Messages in this thread | | | Date | Fri, 23 Jun 2023 18:34:31 +0100 | Subject | Re: [PATCH V4] thermal/core/power_allocator: reset thermal governor when trip point is changed | From | Lukasz Luba <> |
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On 6/23/23 17:55, Rafael J. Wysocki wrote: > On Fri, Jun 23, 2023 at 9:43 AM Lukasz Luba <lukasz.luba@arm.com> wrote: >> >> >>
[snip]
>> >> I agree, the patch header doesn't explain that properly. Here is the >> explanation for this Intelligent Power Allocator (IPA): >> >> The IPA controls temperature using PID mechanism. It's a closed >> feedback loop. That algorithm can 'learn' from the 'observed' >> in the past reaction for it's control decisions and accumulates that >> information in the part called 'error integral'. Those accumulated >> 'error' gaps are the differences between the set target value and the >> actually achieved value. In our case the target value is the target >> temperature which is coming from the trip point. That part is then used >> with the 'I' (of PID) component, so we can compensate for those >> 'learned' mistakes. >> Now, when you change the target temperature value - all your previous >> learned errors won't help you. That's why Intelligent Power Allocator >> should reset previously accumulated history. > > Right. > > And every other governor using information from the past for control > will have an analogous problem, won't it?
Not necessarily, but to play safe I would go case-by-case and make sure other governors are aligned to this new feature.
E.g. the bang-bang governor operates only on current temperature and current trip value + trip hysteresis. The flow graph describes it [1]. The control (state of the fan: ON or OFF) of that governor could be simply adjusted to the new reality -> new trip point temp. That would just mean 'toggling' the fan if needed. There are only 2 'target' states: 0 or 1 for the fan. You can images a situation when the temperature doesn't change, but we manipulate the trip value for that governor. The governor would react correctly always in such situation w/o a need of a reset IMO.
> >>> >>>>> >>>>>> For the 2nd case IIUC the code, we pass the 'trip.temperature' >>>>>> and should be ready for what you said (modification of that value). >>>>> >>>>> Generally speaking, it needs to be prepared for a simultaneous change >>>>> of multiple trip points (including active), in which case it may not >>>>> be useful to invoke the ->reset() callback for each of them >>>>> individually. >>>> >>>> Although, that looks more cleaner IMO. Resetting one by one in >>>> a temperature order would be easily maintainable, won't be? >>> >>> I wouldn't call it maintainable really. >>> >>> First of all, the trips may not be ordered. There are no guarantees >>> whatsoever that they will be ordered, so the caller may have to >>> determine the temperature order in the first place. This would be an >>> extra requirement that currently is not there. >>> >>> Apart from this, I don't see any fundamental difference between the >>> case when trip points are updated via sysfs and when they are updated >>> by the driver. The governor should reset itself in any of those cases >>> and even if one trip point changes, the temperature order of all of >>> them may change, so the governor reset mechanism should be able to >>> handle the case when multiple trip points are updated at the same >>> time. While you may argue that this is theoretical, the ACPI spec >>> clearly states that this is allowed to happen, for example. >>> >>> If you want a generic reset callback for governors, that's fine, but >>> make it generic and not specific to a particular use case. >> >> I think we agree here, but probably having slightly different >> implementation in mind. Based on you explanation I think you >> want simply this API: >> void (*reset)(struct thermal_zone_device *tz); >> >> 1. no return value >> 2. no specific trip ID as argument >> >> Do you agree? > > Yes, I do.
OK, thanks.
Di could you implement that 'reset()' API according to this description, please?
> >> IMO such implementation and API would also work for this IPA >> purpose. Would that work for the ACPI use case as well? > > It would AFAICS.
Thanks Rafael for the comments and the progress that we made :)
Regards, Lukasz
[1] https://elixir.bootlin.com/linux/v6.3/source/drivers/thermal/gov_bang_bang.c#L80
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