Re: power-efficient scheduling design

From: Morten Rasmussen
Date: Wed Jun 19 2013 - 13:00:51 EST

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.


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