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

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Message-ID: <20180809154156.gdsx2vacjmp6p6dp@darkstar>
Date:   Thu, 9 Aug 2018 16:41:56 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Quentin Perret <quentin.perret@....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>,
        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 v3 06/14] sched/cpufreq: uclamp: add utilization clamping
 for RT tasks

On 07-Aug 14:54, Quentin Perret wrote:
> Hi Patrick,

Hi Quentin!

> On Monday 06 Aug 2018 at 17:39:38 (+0100), Patrick Bellasi wrote:
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index a7affc729c25..bb25ef66c2d3 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -200,6 +200,7 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
> >  static unsigned long sugov_get_util(struct sugov_cpu *sg_cpu)
> >  {
> >  	struct rq *rq = cpu_rq(sg_cpu->cpu);
> > +	unsigned long util_cfs, util_rt;
> >  	unsigned long util, irq, max;
> >  
> >  	sg_cpu->max = max = arch_scale_cpu_capacity(NULL, sg_cpu->cpu);
> 
> IIUC, not far below this you should still have something like:
> 
> 	if (rt_rq_is_runnable(&rq->rt))
> 		return max;

Do you mean that when RT tasks are RUNNABLE we still want to got to
MAX? Not sure to understand... since this patch is actually to clamp
the RT class...

> So you won't reach the actual clamping code at the end of the function
> when a RT task is runnable no?

... mmm... no... this patch is to clamp RT tasks... Am I missing
something?

> Also, I always had the feeling that the default for RT should be
> util_min == 1024, and then users could decide to lower the bar if they
> want to.

Mmm... good point! This would keep the policy unaltered for RT tasks.

I want to keep sched class specific code in uclamp at minimum, but
likely this should be achievable by just properly initializing the
task-specific util_min for RT tasks, if the original task has
UCLAM_NOT_VALID.

> For the specific case of RT, that feels more natural than
> applying a max util clamp IMO. What do you think ?

I'll look better into this for the next posting!

Cheers Patrick

-- 
#include <best/regards.h>

Patrick Bellasi