Re: [RFC] sched: CPU topology try

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jan 08, 2014 at 01:27:39PM +0000, Morten Rasmussen wrote:
> On Wed, Jan 08, 2014 at 12:45:34PM +0000, Peter Zijlstra wrote:
> > On Wed, Jan 08, 2014 at 12:35:34PM +0000, Morten Rasmussen wrote:
> > > > Currently we detect overload by sg.nr_running >= sg.capacity, which can
> > > > be very misleading because while a cpu might have a task running 'now'
> > > > it might be 99% idle.
> > > > 
> > > > At which point I argued we should change the capacity thing anyhow. Ever
> > > > since the runnable_avg patch set I've been arguing to change that into
> > > > an actual utilization test.
> > > > 
> > > > So I think that if we measure overload by something like >95% utilization
> > > > on the entire group the load scaling again makes perfect sense.
> > > 
> > > I agree that it make more sense to change the overload test to be based
> > > on some tracked load. How about the non-overloaded case? Load balancing
> > > would have to be based on unweighted task loads in that case?
> > 
> > Yeah, until we're overloaded our goal is to minimize idle time.
> 
> I would say, make the most of the available cpu cycles. Minimizing idle
> time is not always the right thing to do when considering power
> awareness.
> 
> If we know the actual load of the tasks, we may be able to consolidate

I think we must start to be careful with the word load, I think you
meant to say utilization. 

> them on fewer cpus and save power by idling cpus. In that case the idle
> time (total) is unchanged (unless the P-state is changed). Somewhat
> similar to the video use-case running on 1, 2, and 4 cpu that I reposted
> yesterday.

But fair enough.. Its idle time when you consider CPUs to always run at
max frequency, but clearly I must stop thinking about CPUs like that :-)