On 19 March 2012 17:15, Srivatsa S. Bhat
<srivatsa.b...@linux.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.kach...@linaro.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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