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

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

On 28-Nov 15:55, Vincent Guittot wrote:
> On Wed, 28 Nov 2018 at 15:40, Patrick Bellasi <patrick.bellasi@arm.com> wrote:
> >
> > 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 ?
> 
> In the example of a task that runs 39ms in period of 80ms that we used
> during previous version,
> the utilization on the big core will reach 709 so will util_est too
> When the task migrates on little core (512), util_est is higher than
> current cpu capacity

Right, and what's the problem ?

1) We know that PELT is calibrated to 32ms period task and in your
   example, since the runtime is higher then the half-life, it's
   correct to estimate a utilization higher then 50%.

   PELT utilization is defined _based on the half-life_: thus
   your task having a 50% duty cycle does not mean we are not correct
   if report a utilization != 50%.
   It would be as broken as reporting 10% utilization for a task
   running 100ms every 1s.

2) If it was a 70% task on a previous activation, once it's moved into
   a lower capacity CPU it's still correct to assume that it's likely
   going to require the same bandwidth and thus will be
   under-provisioned.

I still don't see where we are wrong in this case :/

To me it looks different then the problem I described.

> > 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 ?

What's your opinion on the above instead ?

-- 
#include <best/regards.h>

Patrick Bellasi