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..
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