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Date:   Thu, 7 Sep 2023 15:42:13 +0100
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Subject: Re: [RFC PATCH 0/7] sched: cpufreq: Remove magic margins
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To:     Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Qais Yousef <qyousef@layalina.io>
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From:   Lukasz Luba <lukasz.luba@arm.com>
In-Reply-To: <20230907142923.GJ10955@noisy.programming.kicks-ass.net>
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On 9/7/23 15:29, Peter Zijlstra wrote:
> On Thu, Sep 07, 2023 at 02:57:26PM +0100, Lukasz Luba wrote:
>>
>>
>> On 9/7/23 14:26, Peter Zijlstra wrote:
>>> On Wed, Sep 06, 2023 at 10:18:50PM +0100, Qais Yousef wrote:
>>>
>>>> This is probably controversial statement. But I am not in favour of util_est.
>>>> I need to collect the data, but I think we're better with 16ms PELT HALFLIFE as
>>>> default instead. But I will need to do a separate investigation on that.
>>>
>>> I think util_est makes perfect sense, where PELT has to fundamentally
>>> decay non-running / non-runnable tasks in order to provide a temporal
>>> average, DVFS might be best served with a termporal max filter.
>>>
>>>
>>
>> Since we are here...
>> Would you allow to have a configuration for
>> the util_est shifter: UTIL_EST_WEIGHT_SHIFT ?
>>
>> I've found other values than '2' better in some scenarios. That helps
>> to prevent a big task to 'down' migrate from a Big CPU (1024) to some
>> Mid CPU (~500-700 capacity) or even Little (~120-300).
> 
> Larger values, I'm thinking you're after? Those would cause the new
> contribution to weight less, making the function more smooth, right?

Yes, more smooth, because we only use the 'ewma' goodness for decaying
part (not the raising [1]).

> 
> What task characteristic is tied to this? That is, this seems trivial to
> modify per-task.

In particular Speedometer test and the main browser task, which reaches
~900util, but sometimes vanish and waits for other background tasks
to do something. In the meantime it can decay and wake-up on
Mid/Little (which can cause a penalty to score up to 5-10% vs. if
we pin the task to big CPUs). So, a longer util_est helps to avoid
at least very bad down migration to Littles...

[1] https://elixir.bootlin.com/linux/v6.5.1/source/kernel/sched/fair.c#L4442