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

[Patrick Bellasi <patrick.bellasi@arm.com>]

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.

> > We had an internal discussion and we found pro/cons for both... but

The UCLAMP_SCHED_CLASS policy is thus less energy efficiency but it
should grant a better "isolation" in terms of what is the expected
speed-up a task will get at run-time, independently from higher
priority classes.

Does that make sense?

> > I'm not sure keeping the sched_feat is a good solution on the long
> > run, i.e. mainline merge ;)

This problem still stands...

-- 
#include <best/regards.h>

Patrick Bellasi