[RFC PATCH v3 3/6] sched: pack small tasks

[Peter Zijlstra]

On Wed, Mar 27, 2013 at 03:51:51PM +0530, Preeti U Murthy wrote:
> Hi,
> 
> On 03/26/2013 05:56 PM, Peter Zijlstra wrote:
> > On Fri, 2013-03-22 at 13:25 +0100, Vincent Guittot wrote:
> >> +static bool is_buddy_busy(int cpu)
> >> +{
> >> +       struct rq *rq = cpu_rq(cpu);
> >> +
> >> +       /*
> >> +        * A busy buddy is a CPU with a high load or a small load with
> >> a lot of
> >> +        * running tasks.
> >> +        */
> >> +       return (rq->avg.runnable_avg_sum >
> >> +                       (rq->avg.runnable_avg_period / (rq->nr_running
> >> + 2)));
> >> +}
> > 
> > Why does the comment talk about load but we don't see it in the
> > equation. Also, why does nr_running matter at all? I thought we'd
> > simply bother with utilization, if fully utilized we're done etc..
> > 
> 
> Peter, lets say the run-queue has 50% utilization and is running 2
> tasks. And we wish to find out if it is busy. We would compare this
> metric with the cpu power, which lets say is 100.
> 
> rq->util * 100 < cpu_of(rq)->power.
> 
> In the above scenario would we declare the cpu _not_busy? Or would we do
> the following:
> 
> (rq->util * 100) * #nr_running <  cpu_of(rq)->power and conclude that it
> is just enough _busy_ to not take on more processes?

That is just confused... ->power doesn't have anything to do with a per-cpu
measure. ->power is a inter-cpu measure of relative compute capacity.

Mixing in nr_running confuses things even more; it doesn't matter how many
tasks it takes to push utilization up to 100%; once its there the cpu simply
cannot run more.

So colour me properly confused..