linux-kernel - Re: [PATCH V2 3/6] thermal: Add generic cpuhotplug cooling implementation

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Date:	Tue, 20 Mar 2012 11:36:18 +0530
From:	Amit Kachhap <amit.kachhap@...aro.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	linux-pm@...ts.linux-foundation.org,
	linux-samsung-soc@...r.kernel.org, linux-kernel@...r.kernel.org,
	mjg59@...f.ucam.org, linux-acpi@...r.kernel.org, lenb@...nel.org,
	linaro-dev@...ts.linaro.org, lm-sensors@...sensors.org,
	patches@...aro.org, eduardo.valentin@...com, durgadoss.r@...el.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH V2 3/6] thermal: Add generic cpuhotplug cooling implementation

On 19 March 2012 17:15, Srivatsa S. Bhat
<srivatsa.bhat@...ux.vnet.ibm.com> wrote:
> On 03/19/2012 11:47 AM, Amit Daniel Kachhap wrote:
>
>> This patch adds support for generic cpu thermal cooling low level
>> implementations using cpuhotplug based on the thermal level requested
>> from user. Different cpu related cooling devices can be registered by the
>> user and the binding of these cooling devices to the corresponding
>> trip points can be easily done as the registration APIs return the
>> cooling device pointer. The user of these APIs are responsible for
>> passing the cpumask.
>>
>
>
> I am really not aware of the cpu thermal cooling stuff, but since this patch
> deals with CPU Hotplug (which I am interested in), I have some questions
> below..
>
>
>> Signed-off-by: Amit Daniel Kachhap <amit.kachhap@...aro.org>
>> +
>> +static int cpuhotplug_get_cur_state(struct thermal_cooling_device *cdev,
>> +                              unsigned long *state)
>> +{
>> +     int ret = -EINVAL;
>> +     struct hotplug_cooling_device *hotplug_dev;
>> +
>> +     mutex_lock(&cooling_cpuhotplug_lock);
>> +     list_for_each_entry(hotplug_dev, &cooling_cpuhotplug_list, node) {
>> +             if (hotplug_dev && hotplug_dev->cool_dev == cdev) {
>> +                     *state = hotplug_dev->hotplug_state;
>> +                     ret = 0;
>> +                     break;
>> +             }
>> +     }
>> +     mutex_unlock(&cooling_cpuhotplug_lock);
>> +     return ret;
>> +}
>> +
>> +/*This cooling may be as ACTIVE type*/
>> +static int cpuhotplug_set_cur_state(struct thermal_cooling_device *cdev,
>> +                              unsigned long state)
>> +{
>> +     int cpuid, this_cpu = smp_processor_id();
>
>
> What prevents this task from being migrated to another CPU?
> IOW, what ensures that 'this_cpu' remains valid throughout this function?
>
> I see that you are acquiring mutex locks below.. So this is definitely not
> a preempt disabled section.. so I guess my question above is valid..
>
> Or is this code bound to a particular cpu?

No this thread is not bound to a cpu. Your comment is valid and some
synchronization is needed for preemption. Thanks for pointing this
out.

>
>> +     struct hotplug_cooling_device *hotplug_dev;
>> +
>> +     mutex_lock(&cooling_cpuhotplug_lock);
>> +     list_for_each_entry(hotplug_dev, &cooling_cpuhotplug_list, node)
>> +             if (hotplug_dev && hotplug_dev->cool_dev == cdev)
>> +                     break;
>> +
>> +     mutex_unlock(&cooling_cpuhotplug_lock);
>> +     if (!hotplug_dev || hotplug_dev->cool_dev != cdev)
>> +             return -EINVAL;
>> +
>> +     if (hotplug_dev->hotplug_state == state)
>> +             return 0;
>> +
>> +     /*
>> +     * This cooling device may be of type ACTIVE, so state field can
>> +     * be 0 or 1
>> +     */
>> +     if (state == 1) {
>> +             for_each_cpu(cpuid, hotplug_dev->allowed_cpus) {
>> +                     if (cpu_online(cpuid) && (cpuid != this_cpu))
>
>
> What prevents the cpu numbered cpuid from being taken down right at this moment?
> Don't you need explicit synchronization with CPU Hotplug using something like
> get_online_cpus()/put_online_cpus() here?
>
>> +                             cpu_down(cpuid);
>> +             }
>> +     } else if (state == 0) {
>> +             for_each_cpu(cpuid, hotplug_dev->allowed_cpus) {
>> +                     if (!cpu_online(cpuid) && (cpuid != this_cpu))
>
>
> Same here.
>
>> +                             cpu_up(cpuid);
>> +             }
>> +     } else {
>> +             return -EINVAL;
>> +     }
>> +
>> +     hotplug_dev->hotplug_state = state;
>> +
>> +     return 0;
>> +}
>> +/* bind hotplug callbacks to cpu hotplug cooling device */
>> +static struct thermal_cooling_device_ops cpuhotplug_cooling_ops = {
>> +     .get_max_state = cpuhotplug_get_max_state,
>> +     .get_cur_state = cpuhotplug_get_cur_state,
>> +     .set_cur_state = cpuhotplug_set_cur_state,
>> +};
>> +
>
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