linux-kernel - Re: [PATCH v2 2/2] thermal: power_allocator: update once cooling devices when temp is low

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Date:   Tue, 20 Apr 2021 17:24:31 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        amitk@...nel.org, rui.zhang@...el.com
Subject: Re: [PATCH v2 2/2] thermal: power_allocator: update once cooling
 devices when temp is low

On 20/04/2021 16:21, Lukasz Luba wrote:
> Hi Daniel,
> 
> On 4/20/21 2:30 PM, Daniel Lezcano wrote:
>> On 19/04/2021 10:45, Lukasz Luba wrote:
> 
> [snip]
> 
>>> -        instance->cdev->updated = false;
>>> +        if (update)
>>> +            instance->cdev->updated = false;
>>> +
>>>           mutex_unlock(&instance->cdev->lock);
>>> -        (instance->cdev);
>>> +
>>> +        if (update)
>>> +            thermal_cdev_update(instance->cdev);
>>
>> This cdev update has something bad IMHO. It is protected by a mutex but
>> the 'updated' field is left unprotected before calling
>> thermal_cdev_update().
>>
>> It is not the fault of this code but how the cooling device are updated
>> and how it interacts with the thermal instances.
>>
>> IMO, part of the core code needs to revisited.
> 
> I agree, but please check my comments below.
> 
>>
>> This change tight a bit more the knot.
>>
>> Would it make sense to you if we create a function eg.
>> __thermal_cdev_update()
> 
> I'm not sure if I assume it right that the function would only have the:
> list_for_each_entry(instance, &cdev->thermal_instances, cdev_node)
> 
> loop from the thermal_cdev_update(). But if it has only this loop then
> it's too little.
> 
>>
>> And then we have:
>>
>> void thermal_cdev_update(struct thermal_cooling_device *cdev)
>> {
>>          mutex_lock(&cdev->lock);
>>          /* cooling device is updated*/
>>          if (cdev->updated) {
>>                  mutex_unlock(&cdev->lock);
>>                  return;
>>          }
>>
>>     __thermal_cdev_update(cdev);
>>
>>          thermal_cdev_set_cur_state(cdev, target);
> 
> Here we are actually setting the 'target' state via:
> cdev->ops->set_cur_state(cdev, target)
> 
> then we notify, then updating stats.
> 
>>
>>          cdev->updated = true;
>>          mutex_unlock(&cdev->lock);
>>          trace_cdev_update(cdev, target);
> 
> Also this trace is something that I'm using in my tests...

Yeah, I noticed right after sending the comments. All that should be
moved in the lockless function.

So this function becomes:

void thermal_cdev_update(struct thermal_cooling_device *cdev)
{
	mutex_lock(&cdev->lock);
	if (!cdev->updated) {
		__thermal_cdev_update(cdev);
		cdev->updated = true;
	}
	mutex_unlock(&cdev->lock);

	dev_dbg(&cdev->device, "set to state %lu\n", target);
}

We end up with the trace_cdev_update(cdev, target) inside the mutex
section but that should be fine.

>>          dev_dbg(&cdev->device, "set to state %lu\n", target);
>> }
>>
>> And in this file we do instead:
>>
>> -        instance->cdev->updated = false;
>> +        if (update)
>> +            __thermal_cdev_update(instance->cdev);
>>            mutex_unlock(&instance->cdev->lock);
>> -        thermal_cdev_update(instance->cdev);
> 
> Without the line above, we are not un-throttling the devices.

Is it still true with the amended function thermal_cdev_update() ?


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