linux-kernel - Re: [PATCH v7 01/15] sched/core: uclamp: Add CPU's clamp buckets refcounting

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Date:   Wed, 13 Mar 2019 14:08:21 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Patrick Bellasi <patrick.bellasi@....com>,
        LKML <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>
Subject: Re: [PATCH v7 01/15] sched/core: uclamp: Add CPU's clamp buckets refcounting

On Wed, Mar 13, 2019 at 12:46 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Mar 13, 2019 at 03:23:59PM +0000, Patrick Bellasi wrote:
> > On 13-Mar 15:09, Peter Zijlstra wrote:
> > > On Fri, Feb 08, 2019 at 10:05:40AM +0000, Patrick Bellasi wrote:
>
> > > > +static inline void uclamp_rq_update(struct rq *rq, unsigned int clamp_id)
> > > > +{
> > > > + struct uclamp_bucket *bucket = rq->uclamp[clamp_id].bucket;
> > > > + unsigned int max_value = uclamp_none(clamp_id);
> > >
> > > That's 1024 for uclamp_max
> > >
> > > > + unsigned int bucket_id;
> > > > +
> > > > + /*
> > > > +  * Both min and max clamps are MAX aggregated, thus the topmost
> > > > +  * bucket with some tasks defines the rq's clamp value.
> > > > +  */
> > > > + bucket_id = UCLAMP_BUCKETS;
> > > > + do {
> > > > +         --bucket_id;
> > > > +         if (!rq->uclamp[clamp_id].bucket[bucket_id].tasks)
> > > > +                 continue;
> > > > +         max_value = bucket[bucket_id].value;
> > >
> > > but this will then _lower_ it. That's not a MAX aggregate.
> >
> > For uclamp_max we want max_value=1024 when there are no active tasks,
> > which means: no max clamp enforced on CFS/RT "idle" cpus.
> >
> > If instead there are active RT/CFS tasks then we want the clamp value
> > of the max group, which means: MAX aggregate active clamps.
> >
> > That's what the code above does and the comment says.
>
> That's (obviously) not how I read it... maybe something like:
>
> static inline void uclamp_rq_update(struct rq *rq, unsigned int clamp_id)
> {
>         struct uclamp_bucket *bucket = rq->uclamp[clamp_id].bucket;
>         int i;
>
>         /*
>          * Since both min and max clamps are max aggregated, find the
>          * top most bucket with tasks in.
>          */
>         for (i = UCLMAP_BUCKETS-1; i>=0; i--) {
>                 if (!bucket[i].tasks)
>                         continue;
>                 return bucket[i].value;
>         }
>
>         /* No tasks -- default clamp values */
>         return uclamp_none(clamp_id);
> }
>
> would make it clearer?

This way it's also more readable/obvious when it's used inside
uclamp_rq_dec_id, assuming uclamp_rq_update is renamed into smth like
get_max_rq_uclamp.