Re: [PATCH v7 2/2] sched/fair: update scale invariance of PELT

[Patrick Bellasi <patrick.bellasi@arm.com>]

On 28-Nov 14:33, Vincent Guittot wrote:
> On Wed, 28 Nov 2018 at 12:53, Patrick Bellasi <patrick.bellasi@arm.com> wrote:
> >
> > On 28-Nov 11:02, Peter Zijlstra wrote:
> > > On Wed, Nov 28, 2018 at 10:54:13AM +0100, Vincent Guittot wrote:
> > >
> > > > Is there anything else that I should do for these patches ?
> > >
> > > IIRC, Morten mention they break util_est; Patrick was going to explain.
> >
> > I guess the problem is that, once we cross the current capacity,
> > strictly speaking util_avg does not represent anymore a utilization.
> >
> > With the new signal this could happen and we end up storing estimated
> > utilization samples which will overestimate the task requirements.
> >
> > We will have a spike in estimated utilization at next wakeup, since we
> > use MAX(util_avg@dequeue_time, ewma). Potentially we also inflate the EWMA in
> > case we collect multiple samples above the current capacity.
> 
> TBH I don't see how it's different from current implementation with a
> task that was scheduled on big core and now wakes up on little core.
> The util_est is overestimated as well.

While running below the capacity of a CPU, either big or LITTLE, we
can still measure the actual used bandwidth as long as we have idle
time. If the task is then moved into a lower capacity core, I think
it's still safe to assume that, likely, it would need more capacity.

Why do you say it's the same ?

With your new signal instead, once we cross the current capacity,
utilization is just not anymore utilization. Thus, IMHO it make sense
avoid to accumulate a sample for what we call "estimated utilization".

I would also say that, with the current implementation which caps
utilization to the current capacity, we get better estimation in
general. At least we can say with absolute precision:

   "the task needs _at least_ that amount of capacity".

Potentially we can also flag the task as being under-provisioned, in
case there was not idle time, and _let a policy_ decide what to do
with it and the granted information we have.

While, with your new signal, once we are over the current capacity,
the "utilization" is just a sort of "random" number at best useful to
drive some conclusions about how long the task has been delayed.

IOW, I fear that we are embedding a policy within a signal which is
currently representing something very well defined: how much cpu
bandwidth a task used. While, latency/under-provisioning policies
perhaps should be better placed somewhere else.

Perhaps I've missed it in some of the previous discussions:
have we have considered/discussed this signal-vs-policy aspect ?

-- 
#include <best/regards.h>

Patrick Bellasi