linux-kernel - Re: [PATCH v2 2/5] thermal: devfreq_cooling: get a copy of device status

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Date:   Mon, 7 Dec 2020 12:41:04 +0000
From:   Lukasz Luba <lukasz.luba@....com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, rui.zhang@...el.com,
        amit.kucheria@...durent.com, orjan.eide@....com, robh@...nel.org,
        alyssa.rosenzweig@...labora.com, steven.price@....com,
        airlied@...ux.ie, daniel@...ll.ch, ionela.voinescu@....com
Subject: Re: [PATCH v2 2/5] thermal: devfreq_cooling: get a copy of device
 status



On 12/3/20 4:09 PM, Daniel Lezcano wrote:
> On 03/12/2020 16:38, Lukasz Luba wrote:
>>
>>
>> On 12/3/20 1:09 PM, Daniel Lezcano wrote:
>>> On 18/11/2020 13:03, Lukasz Luba wrote:
>>>> Devfreq cooling needs to now the correct status of the device in order
>>>> to operate. Do not rely on Devfreq last_status which might be a stale
>>>> data
>>>> and get more up-to-date values of the load.
>>>>
>>>> Devfreq framework can change the device status in the background. To
>>>> mitigate this situation make a copy of the status structure and use it
>>>> for internal calculations.
>>>>
>>>> In addition this patch adds normalization function, which also makes
>>>> sure
>>>> that whatever data comes from the device, it is in a sane range.
>>>>
>>>> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
>>>> ---
>>>>    drivers/thermal/devfreq_cooling.c | 52 +++++++++++++++++++++++++------
>>>>    1 file changed, 43 insertions(+), 9 deletions(-)
>>>>
>>>> diff --git a/drivers/thermal/devfreq_cooling.c
>>>> b/drivers/thermal/devfreq_cooling.c
>>>> index 659c0143c9f0..925523694462 100644
>>>> --- a/drivers/thermal/devfreq_cooling.c
>>>> +++ b/drivers/thermal/devfreq_cooling.c
>>>> @@ -227,20 +227,46 @@ static inline unsigned long
>>>> get_total_power(struct devfreq_cooling_device *dfc,
>>>>                                       voltage);
>>>>    }
>>>>    +static void _normalize_load(struct devfreq_dev_status *status)
>>>> +{
>>>> +    /* Make some space if needed */
>>>> +    if (status->busy_time > 0xffff) {
>>>> +        status->busy_time >>= 10;
>>>> +        status->total_time >>= 10;
>>>> +    }
>>>> +
>>>> +    if (status->busy_time > status->total_time)
>>>> +        status->busy_time = status->total_time;
>>>
>>> How the condition above is possible?
>>
>> They should, be checked by the driver, but I cannot trust
>> and have to check for all corner cases: (div by 0, overflow
>> one of them, etc). The busy_time and total_time are unsigned long,
>> which means 4B on 32bit machines.
>> If these values are coming from device counters, which count every
>> busy cycle and total cycles of a clock of a device running at e.g.
>> 1GHz they would overflow every ~4s.
> 
> I don't think it is up to this routine to check the driver is correctly
> implemented, especially at every call to get_requested_power.
> 
> If the normalization ends up by doing this kind of thing, there is
> certainly something wrong in the 'status' computation to be fixed before
> submitting this series.
> 
> 
>> Normally IPA polling are 1s and 100ms, it's platform specific. But there
>> are also 'empty' periods when IPA sees temperature very low and does not
>> even call the .get_requested_power() callbacks for the cooling devices,
>> just grants max freq to all. This is problematic. I am investigating it
>> and will propose a solution for IPA soon.
>>
>> I would avoid all of this if devfreq core would have default for all
>> devices a reliable polling timer... Let me check some possibilities also
>> for this case.
> 
> Ok, may be create an API to compute the 'idle,busy,total times' to be
> used by the different the devfreq drivers and then fix the overflow in
> this common place.

Yes, I have this plan, but I have to close this patch series. To go
forward with this, I will drop the normalization function and will keep
only the code of safe copy of the 'status', so using busy_time and
total_time will be safe.

I will address this computation and normalization in different patch
series. There might be a need of a new API as you pointed out, which
is out-of-scope of this patch set.

> 
>>>> +    status->busy_time *= 100;
>>>> +    status->busy_time /= status->total_time ? : 1;
>>>> +
>>>> +    /* Avoid division by 0 */
>>>> +    status->busy_time = status->busy_time ? : 1;
>>>> +    status->total_time = 100;
>>>
>>> Why not base the normalization on 1024? and use an intermediate u64.
>>
>> You are the 2nd reviewer who is asking this. I tried to keep 'load' as
>> in range [0, 100] since we also have 'load' in cpufreq cooling in this
>> range. Maybe I should switch to 1024 (Ionela was also asking for this).
> 
> Well it is common practice to compute normalization with 1024 because
> the division is a bit shift and the compiler optimize the code very well
> with that value.
> 

I will keep this 1024 in mind for the next topic series.

Regards,
Lukasz