linux-kernel - Re: [RFC PATCH 0/1] sched/pelt: Change PELT halflife at runtime

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Message-ID: <20221108194843.i4qckcu7zwqstyis@airbuntu>
Date:   Tue, 8 Nov 2022 19:48:43 +0000
From:   Qais Yousef <qyousef@...alina.io>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Kajetan Puchalski <kajetan.puchalski@....com>,
        Jian-Min Liu <jian-min.liu@...iatek.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Vincent Donnefort <vdonnefort@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Abhijeet Dharmapurikar <adharmap@...cinc.com>,
        Qais Yousef <qais.yousef@....com>,
        linux-kernel@...r.kernel.org,
        Jonathan JMChen <jonathan.jmchen@...iatek.com>
Subject: Re: [RFC PATCH 0/1] sched/pelt: Change PELT halflife at runtime

On 11/07/22 14:41, Peter Zijlstra wrote:
> On Thu, Sep 29, 2022 at 03:41:47PM +0100, Kajetan Puchalski wrote:
> 
> > Based on all the tests we've seen, jankbench or otherwise, the
> > improvement can mainly be attributed to the faster ramp up of frequency
> > caused by the shorter PELT window while using schedutil.
> 
> Would something terrible like the below help some?
> 
> If not, I suppose it could be modified to take the current state as
> history. But basically it runs a faster pelt sum along side the regular
> signal just for ramping up the frequency.

A bit of a tangent, but this reminded me of this old patch:

	https://lore.kernel.org/lkml/1623855954-6970-1-git-send-email-yt.chang@mediatek.com/

I think we have a bit too many moving cogs that might be creating undesired
compound effect.

Should we consider removing margins in favour of improving util ramp up/down?
(whether via util_est or pelt hf).

Only worry is lower end devices; but it seems to me better improve util
response and get rid of these magic numbers - if we can. Having the ability to
adjust them at runtime will help defer the trade-offs to sys admins.


Thanks

--
Qais Yousef