Re: [RFC PATCH 06/16] arm: topology: Define TC2 sched energy and provide it to scheduler

[Peter Zijlstra <peterz@infradead.org>]

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On Mon, Jun 02, 2014 at 03:15:36PM +0100, Morten Rasmussen wrote:
> > 
> > Talk to me about this core vs cluster thing.
> > 
> > Why would an architecture have multiple energy domains like this?

> The reason is that power domains are often organized in a hierarchy
> where you may be able to power down just a cpu or the entire cluster
> along with cluster wide shared resources. This is quite typical for ARM
> systems. Frequency domains (P-states) typically cover the same hardware
> as one of the power domain levels. That is, there might be several
> smaller power domains sharing the same frequency (P-state) or there
> might be a power domain spanning multiple frequency domains.
> 
> The main reason why we need to worry about all this is that it typically
> cost a lot more energy to use the first cpu in a cluster since you
> also need to power up all the shared hardware resources than the energy
> cost of waking and using additional cpus in the same cluster.
> 
> IMHO, the most natural way to model the energy is therefore something
> like:
> 
>     energy = energy_cluster + n * energy_cpu
> 
> Where 'n' is the number of cpus powered up and energy_cluster is the
> cost paid as soon as any cpu in the cluster is powered up.

OK, that makes sense, thanks! Maybe expand the doc/changelogs with this
because it wasn't immediately clear to me.

> > Also, in general, why would we need to walk the domain tree all the way
> > up, typically I would expect to stop walking once we've covered the two
> > cpu's we're interested in, because above that nothing changes.
> 
> True. In some cases we don't have to go all the way up. There is a
> condition in energy_diff_load() that bails out if the energy doesn't
> change further up the hierarchy. There might be scope for improving that
> condition though.
> 
> We can basically stop going up if the utilization of the domain is
> unchanged by the change we want to do. For example, we can ignore the
> next level above if a third cpu is keeping the domain up all the time
> anyway. In the 100% + 50% case above, putting another 50% task on the
> 50% cpu wouldn't affect the cluster according the proposed model, so it
> can be ignored. However, if we did the same on any of the two cpus in
> the 50% + 25% example we affect the cluster utilization and have to do
> the cluster level maths.
> 
> So we do sometimes have to go all the way up even if we are balancing
> two sibling cpus to determine the energy implications. At least if we
> want an energy score like energy_diff_load() produces. However, we might
> be able to take some other shortcuts if we are balancing load between
> two specific cpus (not wakeup/fork/exec balancing) as you point out. But
> there are cases where we need to continue up until the domain
> utilization is unchanged.

Right.. so my worry with this is scalability. We typically want to avoid
having to scan the entire machine, even for power aware balancing.

That said, I don't think we have a 'sane' model for really big hardware
(yet). Intel still hasn't really said anything much on that iirc, as
long as a single core is up, all the memory controllers in the numa
fabric need to be awake, not to mention to cost of keeping the dram
alive.



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