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

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Date:   Mon, 20 Feb 2023 11:13:18 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Qais Yousef <qyousef@...alina.io>,
        Kajetan Puchalski <kajetan.puchalski@....com>,
        Jian-Min Liu <jian-min.liu@...iatek.com>,
        Ingo Molnar <mingo@...nel.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 Thu, Feb 09, 2023 at 05:16:46PM +0100, Vincent Guittot wrote:

> > The results is very similar to PELT halflife reduction. The advantage is
> > that 'util_est_faster' is only activated selectively when the runtime of
> > the current task in its current activation is long enough to create this
> > CPU util boost.
> 
> IIUC how util_est_faster works, it removes the waiting time when
> sharing cpu time with other tasks. So as long as there is no (runnable
> but not running time), the result is the same as current util_est.

Uh.. it's double the speed, no? Even if there is no contention, the
fake/in-situ pelt sum runs at double time and thus will ramp up faster
than normal.

> util_est_faster makes a difference only when the task alternates
> between runnable and running slices.

UTIL_EST was supposed to help mitigate some of that, but yes. Also note
that _FASTER sorta sucks here because it starts from 0 every time, if it
were to start from the state saved by util_est_dequeue(), it would ramp
up faster still.

Patch has a comment along those lines I think.

> Have you considered using runnable_avg metrics in the increase of cpu
> freq ? This takes into the runnable slice and not only the running
> time and increase faster than util_avg when tasks compete for the same
> CPU

Interesting! Indeed, that's boosting the DVFS for contention. And as
deggeman's reply shows, it seems to work well.

I wonder if that one place where it regresses is exactly the case
without contention.