linux-kernel - Re: [PATCH 2/2] sched/fair: util_est: add running

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Message-ID: <20180605194344.GB239272@joelaf.mtv.corp.google.com>
Date:   Tue, 5 Jun 2018 12:43:44 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>, Todd Kjos <tkjos@...gle.com>
Subject: Re: [PATCH 2/2] sched/fair: util_est: add running_sum tracking

On Tue, Jun 05, 2018 at 12:33:17PM -0700, Joel Fernandes wrote:
> On Tue, Jun 05, 2018 at 04:21:56PM +0100, Patrick Bellasi wrote:
[..]
> > To be more precise, at each ___update_load_avg we should really update
> > running_avg by:
> > 
> >    u32 divider = LOAD_AVG_MAX - 1024 + sa->period_contrib;
> >    sa->running_avg = sa->running_sum / divider;
> > 
> > but, this would imply tracking an additional signal in sched_avg and
> > doing an additional division at ___update_load_avg() time.
> > 
> > Morten suggested that, if we accept the rounding errors due to
> > considering
> > 
> >       divider ~= LOAD_AVG_MAX
> > 
> > thus discarding the (sa->period_contrib - 1024) correction, then we
> > can completely skip the tracking of running_avg (thus saving space in
> > sched_avg) and approximate it at dequeue time as per the code line,
> > just to compute the new util_est sample to accumulate.
> > 
> > Does that make sense now?
> 
> The patch always made sense to me.. I was just pointing out the extra
> division this patch adds. I agree since its done on dequeue-only, then its
> probably Ok to do..
> 

One thing to note about this error is I remember not compensating for it
would make the utilization reduce... which is kind of weird. But yeah if we
can find a way compensate for the error and also keep the overhead low,
that's super.. I know this is probably low in priority considering the other
concerns Vincent brought up which are being discussed in the other thread,..
but just saying. :)

 - Joel