linux-kernel - Re: [PATCH v3 06/14] sched/cpufreq: uclamp: add utilization clamping for RT tasks

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Message-ID: <20180809153423.nsoepprhut3dv4u2@darkstar>
Date:   Thu, 9 Aug 2018 16:34:23 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v3 06/14] sched/cpufreq: uclamp: add utilization clamping
 for RT tasks

On 07-Aug 15:26, Juri Lelli wrote:
> Hi,
> 
> On 06/08/18 17:39, Patrick Bellasi wrote:
> 
> [...]
> 
> > @@ -223,13 +224,25 @@ static unsigned long sugov_get_util(struct sugov_cpu *sg_cpu)
> >  	 * utilization (PELT windows are synchronized) we can directly add them
> >  	 * to obtain the CPU's actual utilization.
> >  	 *
> > -	 * CFS utilization can be boosted or capped, depending on utilization
> > -	 * clamp constraints configured for currently RUNNABLE tasks.
> > +	 * CFS and RT utilizations can be boosted or capped, depending on
> > +	 * utilization constraints enforce by currently RUNNABLE tasks.
> > +	 * They are individually clamped to ensure fairness across classes,
> > +	 * meaning that CFS always gets (if possible) the (minimum) required
> > +	 * bandwidth on top of that required by higher priority classes.
> 
> Is this a stale comment written before UCLAMP_SCHED_CLASS was
> introduced? It seems to apply to the below if branch only.

Yes, you right... I'll update the comment.

> >  	 */
> > -	util = cpu_util_cfs(rq);
> > -	if (util)
> > -		util = uclamp_util(cpu_of(rq), util);
> > -	util += cpu_util_rt(rq);
> > +	util_cfs = cpu_util_cfs(rq);
> > +	util_rt  = cpu_util_rt(rq);
> > +	if (sched_feat(UCLAMP_SCHED_CLASS)) {
> > +		util = 0;
> > +		if (util_cfs)
> > +			util += uclamp_util(cpu_of(rq), util_cfs);
> > +		if (util_rt)
> > +			util += uclamp_util(cpu_of(rq), util_rt);
> > +	} else {
> > +		util  = cpu_util_cfs(rq);
> > +		util += cpu_util_rt(rq);
> > +		util  = uclamp_util(cpu_of(rq), util);
> > +	}

Regarding the two policies, do you have any comment?

We had an internal discussion and we found pro/cons for both... but
I'm not sure keeping the sched_feat is a good solution on the long
run, i.e. mainline merge ;)

-- 
#include <best/regards.h>

Patrick Bellasi