Re: [PATCH 1/2] thermal: power_allocator: maintain the device statistics from going stale

[Lukasz Luba <lukasz.luba@arm.com>]

On 4/6/21 12:24 PM, Daniel Lezcano wrote:
> On 06/04/2021 12:39, Lukasz Luba wrote:
>>
>>
>> On 4/6/21 11:16 AM, Daniel Lezcano wrote:
>>> On 06/04/2021 10:44, Lukasz Luba wrote:
>>>>
>>>>
>>>> On 4/2/21 4:54 PM, Daniel Lezcano wrote:
>>>>> On 31/03/2021 18:33, Lukasz Luba wrote:
>>>>>> When the temperature is below the first activation trip point the
>>>>>> cooling
>>>>>> devices are not checked, so they cannot maintain fresh statistics. It
>>>>>> leads into the situation, when temperature crosses first trip
>>>>>> point, the
>>>>>> statistics are stale and show state for very long period.
>>>>>
>>>>> Can you elaborate the statistics you are referring to ?
>>>>>
>>>>> I can understand the pid controller needs temperature but I don't
>>>>> understand the statistics with the cooling device.
>>>>>
>>>>>
>>>>
>>>> The allocate_power() calls cooling_ops->get_requested_power(),
>>>> which is for CPUs cpufreq_get_requested_power() function.
>>>> In that cpufreq implementation for !SMP we still has the
>>>> issue of stale statistics. Viresh, when he introduced the usage
>>>> of sched_cpu_util(), he fixed that 'long non-meaningful period'
>>>> of the broken statistics and it can be found since v5.12-rc1.
>>>>
>>>> The bug is still there for the !SMP. Look at the way how idle time
>>>> is calculated in get_load() [1]. It relies on 'idle_time->timestamp'
>>>> for calculating the period. But when this function is not called,
>>>> the value can be very far away in time, e.g. a few seconds back,
>>>> when the last allocate_power() was called.
>>>>
>>>> The bug is there for both SMP and !SMP [2] for older kernels, which can
>>>> be used in Android or ChromeOS. I've been considering to put this simple
>>>> IPA fix also to some other kernels, because Viresh's change is more
>>>> a 'feature' and does not cover both platforms.
>>>
>>> Ok, so IIUC, the temperature is needed as well as the power to do the
>>> connection for the pid loop (temp vs power).
>>>
>>> I'm trying to figure out how to delegate the mitigation switch on/off to
>>> the core code instead of the governor (and kill tz->passive) but how
>>> things are implemented for the IPA makes this very difficult.
>>>
>>> AFAICT, this fix is not correct.
>>>
>>> If the temperature is below the 'switch_on_temp' the passive is set to
>>> zero and the throttle function is not called anymore (assuming it is
>>> interrupt mode not polling mode), so the power number is not updated
>>> also.
>>
>> IPA doesn't work well in asynchronous mode, because it needs this fixed
>> length for the period. I have been experimenting with tsens IRQs and
>> also both fixed polling + sporadic asynchronous IRQs, trying to fix it
>> and have 'predictable' IPA, but without a luck.
>> IPA needs synchronous polling events like we have for high temp e.g.
>> 100ms and low temp e.g. 1000ms. The asynchronous events are root of
>> undesirable states (too low or to high) calculated and set for cooling
>> devices. It's also harder to escape these states with asynchronous
>> events. I don't recommend using IPA with asynchronous events from IRQs,
>> for now. It might change in future, though.
> 
> I understand that but there is the 'switch_on_temp' trip point which is
> supposed to begin to collect the power values ahead of the
> 'desired_temp' (aka mitigation trip point / sustainable power).
> 
> 
>> The patch 2/2 should calm down the unnecessary updates/notifications so
>> your request.
>> The longer polling, which we have for temperature below 'switch_on_temp'
>> (e.g. every 1sec) shouldn't harm the performance of the system, but
>> definitely makes IPA more predictable and stable.
> 
> The change I proposed is correct then no ? The polling is still effective.

In your proposed code there is 'tz->last_temperature < switch_on_temp'
which then return 0 immediately. So we don't poke the devices.

> 
> If the IPA needs a sampling, it may be more adequate to separate the
> sampling from the polling. So the other governors won't be impacted by
> the IPA specific setup, and we do not end up with polling/passive delays
> tricks for a governor. The IPA would have its own self-encapsulated
> sampling rate and the thermal zone setup won't depend on the governor.
> 
> What do you think ?
> 

IMHO having a private timer in the governor creates another complexity
and confusion.

What we have in thermal now is good enough. We have DT support for both
periods so there is need even to write via sysfs:
polling-delay-passive
polling-delay
The device driver developers can rely on this reliable check in the
thermal framework.

I don't agree that IPA forces any specific setup. If the thermal is
configured to do the polling of the temp sensor, because maybe there
are no HW interrupts, then there is no other way. The Arm SCMI sensors
were one of them, where we had to send a SCMI request. There was no
notifications/IRQs that temp crossed some trip point. Now it should be
better, but still it depends on vendor's FW implementation if there is
IRQ.

The reliable polling is not IPA 'feature request'.
We cannot avoid polling in some configurations. Thermal framework
must support this scenario: polling/checking temp sensor even when
the temp is low.

Thus, since framework must check the temp, calling
the governor->throttle() doesn't harm (as I said every 1sec).
Furthermore, the governor interprets what trip point and temperature to
interpret and how to react.