linux-kernel - Re: [RFCv3 PATCH 33/48] sched: Energy-aware wake-up task placement

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Message-ID: <20150427160113.16410.10935@quantum>
Date:	Mon, 27 Apr 2015 09:01:13 -0700
From:	Michael Turquette <mturquette@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>,
	"Juri Lelli" <juri.lelli@....com>
Cc:	"Morten Rasmussen" <Morten.Rasmussen@....com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"Dietmar Eggemann" <Dietmar.Eggemann@....com>,
	"yuyang.du@...el.com" <yuyang.du@...el.com>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"nico@...aro.org" <nico@...aro.org>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFCv3 PATCH 33/48] sched: Energy-aware wake-up task placement

Quoting Peter Zijlstra (2015-03-26 03:41:50)
> On Thu, Mar 26, 2015 at 10:21:24AM +0000, Juri Lelli wrote:
> >  - what about other sched classes? I know that this is very premature,
> >    but I can help but thinking that we'll need to do some sort of
> >    aggregation of requests, and if we put triggers in very specialized
> >    points we might lose some of the sched classes separation
> 
> So for deadline we can do P state selection (as you're well aware) based
> on the requested utilization. Not sure what to do for fifo/rr though,
> they lack much useful information (as always).
> 
> Now if we also look ahead to things like the ACPI CPPC stuff we'll see
> that CFS and DL place different requirements on the hints. Where CFS
> would like to hint a max perf (the hardware going slower due to the code
> consisting of mostly stalls is always fine from a best effort energy
> pov), the DL stuff would like to hint a min perf, seeing how it 'needs'
> to provide a QoS.
> 
> So we either need to carry this information along in a 'generic' way
> between the various classes or put the hinting in every class.
> 
> But yes, food for thought for sure.

I am a fan of putting the hints in every class. One idea I've been
considering is that each sched class could have a small, simple cpufreq
governor that expresses its constraints (max for cfs, min qos for dl)
and then the cpufreq core Does The Right Thing.

This would be a multi-governor approach, which requires some surgery to
cpufreq core code, but I like the modularity and maintainability of it
more than having one big super governor that has to satisfy every need.

Regards,
Mike
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