linux-kernel - Re: [PATCH 3/6] cpufreq: schedutil: ensure max frequency while running RT/DL tasks

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Date:   Fri, 3 Mar 2017 12:38:30 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        John Stultz <john.stultz@...aro.org>,
        Juri Lelli <juri.lelli@....com>, Todd Kjos <tkjos@...roid.com>,
        Tim Murray <timmurray@...gle.com>,
        Andres Oportus <andresoportus@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Chris Redpath <chris.redpath@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH 3/6] cpufreq: schedutil: ensure max frequency while
 running RT/DL tasks

On 03-Mar 14:01, Viresh Kumar wrote:
> On 02-03-17, 15:45, Patrick Bellasi wrote:
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > @@ -293,15 +305,29 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
> >  	if (curr == sg_policy->thread)
> >  		goto done;
> >  
> > +	/*
> > +	 * While RT/DL tasks are running we do not want FAIR tasks to
> > +	 * overwrite this CPU's flags, still we can update utilization and
> > +	 * frequency (if required/possible) to be fair with these tasks.
> > +	 */
> > +	rt_mode = task_has_dl_policy(curr) ||
> > +		  task_has_rt_policy(curr) ||
> > +		  (flags & SCHED_CPUFREQ_RT_DL);
> > +	if (rt_mode)
> > +		sg_cpu->flags |= flags;
> > +	else
> > +		sg_cpu->flags = flags;
> 
> This looks so hacked up :)

It is... a bit... :)

> Wouldn't it be better to let the scheduler tell us what all kind of tasks it has
> in the rq of a CPU and pass a mask of flags?

That would definitively report a more consistent view of what's going
on on each CPU.

> I think it wouldn't be difficult (or time consuming) for the
> scheduler to know that, but I am not 100% sure.

Main issue perhaps is that cpufreq_update_{util,this_cpu} are
currently called by the scheduling classes codes and not from the core
scheduler. However I agree that it should be possible to build up such
information and make it available to the scheduling classes code.

I'll have a look at that.

> IOW, the flags field in cpufreq_update_util() will represent all tasks in the
> rq, instead of just the task that is getting enqueued/dequeued..
> 
> And obviously we need to get some utilization numbers for the RT and DL tasks
> going forward, switching to max isn't going to work for ever :)

Regarding this last point, there are WIP patches Juri is working on to
feed DL demands to schedutil, his presentation at last ELC partially
covers these developments:
  https://www.youtube.com/watch?v=wzrcWNIneWY&index=37&list=PLbzoR-pLrL6pSlkQDW7RpnNLuxPq6WVUR

Instead, RT tasks are currently covered by an rt_avg metric which we
already know is not fitting for most purposes.
It seems that the main goal is twofold: move people to DL whenever
possible otherwise live with the go-to-max policy which is the only
sensible solution to satisfy the RT's class main goal, i.e. latency
reduction.

Of course such a go-to-max policy for all RT tasks we already know
that is going to destroy energy on many different mobile scenarios.

As a possible mitigation for that, while still being compliant with
the main RT's class goal, we recently posted the SchedTune v3
proposal:
  https://lkml.org/lkml/2017/2/28/355

In that proposal, the simple usage of CGroups and the new capacity_max
attribute of the (existing) CPU controller should allow to define what
is the "max" value which is just enough to match the latency
constraints of a mobile application without sacrificing too much
energy.

> -- 
> viresh

Cheers Patrick

-- 
#include <best/regards.h>

Patrick Bellasi