linux-kernel - Re: [RFC PATCH 0/7] sched: cpufreq: Remove magic margins

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Message-ID: <20230908135843.GC24372@noisy.programming.kicks-ass.net>
Date:   Fri, 8 Sep 2023 15:58:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Qais Yousef <qyousef@...alina.io>
Cc:     Ingo Molnar <mingo@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Lukasz Luba <lukasz.luba@....com>
Subject: Re: [RFC PATCH 0/7] sched: cpufreq: Remove magic margins

On Fri, Sep 08, 2023 at 02:33:36PM +0100, Qais Yousef wrote:

> > > UTIL_EST_FASTER moves in one direction. And it's a constant response too, no?
> > 
> > The idea of UTIL_EST_FASTER was that we run a PELT sum on the current
> > activation runtime, all runtime since wakeup and take the max of this
> > extra sum and the regular thing.
> > 
> > On top of that this extra PELT sum can/has a time multiplier and thus
> > ramps up faster (this multiplies could be per task). Nb.:
> > 
> > 	util_est_fast = faster_est_approx(delta * 2);
> > 
> > is a state-less expression -- by making
> > 
> > 	util_est_fast = faster_est_approx(delta * curr->se.faster_mult);
> > 
> > only the current task is affected.
> 
> Okay; maybe I didn't understand this fully and will go back and study it more.
> 
> Maybe the word faster is what makes me worried as I really see faster is not
> what people want on a class of systems; or at least CPUs if you think of HMP.
> Taming the beast is a more difficult problem in this class of systems.

The faster refers to the ramp-up. Which was the issue identified in that
earlier thread. The game thing wanted to ramp up more agressive.