Re: [RFCv2 PATCH 00/23] sched: Energy cost model for energy-aware scheduling

[Peter Zijlstra]

On Mon, Jul 07, 2014 at 03:00:18PM +0100, Morten Rasmussen wrote:
> > Could this be addressed by making the scheduler more "proactive" and,
> > rather than just looking at the current energy diff, guesstimate what it
> > would be if not placing a task at all on the CPU? If for example there
> > is no other task running on that CPU, could energy_diff_task() take into
> > account the next deeper C-state rather than just the current one? This
> > way we may be able to achieve more packing even on fully symmetric
> > systems and allow CPUs to go into deeper sleep states.
> 
> I think it would be possible to bias the choice of cpu either by
> considering potential energy savings by letting some cpus get into a
> deeper C-state, or applying a static bias towards some cpus (lower cpuid
> for example). Since it is in the wakeup path it must not be too complex
> to figure out though.
> 
> I haven't seen the problem in reality yet. When I tried the short tasks
> test with all cpus using the same energy model I got tasks consolidated
> on either of the clusters. The consolidation cluster sometimes changed
> during the test.
> 
> There is a lot of tuning to be done, that is for sure. We will have to
> make similar decisions for the periodic/idle balance path as well.

So one of the things I mentioned previously (on IRC, to Morton) is that
we can use the energy numbers (P and C state) to precompute whether or
not race-to-idle makes sense for the platform. Or if it benefits from
packing etc..

So at topology setup time we can statically determine some of these
policies (maybe with a few parameters) and take it from there.

So if the platform benefits from packing, we can set the appropriate
topology bits to do so. If it benefits from race-to-idle, it can select
that, etc.

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