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

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Date:   Wed, 9 Nov 2022 16:49:25 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Qais Yousef <qyousef@...alina.io>
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 Tue, Nov 08, 2022 at 07:48:43PM +0000, Qais Yousef wrote:
> 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).

Yeah, possibly.

So one thing that was key to that hack I proposed is that it is
per-task. This means we can either set or detect the task activation
period and use that to select an appropriate PELT multiplier.

But please explain; once tasks are in a steady state (60HZ, 90HZ or god
forbit higher), the utilization should be the same between the various
PELT window sizes, provided the activation period isn't *much* larger
than the window.

Are these things running a ton of single shot tasks or something daft
like that?