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 16:27:44 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Vincent Guittot <vincent.guittot@...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 31/01/2018 10:56, Vincent Guittot wrote:
> 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

That's what I did initially, but for consistency reasons with the
cpufreq cpu cooling device which is stored in a list and the combo cpu
cooling device, the cpuidle cooling device must be per cluster and
stored in a list.

Alternatively I can do:

struct cpuidle_cooling_device {
	struct thermal_cooling_device *cdev;
-	struct task_struct **tsk;
+	struct task_struct __percpu *tsk;
	struct cpumask *cpumask;
	struct list_head node;
	struct hrtimer timer;
	struct kref kref;
-	wait_queue_head_t *waitq;
+	wait_queue_head_t __percpu waitq;
	atomic_t count;
	unsigned int idle_cycle;
	unsigned int state;
};


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