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Date: Tue, 21 Aug 2012 12:39:51 +0100
From: Matthew Garrett <mjg59@srcf.ucam.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, Alex Shi <alex.shi@intel.com>,
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Subject: Re: [discussion]sched: a rough proposal to enable power saving in
 scheduler
Message-ID: <20120821113951.GA22436@srcf.ucam.org>
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On Tue, Aug 21, 2012 at 11:42:04AM +0200, Ingo Molnar wrote:
> * Matthew Garrett <mjg59@srcf.ucam.org> wrote:
> > [...] 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.

That's a fundamentally uninteresting thing for the kernel to know about. 
AC/battery is just not an important power management policy input when 
compared to various other things.

> > [...] 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 ...

Userspace has been doing a perfectly reasonable job of determining 
policy here.
 
> > [...] 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.

Your suggestions aren't a working default mechanism.

-- 
Matthew Garrett | mjg59@srcf.ucam.org
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