linux-kernel - Re: [PATCH v4 06/16] sched/cpufreq: uclamp: add utilization clamping for FAIR tasks

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Date:   Fri, 14 Sep 2018 15:36:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v4 06/16] sched/cpufreq: uclamp: add utilization clamping
 for FAIR tasks

On Fri, Sep 14, 2018 at 02:19:19PM +0100, Patrick Bellasi wrote:
> On 14-Sep 11:32, Peter Zijlstra wrote:

> > Should that not be:
> > 
> > 	util = clamp_util(rq, cpu_util_cfs(rq));
> > 
> > Because if !util might we not still want to enforce the min clamp?
> 
> If !util CFS tasks should have been gone since a long time
> (proportional to their estimated utilization) and thus it probably
> makes sense to not affect further energy efficiency for tasks of other
> classes.

I don't remember what we do for util for new tasks; but weren't we
talking about setting that to 0 recently? IIRC the problem was that if
we start at 1 with util we'll always run new tasks on big cores, or
something along those lines.

So new tasks would still trigger this case until they'd accrued enough
history.

Either way around, I don't much care at this point except I think it
would be good to have a comment to record the assumptions.

> > Would that not be more readable as:
> > 
> > static inline unsigned int uclamp_value(struct rq *rq, int clamp_id)
> > {
> > 	unsigned int val = rq->uclamp.value[clamp_id];
> > 
> > 	if (unlikely(val == UCLAMP_NOT_VALID))
> > 		val = uclamp_none(clamp_id);
> > 
> > 	return val;
> > }
> 
> I'm trying to keep consistency in variable names usages by always
> accessing the rq's clamps via a *uc_cpu to make it easy grepping the
> code. Does this argument make sense ?
> 
> On the other side, what you propose above is more easy to read
> by looking just at that function.... so, if you prefer it better, I'll
> update it on v5.

I prefer my version, also because it has a single load of the value (yes
I know about CSE passes). I figure one can always grep for uclamp or
something.

> > And how come NOT_VALID is possible? I thought the idea was to always
> > have all things a valid value.
> 
> When we update the CPU's clamp for a "newly idle" CPU, there are not
> tasks refcounting clamps and thus we end up with UCLAMP_NOT_VALID for
> that CPU. That's how uclamp_cpu_update() is currently encoded.
> 
> Perhaps we can set the value to uclamp_none(clamp_id) from that
> function, but I was thinking that perhaps it could be useful to track
> explicitly that the CPU is now idle.

IIRC you added an explicit flag to track idle somewhere.. to keep the
last max clamp in effect or something.

I think, but haven't overly thought about this, that if you always
ensure these things are valid you can avoid a bunch of NOT_VALID
conditions. And less conditions is always good, right? :-)