linux-kernel - Re: [discussion]sched: a rough proposal to enable power saving in scheduler

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Message-ID: <20120821094203.GB12385@gmail.com>
Date:	Tue, 21 Aug 2012 11:42:04 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Matthew Garrett <mjg59@...f.ucam.org>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Alex Shi <alex.shi@...el.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	vincent.guittot@...aro.org, svaidy@...ux.vnet.ibm.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [discussion]sched: a rough proposal to enable power saving in
 scheduler


* Matthew Garrett <mjg59@...f.ucam.org> wrote:

> On Mon, Aug 20, 2012 at 10:06:06AM +0200, Ingo Molnar wrote:
> 
> > If the answer is 'yes' then there's clear cases where the kernel 
> > (should) automatically know the events where we switch from 
> > balancing for performance to balancing for power:
> 
> No. We can't identify all of these cases and we can't identify 
> corner cases. [...]

There's no need to identify 'all' of these cases - but if the 
kernel knows then it can have intelligent default behavior.

> [...] Putting this kind of policy in the kernel is an awful 
> idea. [...]

A modern kernel better know what state the system is in: on 
battery or on AC power.

> [...] It should never be altering policy itself, [...]

The kernel/scheduler simply offers sensible defaults where it 
can. User-space can augment/modify/override that in any which 
way it wishes to.

This stuff has not been properly sorted out in the last 10+ 
years since we have battery driven devices, so we might as well 
start with the kernel offering sane default behavior where it 
can ...

> [...] because it'll get it wrong and people will file bugs 
> complaining that it got it wrong and the biggest case where 
> you *need* to be able to handle switching between performance 
> and power optimisations (your rack management unit just told 
> you that you're going to have to drop power consumption by 
> 20W) is one where the kernel doesn't have all the information 
> it needs to do this. So why bother at all?

The point is to have a working default mechanism.

Thanks,

	Ingo
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