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

[Ingo Molnar <mingo@kernel.org>]

* Matthew Garrett <mjg59@srcf.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