linux-kernel - Re: [PATCH 1/1] thermal: sysfs: avoid actual readings from sysfs

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Date:   Sat, 10 Jun 2023 12:24:39 -0500
From:   Russell Haley <yumpusamongus@...il.com>
To:     Eduardo Valentin <evalenti@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     eduval@...zon.com, rafael@...nel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Amit Kucheria <amitk@...nel.org>,
        Zhang Rui <rui.zhang@...el.com>
Subject: Re: [PATCH 1/1] thermal: sysfs: avoid actual readings from sysfs

On 6/7/23 11:38, Eduardo Valentin wrote:
>> Can you elaborate 'the timing requirement for the governors' ? I'm
>> missing the point
> 
> 
> The point is to avoid contention on the device update path.
> Governor that use differential equations on temperature over time
> will be very time sensitive. Step wise, power allocator, or any
> PID will be very sensitive to time. So, If userspace is hitting
> this API too often we can see cases where the updates needed to
> service userspace may defer/delay the execution of the governor
> logic.
> 
> Despite that, there is really no point to have more updates than
> what was configured for the thermal zone to support. Say that
> we configure a thermal zone to update itself every 500ms, yet
> userspace keeps sending reads every 100ms, we do not need necessarily
> to do a trip to the device every single time to update the temperature,
> as per the design for the thermal zone.

A userspace governor might *also* use PID or filter multiple samples
taken at high rate. I specifically switched my python fan control script
from the Intel coretemp hwmon to the x86_pkg_tmp thermal zone because of
the coretemp driver's annoying 1-second caching behavior.