Re: power-efficient scheduling design

From: Morten Rasmussen <morten.rasmussen@arm.com>
To: Arjan van de Ven <arjan@linux.intel.com>
Cc: David Lang <david@lang.hm>, Ingo Molnar <mingo@kernel.org>,
	"alex.shi@intel.com" <alex.shi@intel.com>,
	"peterz@infradead.org" <peterz@infradead.org>,
	"preeti@linux.vnet.ibm.com" <preeti@linux.vnet.ibm.com>,
	"vincent.guittot@linaro.org" <vincent.guittot@linaro.org>,
	"efault@gmx.de" <efault@gmx.de>,
	"pjt@google.com" <pjt@google.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linaro-kernel@lists.linaro.org" <linaro-kernel@lists.linaro.org>,
	"len.brown@intel.com" <len.brown@intel.com>,
	"corbet@lwn.net" <corbet@lwn.net>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	Catalin Marinas <Catalin.Marinas@arm.com>
Subject: Re: power-efficient scheduling design
Date: Wed, 19 Jun 2013 18:00:43 +0100	[thread overview]
Message-ID: <20130619170042.GH5460@e103034-lin> (raw)
In-Reply-To: <51C1D0BB.3040705@linux.intel.com>

On Wed, Jun 19, 2013 at 04:39:39PM +0100, Arjan van de Ven wrote:
> On 6/18/2013 10:47 AM, David Lang wrote:
> 
> >
> > It's bad enough trying to guess the needs of the processes, but if you also are reduced to guessing the capabilities of the cores, how can anything be made to work?
> 
> btw one way to look at this is to assume that (with some minimal hinting)
> the CPU driver will do the right thing and get you just about the best performance you can get
> (that is appropriate for the task at hand)...
> ... and don't do anything in the scheduler proactively.

If I understand correctly, you mean if your hardware/firmware is fully
in control of the p-state selection and changes it fast enough to match
the current load, the scheduler doesn't have to care? By fast enough I
mean, faster than the scheduler would notice if a cpu was temporarily
overloaded at a low p-state. In that case, you wouldn't need
cpufreq/p-state hints, and the scheduler would only move tasks between
cpus when cpus are fully loaded at their max p-state.

> 
> Now for big.little and other temporary or permanent asymmetries, we may want to
> have a "max performance level" type indicator, and that's fair enough
> (and this can be dynamic, since it for thermal reasons this can change over time,
> but on a somewhat slower timescale)
> 
> 
> the hints I have in mind are not all that complex; we have the biggest issues today
> around task migration (the task migrates to a cold cpu... so a simple notifier chain
> on the new cpu as it is accepting a task and we can bump it up), real time tasks
> (again, simple notifier chain to get you to a predictably high performance level)
> and we're a long way better than we are today in terms of actual problems.
> 
> For all the talk of ondemand (as ARM still uses that today)... that guy puts you in
> either the lowest or highest frequency over 95% of the time. Other non-cpufreq solutions
> like on Intel are bit more advanced (and will grow more so over time), but even there,
> in the grand scheme of things, the scheduler shouldn't have to care anymore with those
> two notifiers in place.

You would need more than a few hints to implement more advanced capacity
management like proposed for the power scheduler. I believe that Intel
would benefit as well from guiding the scheduler to idle the right cpu
to enable deeper idle states and/or enable turbo-boost for other cpus.

Morten