linux-kernel - Re: [PATCH v5 05/10] cpufreq/schedutil: get max utilization

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Message-ID: <20180530083715.GB2174@e108498-lin.cambridge.arm.com>
Date:   Wed, 30 May 2018 09:37:15 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>, peterz@...radead.org,
        mingo@...nel.org, linux-kernel@...r.kernel.org, rjw@...ysocki.net,
        dietmar.eggemann@....com, Morten.Rasmussen@....com,
        viresh.kumar@...aro.org, valentin.schneider@....com
Subject: Re: [PATCH v5 05/10] cpufreq/schedutil: get max utilization

On Tuesday 29 May 2018 at 11:52:03 (+0200), Juri Lelli wrote:
> On 29/05/18 09:40, Quentin Perret wrote:
> > Hi Vincent,
> > 
> > On Friday 25 May 2018 at 15:12:26 (+0200), Vincent Guittot wrote:
> > > Now that we have both the dl class bandwidth requirement and the dl class
> > > utilization, we can use the max of the 2 values when agregating the
> > > utilization of the CPU.
> > > 
> > > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > > ---
> > >  kernel/sched/sched.h | 6 +++++-
> > >  1 file changed, 5 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > > index 4526ba6..0eb07a8 100644
> > > --- a/kernel/sched/sched.h
> > > +++ b/kernel/sched/sched.h
> > > @@ -2194,7 +2194,11 @@ static inline void cpufreq_update_util(struct rq *rq, unsigned int flags) {}
> > >  #ifdef CONFIG_CPU_FREQ_GOV_SCHEDUTIL
> > >  static inline unsigned long cpu_util_dl(struct rq *rq)
> > >  {
> > > -	return (rq->dl.running_bw * SCHED_CAPACITY_SCALE) >> BW_SHIFT;
> > > +	unsigned long util = (rq->dl.running_bw * SCHED_CAPACITY_SCALE) >> BW_SHIFT;
> > > +
> > > +	util = max_t(unsigned long, util, READ_ONCE(rq->avg_dl.util_avg));
> > 
> > Would it make sense to use a UTIL_EST version of that signal here ? I
> > don't think that would make sense for the RT class with your patch-set
> > since you only really use the blocked part of the signal for RT IIUC,
> > but would that work for DL ?
> 
> Well, UTIL_EST for DL looks pretty much what we already do by computing
> utilization based on dl.running_bw. That's why I was thinking of using
> that as a starting point for dl.util_avg decay phase.

Hmmm I see your point, but running_bw and the util_avg are fundamentally
different ... I mean, the util_avg doesn't know about the period, which is
an issue in itself I guess ...

If you have a long running DL task (say 100ms runtime) with a long period
(say 1s), the running_bw should represent ~1/10 of the CPU capacity, but
the util_avg can go quite high, which means that you might end up
executing this task at max OPP. So if we really want to drive OPPs like
that for deadline, a util_est-like version of this util_avg signal
should help. Now, you can also argue that going to max OPP for a task
that _we know_ uses 1/10 of the CPU capacity isn't right ...