linux-kernel - Re: [RFC PATCH 0/6] sched: uclamp sum aggregation

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Date:   Tue, 5 Dec 2023 15:18:58 +0000
From:   Hongyan Xia <hongyan.xia2@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Qais Yousef <qyousef@...alina.io>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Lukasz Luba <lukasz.luba@....com>,
        Christian Loehle <christian.loehle@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/6] sched: uclamp sum aggregation

On 04/12/2023 16:12, Vincent Guittot wrote:
> On Mon, 4 Dec 2023 at 02:48, Hongyan Xia <hongyan.xia2@....com> wrote:
>>
>> [...]
>>
>> Other shortcomings are not that critical, but the fact that uclamp_min's
>> effectiveness is divided by N under max aggregation I think is not
>> acceptable.
> 
> Change EAS task placement policy in this case to take into account
> actual utilization and uclamp_min/max

Thank you. I agree. I want to emphasize this specifically because this 
is exactly what I'm trying to do. The whole series can be rephrased in a 
different way:

- The PELT signal is distorted when uclamp is active.
- Let's consider the [PELT, uclamp_min, uclamp_max] tuple.
- Always carrying all three variables is too much, but [PELT, 
clamped(PELT)] is an approximation that works really well.

Of course, I'll explore if there's a way to make things less messy. I 
just realized why I didn't do things util_est way but instead directly 
clamping on PELT, it's because util_est boosts util_avg and can't work 
for uclamp_max. I'll keep exploring options.

>> [...]