linux-kernel - Re: [PATCH v2] PM: EM: Add inotify support when the energy model is updated.

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Message-ID: <96b4ae67-b9a8-47d3-a951-a880848e6719@arm.com>
Date: Thu, 22 May 2025 09:19:52 +0100
From: Lukasz Luba <lukasz.luba@....com>
To: Changwoo Min <changwoo@...lia.com>
Cc: christian.loehle@....com, tj@...nel.org, pavel@...nel.org,
 kernel-dev@...lia.com, linux-pm@...r.kernel.org,
 linux-kernel@...r.kernel.org, len.brown@...el.com,
 "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH v2] PM: EM: Add inotify support when the energy model is
 updated.

On 5/10/25 12:34, Rafael J. Wysocki wrote:
> On Sat, May 10, 2025 at 7:07 AM Changwoo Min <changwoo@...lia.com> wrote:
>>
>> Thank you, Rafael, for the pointer.
>>
>> On 5/10/25 01:41, Rafael J. Wysocki wrote:
>>>>
>>>> I have discussed that with Rafael and we have similar view.
>>>> The EM debugfs is not the right interface for this purpose.
>>>>
>>>> A better design and mechanism for your purpose would be the netlink
>>>> notification. It is present in the kernel in thermal framework
>>>> and e.g. is used by Intel HFI
>>>> - drivers/thermal/intel/intel_hfi.c
>>>> - drivers/thermal/thermal_netlink.c
>>>> It's able to send to the user space the information from FW about
>>>> the CPUs' efficiency changes, which is similar to this EM modification.
>>>
>>> In addition, after this patch
>>>
>>> https://lore.kernel.org/linux-pm/3637203.iIbC2pHGDl@rjwysocki.net/
>>>
>>> which is about to get into linux-next, em_dev_update_perf_domain()
>>> will not be the only place where the Energy Model can be updated.
>>
>> I am curious about whether the energy mode is likely to be updated more
>> often with this change. How often the energy model is likely to be
>> updated is the factor to be considered for the interface and the model
>> to post-processing the eneergy model (in the BPF schedulers).
> 
> It really is hard to say precisely because eventually this will depend
> on the platform firmware.  Hopefully, this is not going to happen too
> often, but if the thermal envelope of the platform is tight, for
> instance, it may not be the case.

It's hard to say for all use cases, but there are some easy to measure
and understand:

1. Long scenarios with heavy GPU usage (e.g. gaming). Power on CPUs
    built from High-Performance cells can be affected by +20% and after
    ~1min
2. Longer recording with heavy ISP usage, similar to above

In those two, it's sufficient to update the EM every 1-3sec to reach
this +20% after 60sec. Although, at the beginning when the GPU starts
heating the updates should happen a bit more often.

There are some more complex cases, e.g. when more than 1 Big CPU does
heavy computations and the heat is higher than normal EM model of
single CPU (even for that scenario profile). Then the updates to EM
can go a bit more often (it depends what the platform would like
to leverage and achieve w/ SW).