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

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

Hi Peter, Vincent,
is there anything different I can do on this?

Cheers,
Patrick

On 28-Jun 15:00, Patrick Bellasi wrote:
> On 28-Jun 14:38, Peter Zijlstra 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
> > 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.
> 
> Sure, we like the new time-scaling PELT which ramps up faster and, as
> long as we have idle time, it's better in predicting what would be the
> utilization as if we was running at max OPP.
> 
> However, the experiment above shows that:
> 
>  - despite the task being a 75% after a certain activation, it takes
>    multiple activations for PELT to actually enter that range.
> 
>  - the first activation ends at 665, 10% short wrt the configured
>    utilization
> 
>  - while the PELT signal converge toward the 75%, we have some pretty
>    consistent drops at wakeup time, especially after the first big
>    activation.
> 
> > Or am I missing something?
> 
> I'm not sure the above happens because of a problem in the new
> time-scaling PELT, I actually think it's kind of expected given the
> way we re-scale time contributions depending on the current OPPs.
> 
> It's just that a 375 drops in utilization with just 1.1ms sleep time
> looks to me more related to the time-scaling invariance then just the
> normal/expected PELT decay.
> 
>    Could it be an out-of-sync issue between the PELT time scaling code
>    and capacity scaling code?
>    Perhaps due to some OPP changes/notification going wrong?
> 
> Sorry for not being much more useful on that, maybe Vincent has some
> better ideas.
> 
> The only thing I've kind of convinced myself is that an EWMA on
> util_est does not make a lot of sense for increasing utilization
> tracking.
> 
> Best,
> Patrick
> 
> -- 
> #include <best/regards.h>
> 
> Patrick Bellasi

-- 
#include <best/regards.h>

Patrick Bellasi