordering
>>>
>>> Can you elaborate ? I don't get the part with the percpu structure.
>>
>> Something like:
>>
>> struct cpuidle_cooling_cpu {
>>        struct task_struct *tsk;
>>        wait_queue_head_t waitq;
>> };
>>
>> DECLARE_PER_CPU(struct cpuidle_cooling_cpu *, cpu_data);
>
> I got this part but I don't get how that fixes the ordering thing.

Because you don't care of the CPU ordering to retrieve the data as
they are stored per cpu directly

>
>
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