linux-kernel - Re: power-efficient scheduling design

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Date:	Tue, 11 Jun 2013 18:48:58 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	David Lang <david@...g.hm>
CC:	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Catalin Marinas <catalin.marinas@....com>,
	Ingo Molnar <mingo@...nel.org>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	"alex.shi@...el.com" <alex.shi@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Mike Galbraith <efault@....de>,
	"pjt@...gle.com" <pjt@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linaro-kernel <linaro-kernel@...ts.linaro.org>,
	"len.brown@...el.com" <len.brown@...el.com>,
	"corbet@....net" <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: power-efficient scheduling design

On 6/11/2013 5:27 PM, David Lang wrote:
>
> Nobody is saying that this sort of thing should be in the fastpath of the scheduler.
>
> But if the scheduler has a table that tells it the possible states, and the cost to get from the current state to each of these states (and to get back and/or wake up to
> full power), then the scheduler can make the decision on what to do, invoke a routine to make the change (and in the meantime, not be fighting the change by trying to
> schedule processes on a core that's about to be powered off), and then when the change happens, the scheduler will have a new version of the table of possible states and costs
>
> This isn't in the fastpath, it's in the rebalancing logic.

the reality is much more complex unfortunately.
C and P states hang together tightly, and even C state on
one core impacts other cores' performance, just like P state selection
on one core impacts other cores.

(at least for x86, we should really stop talking as if the OS picks the "frequency",
that's just not the case anymore)

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