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

[Juri Lelli]

On 13/08/18 11:12, Patrick Bellasi wrote:
> Hi Vincent!
> 
> On 09-Aug 18:03, Vincent Guittot wrote:
> > > On 07-Aug 15:26, Juri Lelli wrote:
> 
> [...]
> 
> > > > > +   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?
> > 
> > Does the policy for (sched_feat(UCLAMP_SCHED_CLASS)== true) really
> > make sense as it is ?
> > I mean, uclamp_util doesn't make any difference between rt and cfs
> > tasks when clamping the utilization so why should be add twice the
> > returned value ?
> > IMHO, this policy would make sense if there were something like
> > uclamp_util_rt() and a uclamp_util_cfs()
> 
> The idea for the UCLAMP_SCHED_CLASS policy is to improve fairness on
> low-priority classese, especially when we have high RT utilization.
> 
> Let say we have:
> 
>  util_rt  = 40%, util_min=0%
>  util_cfs = 10%, util_min=50%
> 
> the two policies will select:
> 
>   UCLAMP_SCHED_CLASS: util = uclamp(40) + uclamp(10) = 50 + 50   = 100%
>  !UCLAMP_SCHED_CLASS: util = uclamp(40 + 10)         = uclmp(50) =  50%
> 
> Which means that, despite the CPU's util_min will be set to 50% when
> CFS is running, these tasks will have almost no boost at all, since
> their bandwidth margin is eclipsed by RT tasks.

Ah, right. But isn't possible to distinguish between classes? I mean, if
you would know that only CFS is clamped (boosted) in this case, you
could have:

 util = util_rt + uclamp(util_cfs) = 40 + 50 = 90%

Which should do what one expects w/o energy side effects?