linux-kernel - Re: [PATCH v8 04/16] sched/core: uclamp: Add system default clamps

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Message-ID: <20190508191529.GA26813@worktop.programming.kicks-ass.net>
Date:   Wed, 8 May 2019 21:15:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
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 v8 04/16] sched/core: uclamp: Add system default clamps

On Wed, May 08, 2019 at 09:07:33PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 02, 2019 at 11:41:40AM +0100, Patrick Bellasi wrote:
> > +static inline struct uclamp_se
> > +uclamp_eff_get(struct task_struct *p, unsigned int clamp_id)
> > +{
> > +	struct uclamp_se uc_req = p->uclamp_req[clamp_id];
> > +	struct uclamp_se uc_max = uclamp_default[clamp_id];
> > +
> > +	/* System default restrictions always apply */
> > +	if (unlikely(uc_req.value > uc_max.value))
> > +		return uc_max;
> > +
> > +	return uc_req;
> > +}
> > +
> > +static inline unsigned int
> > +uclamp_eff_bucket_id(struct task_struct *p, unsigned int clamp_id)
> > +{
> > +	struct uclamp_se uc_eff;
> > +
> > +	/* Task currently refcounted: use back-annotated (effective) bucket */
> > +	if (p->uclamp[clamp_id].active)
> > +		return p->uclamp[clamp_id].bucket_id;
> > +
> > +	uc_eff = uclamp_eff_get(p, clamp_id);
> > +
> > +	return uc_eff.bucket_id;
> > +}
> > +
> > +unsigned int uclamp_eff_value(struct task_struct *p, unsigned int clamp_id)
> > +{
> > +	struct uclamp_se uc_eff;
> > +
> > +	/* Task currently refcounted: use back-annotated (effective) value */
> > +	if (p->uclamp[clamp_id].active)
> > +		return p->uclamp[clamp_id].value;
> > +
> > +	uc_eff = uclamp_eff_get(p, clamp_id);
> > +
> > +	return uc_eff.value;
> > +}
> 
> This is 'wrong' because:
> 
>   uclamp_eff_value(p,id) := uclamp_eff(p,id).value

Clearly I means to say the above does not hold with the given
implementation, while the naming would suggest it does.

> Which seems to suggest the uclamp_eff_*() functions want another name.
> 
> Also, suppose the above would be true; does GCC really generate better
> code for the LHS compared to the RHS?