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

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Date:   Wed, 18 Mar 2020 15:38:23 +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 Wed, Mar 18, 2020 at 11:17:49AM +0100, Daniel Lezcano wrote:
> On 18/03/2020 09:24, Morten Rasmussen wrote:
> > 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.
> 
> It is a counter-intuitive comment, now I get it, thanks for the
> clarification. It is a good point.

No problem.

> Taking into consideration the dynamic, in the case #1, the break even is
> not reached, the idle duration is smaller and that leads the governor to
> choose shallower idle states after and consequently CPU0 will be used in
> priority. We end up with CPU0 in a shallow state and CPU1 in a deep state.

Indeed. I was speculating earlier if the opposite could happen too. If
we extended the second case to form a repeating pattern, could we
prevent somehow prevent CPU1 from reaching a deeper state? Could we have
pattern that would keep both CPUs in shallow state where it would have
been more efficient to consolidate the wake-ups on CPU0 and let CPU1
enter deeper states?

> 
> With the case #2, we can have the CPUs in both deep state and the
> governor should be keeping choosing the same idle state.

Ideally yes. However it depends on the break-even times of the deeper
states and when the next wake-ups happen.

> I don't know what is more energy/perf efficient. IMO this is very
> workload dependent. The only way to check is to test. Hopefully I can
> find a platform for that.

Moving the wake-up shouldn't impact energy directly, although it have a
positive latency impact as you are more likely to avoid waking up CPUs
that haven't finished the idle entry sequence. However, changing the
wake-up pattern could have an indirect energy impact, positive or
negative. It isn't clear to me either what outcome to expect.

Morten