linux-kernel - Re: [PATCH v6 09/16] sched/cpufreq: uclamp: Add utilization clamping for RT tasks

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Date:   Wed, 23 Jan 2019 14:40:11 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-api@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...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 v6 09/16] sched/cpufreq: uclamp: Add utilization clamping
 for RT tasks

On 23-Jan 11:49, Peter Zijlstra wrote:
> On Tue, Jan 15, 2019 at 10:15:06AM +0000, Patrick Bellasi wrote:
> > @@ -858,16 +859,23 @@ static inline void
> >  uclamp_effective_get(struct task_struct *p, unsigned int clamp_id,
> >  		     unsigned int *clamp_value, unsigned int *bucket_id)
> >  {
> > +	struct uclamp_se *default_clamp;
> > +
> >  	/* Task specific clamp value */
> >  	*clamp_value = p->uclamp[clamp_id].value;
> >  	*bucket_id = p->uclamp[clamp_id].bucket_id;
> >  
> > +	/* RT tasks have different default values */
> > +	default_clamp = task_has_rt_policy(p)
> > +		? uclamp_default_perf
> > +		: uclamp_default;
> > +
> >  	/* System default restriction */
> > -	if (unlikely(*clamp_value < uclamp_default[UCLAMP_MIN].value ||
> > -		     *clamp_value > uclamp_default[UCLAMP_MAX].value)) {
> > +	if (unlikely(*clamp_value < default_clamp[UCLAMP_MIN].value ||
> > +		     *clamp_value > default_clamp[UCLAMP_MAX].value)) {
> >  		/* Keep it simple: unconditionally enforce system defaults */
> > -		*clamp_value = uclamp_default[clamp_id].value;
> > -		*bucket_id = uclamp_default[clamp_id].bucket_id;
> > +		*clamp_value = default_clamp[clamp_id].value;
> > +		*bucket_id = default_clamp[clamp_id].bucket_id;
> >  	}
> >  }
> 
> So I still don't much like the whole effective thing;

:/

I find back-annotation useful in many cases since we have different
sources for possible clamp values:

 1. task specific
 2. cgroup defined
 3. system defaults
 4. system power default

I don't think we can avoid to somehow back annotate on which bucket a
task has been refcounted... it makes dequeue so much easier, it helps
in ensuring that the refcouning is consistent and enable lazy updates.

> but I think you should use rt_task() instead of
> task_has_rt_policy().

Right... will do.

-- 
#include <best/regards.h>

Patrick Bellasi