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

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Date:   Tue, 19 Jun 2018 12:19:01 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Quentin Perret <quentin.perret@....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 19/06/18 11:02, Quentin Perret wrote:
> 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.

It's actually built on the (current) assumption that siblings share
capacity [1], so it seems to align with what EM requires.

[1] https://elixir.bootlin.com/linux/latest/source/drivers/base/arch_topology.c#L71