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

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Date:	Wed, 25 Mar 2015 19:14:13 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	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>,
	"mturquette@...aro.org" <mturquette@...aro.org>,
	"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

On Wed, Mar 25, 2015 at 06:01:22PM +0000, Juri Lelli wrote:

> Yes and no, IMHO. It makes perfect sense to trigger cpufreq on the
> target_cpu's freq domain, as we know that we are going to add p's
> utilization there.

Fair point; I mainly wanted to start this discussion so that seems to
have been a success :-)

> Anyway, I was thinking that we could just
> rely on triggering points in {en,de}queue_task_fair and task_tick_fair.
> We end up calling one of them every time we wake-up a task, perform
> a load balancing decision or just while running the task itself
> (we have to react to tasks phase changes). This way we should be
> able to reduce the number of triggering points and be more general
> at the same time.

The one worry I have with that is that it might need to re-compute which
P state to request, where in the above (now trimmed quoted) code we
already figured out which P state we needed to be in, any hook in
enqueue would have forgotten that.

> > So does it make sense to at least put in the right hooks now? I realize
> > we'll likely take cpufreq out back and feed it to the bears but
> > something managing P states will be there whatever we'll call the new
> > fangled thing and this would be the place to hook it still.
> > 
> 
> We should be able to clean up and post something along this line
> fairly soon.

Grand!
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