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

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Message-ID: <20190313194619.GR2482@worktop.programming.kicks-ass.net>
Date:   Wed, 13 Mar 2019 20:46:19 +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 03:23:59PM +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?