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

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Message-ID: <20180411151450.GK4043@hirez.programming.kicks-ass.net>
Date:   Wed, 11 Apr 2018 17:14:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        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 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.