linux-kernel - Re: [PATCH 1/7] sched/core: uclamp: add CPU clamp groups accounting

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Date:   Fri, 13 Apr 2018 13:15:31 +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>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>
Subject: Re: [PATCH 1/7] sched/core: uclamp: add CPU clamp groups accounting

On Fri, Apr 13, 2018 at 12:04:26PM +0100, Patrick Bellasi wrote:
> On 13-Apr 12:22, Peter Zijlstra wrote:
> > On Fri, Apr 13, 2018 at 10:26:48AM +0200, Peter Zijlstra wrote:
> > > On Mon, Apr 09, 2018 at 05:56:09PM +0100, Patrick Bellasi wrote:
> > > > +static inline void uclamp_cpu_get(struct task_struct *p, int cpu, int clamp_id)
> > > > +{
> > > > +	struct uclamp_cpu *uc_cpu = &cpu_rq(cpu)->uclamp[clamp_id];
> > > 
> > > > +static inline void uclamp_cpu_put(struct task_struct *p, int cpu, int clamp_id)
> > > > +{
> > > > +	struct uclamp_cpu *uc_cpu = &cpu_rq(cpu)->uclamp[clamp_id];
> > > 
> > > That all seems daft, since you already have rq at the call site.
> > > 
> > > > +static inline void uclamp_task_update(struct rq *rq, struct task_struct *p)
> > > > +{
> > > > +	int cpu = cpu_of(rq);
> > > > +	int clamp_id;
> > > > +
> > > > +	/* The idle task does not affect CPU's clamps */
> > > > +	if (unlikely(p->sched_class == &idle_sched_class))
> > > > +		return;
> > > > +	/* DEADLINE tasks do not affect CPU's clamps */
> > > > +	if (unlikely(p->sched_class == &dl_sched_class))
> > > > +		return;
> > > 
> > > This is wrong; it misses the stop_sched_class.
> > > 
> > > And since you're looking at sched_class anyway, maybe put a marker in
> > > there:
> > > 
> > > 	if (!p->sched_class->has_clamping)
> > > 		return;
> > 
> > Alternatively, we could simply add uclamp_task_{en,de}queue() into
> > {en,de}queue_task_{fair,rt}().
> 
> I like better your first proposal, I think it makes sense to factor
> out in core code used by both RT and FAIR the same way.
> 
> Do you have a strong preference?

The second is probably faster as it avoids the load+branch; then again,
without LTO you'll get an actual call in return. Dunno...