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

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Date:   Wed, 13 Mar 2019 16:12:29 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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 13-Mar 14:40, Peter Zijlstra wrote:
> On Fri, Feb 08, 2019 at 10:05:40AM +0000, Patrick Bellasi wrote:
> > +static inline unsigned int uclamp_bucket_id(unsigned int clamp_value)
> > +{
> > +	return clamp_value / UCLAMP_BUCKET_DELTA;
> > +}
> > +
> > +static inline unsigned int uclamp_bucket_value(unsigned int clamp_value)
> > +{
> > +	return UCLAMP_BUCKET_DELTA * uclamp_bucket_id(clamp_value);
> 
> 	return clamp_value - (clamp_value % UCLAMP_BUCKET_DELTA);
> 
> might generate better code; just a single division, instead of a div and
> mult.

Wondering if compilers cannot do these optimizations... but yes, looks
cool and will do it in v8, thanks.

> > +}
> > +
> > +static inline unsigned int uclamp_none(int clamp_id)
> > +{
> > +	if (clamp_id == UCLAMP_MIN)
> > +		return 0;
> > +	return SCHED_CAPACITY_SCALE;
> > +}
> > +
> > +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);
> > +	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;
> > +		break;
> 
> If you flip the if condition the code will be nicer.
> 
> > +	} while (bucket_id);
> 
> But you can also use a for loop:
> 
> 	for (i = UCLAMP_BUCKETS-1; i>=0; i--) {
> 		if (rq->uclamp[clamp_id].bucket[i].tasks) {
> 			max_value = bucket[i].value;
> 			break;
> 		}
> 	}

Yes, the for looks better, but perhaps like that:

	unsigned int bucket_id = UCLAMP_BUCKETS;

 	/*
 	 * Both min and max clamps are MAX aggregated, thus the topmost
 	 * bucket with some tasks defines the rq's clamp value.
 	 */
	for (; bucket_id >= 0; --bucket_id) {
		if (!bucket[bucket_id].tasks)
			continue;
		max_value = bucket[bucket_id].value;
		break;
	}

... just to save a {} block.


> > +	WRITE_ONCE(rq->uclamp[clamp_id].value, max_value);
> > +}

-- 
#include <best/regards.h>

Patrick Bellasi