Re: [PATCH v3 4/7] thermal/drivers/tegra: Add driver for Tegra30 thermal sensor

From: Thara Gopinath
Date: Wed Jun 16 2021 - 10:36:27 EST



On 6/16/21 6:47 AM, Dmitry Osipenko wrote:
16.06.2021 05:50, Thara Gopinath пишет:
...

Hi,

Thermal pressure is letting scheduler know that the max capacity
available for a cpu to schedule tasks is reduced due to a thermal event.
So you cannot have a h/w thermal pressure and s/w thermal pressure.
There is eventually only one capping applied at h/w level and the
frequency corresponding to this capping should be used for thermal
pressure.

Ideally you should not be having both s/w and h/w trying to throttle at
the same time. Why is this a scenario and what prevents you from
disabling s/w throttling when h/w throttling is enabled. Now if there
has to a aggregation for whatever reason this should be done at the
thermal driver level and passed to scheduler.

Hello,

The h/w mitigation is much more reactive than software, in the same time
it's much less flexible than software. It should provide additional
protection in a cases where software isn't doing a good job. Ideally h/w
mitigation should stay inactive all the time, nevertheless it should be
modeled properly by the driver.

Ok. This is kind of opposite to what I am doing on the Qcom platform I am working on. The h/w throttling is the default since like you mentioned it is more reactive. And s/w does only critical trip management.



That is a good question. IMO, first step would be to call
cpufreq_update_limits().

Right

[ Cc Thara who implemented the thermal pressure ]

May be Thara has an idea about how to aggregate both? There is another
series floating around with hardware limiter [1] and the same
problematic.

  [1] https://lkml.org/lkml/2021/6/8/1791

Thanks, it indeed looks similar.

I guess the common thermal pressure update code could be moved out into
a new special cpufreq thermal QoS handler (policy->thermal_constraints),
where handler will select the frequency constraint and set up the
pressure accordingly. So there won't be any races in the code.

It was a conscious decision to keep thermal pressure update out of qos
max freq update because there are platforms that don't use the qos
framework. For eg acpi uses cpufreq_update_policy.
But you are right. We have two platforms now applying h/w throttling and
cpufreq_cooling applying s/w throttling. So it does make sense to have
one api doing all the computation to update thermal pressure. I am not
sure how exactly/where exactly this will reside.

The generic cpufreq_cooling already uses QoS for limiting the CPU
frequency. It could be okay to use QoS for the OF drivers, this needs a
closer look.

We have the case where CPU frequency is changed by the thermal event and
the thermal pressure equation is the same for both s/w cpufreq_cooling
and h/w thermal driver. The pressure is calculated based on the QoS
cpufreq constraint that is already aggregated.

Hence what we may need to do on the thermal event is:

1. Update the QoS request
2. Update the thermal pressure
3. Ensure that updates are not racing

Yes. So the first two steps you mentioned is exactly what cpufreq_cooling.c also does except for the fact that it is a s/w mitigation. Now if you have two sources that is updating the max frequency via qos, I think you can do either of the following before
calculating thermal pressure
1. Read the throttled frequency from h/w if your h/w supports this feature.
or
2. Use freq_qos_read_value to get the max frequency value.

Either way only the correct throttled capacity should be passed to scheduler.

--
Warm Regards
Thara (She/Her/Hers)

So for starters, I think you should replicate the update of thermal
pressure in your h/w driver when you know that h/w is
throttling/throttled the frequency. You can refer to cpufreq_cooling.c
to see how it is done.

Moving to a common api can be done as a separate patch series.


Thank you for the clarification and suggestion.