linux-kernel - Re: [RFC PATCH 4/6] sched/fair: Introduce an energy estimation helper function

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Date:   Thu, 22 Mar 2018 05:05:29 +0000
From:   Quentin Perret <quentin.perret@....com>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        linux-pm@...r.kernel.org,
        Morten Rasmussen <morten.rasmussen@....com>,
        Chris Redpath <chris.redpath@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [RFC PATCH 4/6] sched/fair: Introduce an energy estimation
 helper function

On Wednesday 21 Mar 2018 at 15:54:58 (+0000), Patrick Bellasi wrote:
> On 21-Mar 14:26, Quentin Perret wrote:
> > On Wednesday 21 Mar 2018 at 12:39:21 (+0000), Patrick Bellasi wrote:
> > > On 20-Mar 09:43, Dietmar Eggemann wrote:
> > > > From: Quentin Perret <quentin.perret@....com>

[...]

> > So actually, what I can do is add something like
> > 
> >     fdom_tot_util += util;
> > 
> > to this loop and compute
> > 
> >     energy = cs->power * fdom_tot_util / cs->cap;
> > 
> > only once, instead of having the second loop to compute the energy. We don't
> > have to scale the util for each and every CPU since they share the same
> > cap state. That would save some divisions and ensure the consistency
> > between the selection of the cap state and the associated energy
> > computation. What do you think ?
> 
> Right, would say that under the hypothesis the we are in the same
> frequency domain (and we are because of fdom->span), that's basically
> doing:
> 
>    sum_i(P_x * U_i / C_x) => P_x / C_x * sum_i(U_i)
> 
> Where (C_x, P_x) are the EM reported capacity and power for the
> expected frequency domain OPP.
> 

Yes that's exactly that. I'll do the change in v2.

> > Or maybe you were talking about consistency between several consecutive
> > calls to compute_energy() ?
> 
> Nope, the above +1
> 

[...]

> > I agree that it would be nice to document somewhere that
> > compute_energy() is unsafe to call without sched_energy_present.
> > I can simply add a proper doc comment to this function actually.
> > Would that work ?
> 
> Right, it's just that _maybe_ an explicit BUG_ON is improving the
> documentation by making more explicit the error on testing ?
> 
> Thus, I would probably add both... but Peter will tell you for sure ;)
> 

Right, but I'm still not sure if the BUG_ON is the right thing to do. I
mean, if we really want to make this check, then we could also try
to recover into a working state ... If we enter compute_energy() without
having an energy model, and if we detect it on time, we could bail out
and disable sched_energy_present ASAP with an error message for example.
That would let us know if EAS is broken without making the system
unusable.

Anyways, if there is a general agreement, or if the maintainers think
that the BUG_ON is the right thing to do here, I'm happy to change that
in future versions :)