linux-kernel - Re: [RFC PATCH v3 10/10] arch_topology: Start Energy Aware Scheduling

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Message-ID: <20180619100224.GT17720@e108498-lin.cambridge.arm.com>
Date:   Tue, 19 Jun 2018 11:02:24 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     Pavan Kondeti <pkondeti@...eaurora.org>, peterz@...radead.org,
        rjw@...ysocki.net, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        mingo@...hat.com, dietmar.eggemann@....com,
        morten.rasmussen@....com, chris.redpath@....com,
        patrick.bellasi@....com, valentin.schneider@....com,
        vincent.guittot@...aro.org, thara.gopinath@...aro.org,
        viresh.kumar@...aro.org, tkjos@...gle.com, joelaf@...gle.com,
        smuckle@...gle.com, adharmap@...cinc.com, skannan@...cinc.com,
        edubezval@...il.com, srinivas.pandruvada@...ux.intel.com,
        currojerez@...eup.net, javi.merino@...nel.org
Subject: Re: [RFC PATCH v3 10/10] arch_topology: Start Energy Aware Scheduling

On Tuesday 19 Jun 2018 at 11:47:14 (+0200), Juri Lelli wrote:
> On 19/06/18 10:40, Quentin Perret wrote:
> > Hi Pavan,
> > 
> > On Tuesday 19 Jun 2018 at 14:48:41 (+0530), Pavan Kondeti wrote:
> 
> [...]
> 
> > > There seems to be a sysfs interface exposed by this driver to change cpu_scale.
> > > Should we worry about it? I don't know what is the usecase for changing the
> > > cpu_scale from user space.
> > 
> > This is something I've been wondering as well. TBH, I'm not sure what to
> > do in this case. And I'm not sure to know what is the use-case either.
> > Debugging purpose I assume ?
> > 
> > Juri, did you have a specific use-case for this feature when the
> > arch_topology driver was first introduced ? Or was it just to align
> > with the existing arm/arm64 code ?
> 
> It was requested (IIRC) because DT might have bogus values and not be
> easily modifiable. So, this is another way to get things right for your
> platform at runtime.

Right, but that also allows you to set different capacities to CPUs
inside the same freq domain, which isn't supported by the EM framework,
at least for now. So I would prefer to assume that your values in DT must
to be correct to use EAS, and leave the code as-is for now.