Message-ID: <386d99d3-aa97-4069-8d63-d197262832bf@kylinos.cn>
Date: Thu, 10 Jul 2025 08:47:12 +0800
From: Zihuan Zhang <zhangzihuan@...inos.cn>
To: Christian Loehle <christian.loehle@....com>, xuewen.yan@...soc.com,
 vincent.guittot@...aro.org, mingo@...hat.com, peterz@...radead.org,
 juri.lelli@...hat.com
Cc: rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
 vschneid@...hat.com, hongyan.xia2@....com, linux-kernel@...r.kernel.org,
 ke.wang@...soc.com, di.shen@...soc.com, xuewen.yan94@...il.com,
 kprateek.nayak@....com, kuyo.chang@...iatek.com, juju.sung@...iatek.com,
 qyousef@...alina.io
Subject: Re: [PATCH v1] sched/uclamp: Exclude kernel threads from uclamp logic

Hi Christian,
Apologies for the late reply, and thanks for raising the concerns.

在 2025/7/3 18:17, Christian Loehle 写道:
> On 7/3/25 11:07, Zihuan Zhang wrote:
>> Hi Christian,
>>
>> Thanks for the question!
>>
>> 在 2025/7/3 17:22, Christian Loehle 写道:
>>> On 7/3/25 10:14, Zihuan Zhang wrote:
>>>> Kernel threads (PF_KTHREAD) are not subject to user-defined utilization
>>>> clamping. They do not represent user workloads and should not participate
>>>> in any uclamp logic, including:
>>> Why not?
>>>
>> As Xuewen mentioned, some kernel threads may intentionally set scheduling attributes for performance. So instead of unconditionally excluding all kernel threads, I’m now considering a more conservative approach:
>> skip only those kthreads that haven’t explicitly set any clamp values.
>>
>> This should help avoid unintended clamp aggregation while still supporting performance-tuned kthreads.
> I'm skeptical, fundamentally you cannot exclude some fair tasks from uclamp logic.
> At least the cpufreq part they will be affected by, so if you 'exclude' some
> kthread that doesn't have clamps set (i.e. has min=0, max=1024) its
> utilization may not contribute to sugov frequency selection by being
> clamped by other task(s) (let's say you only have one other task with
> max=0, excluding the unclamped kthread now leads to sugov requesting
> the lowest OPP? Is that always correct/desired?)
>
> Is there a specific issue you're trying to solve?
> FYI there has been discussion around reworking the uclamp mechanism to solve
> some issues you may have been facing, but so far they haven't lead anywhere:
> https://lore.kernel.org/lkml/cover.1741091349.git.hongyan.xia2@arm.com/
Our original motivation stems from the observation that uclamp is 
primarily designed to manage frequency selection based on user-space 
task behavior. Kernel threads typically do not represent user workloads 
and are often not considered meaningful participants in uclamp-driven 
decisions.
To be clear, we are not aiming to exclude all kthreads from affecting 
frequency, but rather to explore ways to avoid unnecessary uclamp 
aggregation overhead from kernel threads that have no explicit clamp 
values set (i.e. uclamp.min=0, max=1024).
As you pointed out, fully excluding these tasks might interfere with 
sugov behavior in certain edge cases. So a more balanced approach might be:

- For kernel threads that do not set any clamp values, skip the clamp 
aggregation step

- If a kernel thread explicitly sets clamp attributes, it should of 
course remain fully visible to uclamp logic.

This would preserve correctness while reducing unnecessary overhead in 
the hot path, especially on systems with many runnable tasks.

That said, we're open to alternative suggestions if there's a better way 
to achieve this optimization. We'll also review the earlier rework 
discussion you linked, thank you for pointing that out.


Best regards,
Zihuan Zhang

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