Re: Thermal notifications without setting thermal governor to userspace?

[Daniel Lezcano <daniel.lezcano@linaro.org>]

Hi Chris,

I'll answer to your email but meanwhile this can have some interest for you:

 
https://lore.kernel.org/all/20220330100444.3846661-1-daniel.lezcano@linaro.org/

Regards

  -- Daniel

On 30/03/2022 17:45, Chris Down wrote:
> Hey thermal folks,
> 
> I'm hoping that you'll be able to provide some guidance on what options 
> are available for getting thermal notifications without changing the 
> policy to userspace control.
> 
> To be clear, my intent is to have a userspace daemon which can set 
> multiple temperature trips at runtime on which it can receive a 
> notification (preferably by a simple mechanism like uevents), and to be 
> able to distinguish which threshold was tripped. For example, to be able 
> to set trips at 50C, 70C, and 90C, getting events both when the 
> temperature exceeds that and when it dips back under the specified 
> threshold.
> 
> Right now I am polling /sys/class/thermal/thermal_zone*/temp but this 
> can be expensive, especially for the ACPI thermal zone.
> 
> As it is, I see four options:
> 
> 1. Set thermal_zone to use user_space governor and use uevents
> 
> As I understand it, this means that the default critical temperature 
> will no longer be respected and user space is now responsible for taking 
> action to control thermal events.
> 
> If that's correct, then it seems dangerous to set it to user_space for 
> userspace applications which only want to monitor their own trip 
> temperatures, but not take direct action.
> 
> 2. Use hwmon thermal events
> 
> I see that commit 1597b374af22 ("hwmon: Add notification support") adds 
> notification support to hwmon, but as far as I can tell this is based on 
> statically defined trip temperatures from ACPI or similar inputs. I see 
> you can change it with the thermal.crt boot option, but that's too early 
> for a normal userspace daemon to do anything (and it doesn't allow any 
> dynamic reconfiguration).
> 
> 3. Use thermal over netlink
> 
> I see that commit 1ce50e7d408e ("thermal: core: genetlink support for 
> events/cmd/sampling") which was merged a couple of years ago adds 
> support for thermal netlink events.
> 
> I also see articles suggesting that support for this new netlink 
> interface will be or was added to libnl, but I can't seem to find 
> anything about it in the libnl sources.
> 
> Is this mature enough to use? If so, does one have to hack up their own 
> userspace netlink library for now, or what's the plan there?
> 
> In general I would prefer something simpler that fits into the existing 
> strong tooling around uevents/etc though, this looks useful, but it does 
> a lot more than I need and requires adding another userspace dependency 
> on libnl.
> 
> 4. Poll /sys/class/thermal/thermal_zone*/temp or hwmon
> 
> This is what I'm currently doing. It's slow, and often unnecessarily 
> costly on weaker systems.
> 
> Are there other options I'm not considering here that might help me out 
> here?
> 
> Thanks,
> 
> Chris


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