linux-kernel - Re: [PATCH v4 03/16] sched/core: uclamp: add CPU's clamp groups accounting

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Message-ID: <20180914115217.GI24124@hirez.programming.kicks-ass.net>
Date:   Fri, 14 Sep 2018 13:52:17 +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 03/16] sched/core: uclamp: add CPU's clamp groups
 accounting

On Fri, Sep 14, 2018 at 10:07:51AM +0100, Patrick Bellasi wrote:
> On 13-Sep 21:12, Peter Zijlstra wrote:
> > On Tue, Aug 28, 2018 at 02:53:11PM +0100, Patrick Bellasi wrote:
> > > +static inline void uclamp_cpu_get_id(struct task_struct *p,
> > > +				     struct rq *rq, int clamp_id)
> > > +{
> > > +	struct uclamp_group *uc_grp;
> > > +	struct uclamp_cpu *uc_cpu;
> > > +	int clamp_value;
> > > +	int group_id;
> > > +
> > > +	/* Every task must reference a clamp group */
> > > +	group_id = p->uclamp[clamp_id].group_id;
> > 
> > > +}
> > > +
> > > +static inline void uclamp_cpu_put_id(struct task_struct *p,
> > > +				     struct rq *rq, int clamp_id)
> > > +{
> > > +	struct uclamp_group *uc_grp;
> > > +	struct uclamp_cpu *uc_cpu;
> > > +	unsigned int clamp_value;
> > > +	int group_id;
> > > +
> > > +	/* New tasks don't have a previous clamp group */
> > > +	group_id = p->uclamp[clamp_id].group_id;
> > > +	if (group_id == UCLAMP_NOT_VALID)
> > > +		return;
> > 
> > *confused*, so on enqueue they must have a group_id, but then on dequeue
> > they might no longer have?
> 
> Why not ?

That's what it says on the tin, right? enqueue: "every task must reference clamp
group" while on dequeue: "new tasks don't have a (previous) clamp group"
and we check for NOT_VALID.