Date: Tue, 21 Aug 2012 11:42:04 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, Alex Shi <alex.shi@intel.com>,
        Suresh Siddha <suresh.b.siddha@intel.com>, vincent.guittot@linaro.org,
        svaidy@linux.vnet.ibm.com, Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, Paul Turner <pjt@google.com>
Subject: Re: [discussion]sched: a rough proposal to enable power saving in
 scheduler
Message-ID: <20120821094203.GB12385@gmail.com>
References: <5028F12C.7080405@intel.com>
 <1345028738.31459.82.camel@twins>
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 <502BB5A3.5000403@linux.intel.com>
 <1345043096.31459.106.camel@twins>
 <502BE38D.9030405@linux.intel.com>
 <20120820080606.GA6931@gmail.com>
 <20120820181651.GA737@srcf.ucam.org>
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* 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
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