Re: [PATCH] sched/fair: util_est: fast ramp-up EWMA on utilization increases

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

On 28-Jun 15:51, Vincent Guittot wrote:
> On Fri, 28 Jun 2019 at 14:38, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Fri, Jun 28, 2019 at 11:08:14AM +0100, Patrick Bellasi wrote:
> > > On 26-Jun 13:40, Vincent Guittot wrote:
> > > > Hi Patrick,
> > > >
> > > > On Thu, 20 Jun 2019 at 17:06, Patrick Bellasi <patrick.bellasi@arm.com> wrote:
> > > > >
> > > > > The estimated utilization for a task is currently defined based on:
> > > > >  - enqueued: the utilization value at the end of the last activation
> > > > >  - ewma:     an exponential moving average which samples are the enqueued values
> > > > >
> > > > > According to this definition, when a task suddenly change it's bandwidth
> > > > > requirements from small to big, the EWMA will need to collect multiple
> > > > > samples before converging up to track the new big utilization.
> > > > >
> > > > > Moreover, after the PELT scale invariance update [1], in the above scenario we
> > > > > can see that the utilization of the task has a significant drop from the first
> > > > > big activation to the following one. That's implied by the new "time-scaling"
> > > >
> > > > Could you give us more details about this? I'm not sure to understand
> > > > what changes between the 1st big activation and the following one ?
> > >
> > > We are after a solution for the problem Douglas Raillard discussed at
> > > OSPM, specifically the "Task util drop after 1st idle" highlighted in
> > > slide 6 of his presentation:
> > >
> > >   http://retis.sssup.it/ospm-summit/Downloads/02_05-Douglas_Raillard-How_can_we_make_schedutil_even_more_effective.pdf
> > >
> >
> > So I see the problem, and I don't hate the patch, but I'm still
> > struggling to understand how exactly it related to the time-scaling
> > stuff. Afaict the fundamental problem here is layering two averages. The
> 
> AFAICT, it's not related to the time-scaling
> 
> In fact the big 1st activation happens because task runs at low OPP
> and hasn't enough time to finish its running phase before the time to
> begin the next one happens. This means that the task will run several
> computations phase in one go which is no more a 75% task.

But in that case, running multiple activations back to back, should we
not expect the util_avg to exceed the 75% mark?


> From a pelt PoV, the task is far larger than a 75% task and its
> utilization too because it runs far longer (even after scaling time
> with frequency).

Which thus should match my expectation above, no?

> Once cpu reaches a high enough OPP that enable to have sleep phase
> between each running phases, the task load tracking comes back to the
> normal slope increase (the one that would have happen if task would
> have jump from 5% to 75% but already running at max OPP)


Indeed, I can see from the plots a change in slope. But there is also
that big drop after the first big activation: 375 units in 1.1ms.

Is that expected? I guess yes, since we fix the clock_pelt with the
lost_idle_time.


> > second (EWMA in our case) will always lag/delay the input of the first
> > (PELT).
> >
> > The time-scaling thing might make matters worse, because that helps PELT
> > ramp up faster, but that is not the primary issue.
> >
> > Or am I missing something?

-- 
#include <best/regards.h>

Patrick Bellasi