linux-kernel - Re: [PATCH] sched/fair: schedutil: update only with all info available

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Date:   Wed, 11 Apr 2018 17:37:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Patrick Bellasi <patrick.bellasi@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH] sched/fair: schedutil: update only with all info
 available

On Wed, Apr 11, 2018 at 05:29:01PM +0200, Vincent Guittot wrote:
> On 11 April 2018 at 17:14, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Tue, Apr 10, 2018 at 12:04:12PM +0100, Patrick Bellasi wrote:
> >> On 09-Apr 10:51, Vincent Guittot wrote:
> >
> >> > Peter,
> >> > what was your goal with adding the condition "if
> >> > (rq->cfs.h_nr_running)" for the aggragation of CFS utilization
> >>
> >> The original intent was to get rid of sched class flags, used to track
> >> which class has tasks runnable from within schedutil. The reason was
> >> to solve some misalignment between scheduler class status and
> >> schedutil status.
> >>
> >> The solution, initially suggested by Viresh, and finally proposed by
> >> Peter was to exploit RQ knowledges directly from within schedutil.
> >>
> >> The problem is that now schedutil updated depends on two information:
> >> utilization changes and number of RT and CFS runnable tasks.
> >>
> >> Thus, using cfs_rq::h_nr_running is not the problem... it's actually
> >> part of a much more clean solution of the code we used to have.
> >>
> >> The problem, IMO is that we now depend on other information which
> >> needs to be in sync before calling schedutil... and the patch I
> >> proposed is meant to make it less likely that all the information
> >> required are not aligned (also in the future).
> >
> > Specifically, the h_nr_running test was get rid of
> >
> >                 if (delta_ns > TICK_NSEC) {
> >                         j_sg_cpu->iowait_boost = 0;
> >                         j_sg_cpu->iowait_boost_pending = false;
> > -                       j_sg_cpu->util_cfs = 0;
> >
> >                         ^^^^^^^^^^^^^^^^^^^^^^^ that..
> >
> > -                       if (j_sg_cpu->util_dl == 0)
> > -                               continue;
> >                 }
> >
> >
> > because that felt rather arbitrary.
> 
> yes I agree.
> 
> With the patch that updates blocked idle load, we should not have the
> problem of blocked utilization anymore and get rid of the code above
> and h_nr_running test

Yes, these patches predate those, but indeed, now that we age the
blocked load consistently it should no longer be required.

Of course, you still have that weird regression report against those
patches... :-)