linux-kernel - Re: [PATCH V2] sched: fair: Use the earliest break even

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Date:   Wed, 18 Mar 2020 09:24:52 +0100
From:   Morten Rasmussen <morten.rasmussen@....com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Qais Yousef <qais.yousef@....com>,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [PATCH V2] sched: fair: Use the earliest break even

On Tue, Mar 17, 2020 at 06:07:43PM +0100, Daniel Lezcano wrote:
> On 17/03/2020 15:30, Morten Rasmussen wrote:
> > On Tue, Mar 17, 2020 at 02:48:51PM +0100, Daniel Lezcano wrote:
> >> On 17/03/2020 08:56, Morten Rasmussen wrote:
> >>> On Thu, Mar 12, 2020 at 11:04:19AM +0100, Daniel Lezcano wrote:
> >>>>>> In order to be more energy efficient but without impacting the
> >>>>>> performances, let's use another criteria: the break even deadline.
> >>>>>>
> >>>>>> At idle time, when we store the idle state the CPU is entering in, we
> >>>>>> compute the next deadline where the CPU could be woken up without
> >>>>>> spending more energy to sleep.
> >>>
> >>> I don't follow the argument that sleeping longer should improve energy
> >>> consumption. 
> >>
> >> May be it is not explained correctly.
> >>
> >> The patch is about selecting a CPU with the smallest break even deadline
> >> value. In a group of idle CPUs in the same idle state, we will pick the
> >> one with the smallest break even dead line which is the one with the
> >> highest probability it already reached its target residency.
> >>
> >> It is best effort.
> > 
> > Indeed. I get what the patch does, I just don't see how the patch
> > improves energy efficiency.
> 
> If the CPU is woken up before it reached the break even, the idle state
> cost in energy is greater than the energy it saved.
> 
> Am I misunderstanding your point?

Considering just the waking then yes, it reaches energy break-even.
However, considering all the CPUs in the system, it just moves the idle
entry/exit energy cost to a different CPU, it doesn't go away.

Whether you have:

               |-BE-|
           ____    ____
CPU0:  ___/    \__/    \___

CPU1:  ____________________

Or:
               |-BE-|
           ____
CPU0:  ___/    \___________
                   ____
CPU1:  ___________/    \___

_
  = CPU busy = P_{busy}
_ = CPU idle = P_{idle}
/ = CPU idle exit = P_{exit}
\ = CPU idle entry = P_{entry}

The sum of areas under the curves is the same, i.e. the total energy is
unchanged.

Morten