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

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Message-ID: <20190313193056.GP2482@worktop.programming.kicks-ass.net>
Date:   Wed, 13 Mar 2019 20:30:56 +0100
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 v7 01/15] sched/core: uclamp: Add CPU's clamp buckets
 refcounting

On Wed, Mar 13, 2019 at 03:59:54PM +0000, Patrick Bellasi wrote:
> On 13-Mar 14:52, Peter Zijlstra wrote:
> > > +static inline void uclamp_rq_dec_id(struct task_struct *p, struct rq *rq,
> > > +				    unsigned int clamp_id)
> > > +{
> > > +	unsigned int bucket_id = p->uclamp[clamp_id].bucket_id;
> > > +	unsigned int rq_clamp, bkt_clamp;
> > > +
> > > +	SCHED_WARN_ON(!rq->uclamp[clamp_id].bucket[bucket_id].tasks);
> > > +	if (likely(rq->uclamp[clamp_id].bucket[bucket_id].tasks))
> > > +		rq->uclamp[clamp_id].bucket[bucket_id].tasks--;
> > > +
> > > +	/*
> > > +	 * Keep "local clamping" simple and accept to (possibly) overboost
> > > +	 * still RUNNABLE tasks in the same bucket.
> > > +	 */
> > > +	if (likely(rq->uclamp[clamp_id].bucket[bucket_id].tasks))
> > > +		return;
> > 
> > (Oh man, I hope that generates semi sane code; long live CSE passes I
> > suppose)
> 
> What do you mean ?

that does: 'rq->uclamp[clamp_id].bucket[bucket_id].tasks' three times in
a row. And yes the compiler _should_ dtrt, but....