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

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Friday, March 03, 2017 12:38:30 PM Patrick Bellasi wrote:
> 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.

And who's going to figure out what "max" value is most suitable?  And how?

Thanks,
Rafael