Re: [RFC] sched: CPU topology try

From: Morten Rasmussen
Date: Tue Jan 07 2014 - 10:37:22 EST

On Tue, Jan 07, 2014 at 02:11:22PM +0000, Vincent Guittot wrote:
> On 7 January 2014 14:22, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Tue, Jan 07, 2014 at 09:32:04AM +0100, Vincent Guittot wrote:
> >> On 6 January 2014 17:31, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >> > On Mon, Jan 06, 2014 at 02:41:31PM +0100, Vincent Guittot wrote:
> >> >> IMHO, these settings will disappear sooner or later, as an example the
> >> >> idle/busy _idx are going to be removed by Alex's patch.
> >> >
> >> > Well I'm still entirely unconvinced by them..
> >> >
> >> > removing the cpu_load array makes sense, but I'm starting to doubt the
> >> > removal of the _idx things.. I think we want to retain them in some
> >> > form, it simply makes sense to look at longer term averages when looking
> >> > at larger CPU groups.
> >> >
> >> > So maybe we can express the things in log_2(group-span) or so, but we
> >> > need a working replacement for the cpu_load array. Ideally some
> >> > expression involving the blocked load.
> >>
> >> Using the blocked load can surely give benefit in the load balance
> >> because it gives a view of potential load on a core but it still decay
> >> with the same speed than runnable load average so it doesn't solve the
> >> issue for longer term average. One way is to have a runnable average
> >> load with longer time window

The blocked load discussion comes up again :)

I totally agree that blocked load would be useful, but only if we get
the priority problem sorted out. Blocked load is the sum of load_contrib
of blocked tasks, which means that a tiny high priority task can have a
massive contribution to the blocked load.

> >
> > Ah, another way of looking at it is that the avg without blocked
> > component is a 'now' picture. It is the load we are concerned with right
> > now.
> >
> > The more blocked we add the further out we look; with the obvious limit
> > of the entire averaging period.
> >
> > So the avg that is runnable is right now, t_0; the avg that is runnable +
> > blocked is t_0 + p, where p is the avg period over which we expect the
> > blocked contribution to appear.
> >
> > So something like:
> >
> > avg = runnable + p(i) * blocked; where p(i) \e [0,1]
> >
> > could maybe be used to replace the cpu_load array and still represent
> > the concept of looking at a bigger picture for larger sets. Leaving open
> > the details of the map p.

Figuring out p is the difficult bit. AFAIK, with blocked load in its
current form we don't have any clue when a task will reappear.

> That needs to be studied more deeply but that could be a way to have a
> larger picture


> Another point is that we are using runnable and blocked load average
> which are the sum of load_avg_contrib of tasks but we are not using
> the runnable_avg_sum of the cpus which is not the now picture but a
> average of the past running time (without taking into account task
> weight)

Yes. The rq runnable_avg_sum is an excellent longer term load indicator.
It can't be compared with the runnable and blocked load though. The
other alternative that I can think of is to introduce an unweighted
alternative to blocked load. That is, sum of load_contrib/priority.

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