On Tue, Jun 03, 2014 at 04:59:39PM +0100, Peter Zijlstra wrote:

On Tue, Jun 03, 2014 at 01:03:54PM +0100, Morten Rasmussen wrote:

quoted

An unweighted version of cfs.runnable_load_avg gives you a metric that
captures cpu utilization to some extend, but not the number of tasks.
And it reflects task migrations immediately unlike the rq
runnable_avg_sum.

So runnable_avg would be equal to the utilization as long as
there's idle time, as soon as we're over-loaded the metric shows how
much extra cpu is required.

That is, runnable_avg - running_avg >= 0 and the amount is the
exact amount of extra cpu required to make all tasks run but not have
idle time.

Yes, roughly. runnable_avg goes up quite steeply if you have many tasks
on a fully utilized cpu, so the actual amount of extra cpu required
might be somewhat lower. I can't come up with something better, so I
agree.

quoted

Agreed, but I think it is quite important to discuss what we understand
by cpu utilization. It seems to be different depending on what you want
to use it for.

I understand utilization to be however much cpu is actually used, so I
would, per the existing naming, call running_avg to be the avg
utilization of a task/group/cpu whatever.

I see your point, but for load balancing purposes we are more intested
in the runnable_avg as it tells us about the cpu capacity requirements.
I don't like to throw more terms into the mix, but you could call
runnable_avg the potential task/group/cpu utilization. This is an
estimate of how much utilization a task would cause if we moved it to an
idle cpu. That might be quite different from running_avg on an
over-utilized cpu.

quoted

We have done experiments internally with rq runnable_avg_sum for
load-balancing decisions in the past and found it unsuitable due to its
slow response to task migrations. That is why I brought it up here.

So I'm not entirely seeing that from the code (I've not traced this),
afaict we actually update the per-cpu values on migration based on the
task values.

old_rq->sum -= p->val;
new_rq->sum += p->val;

like,.. except of course totally obscured.

Yes, for cfs.runnable_load_avg, rq->avg.runnable_avg_sum is different.
See the other reply.