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

Vincent Guittot
2018-08-09T16:04:12Z

Hi Patrick,

On Thu, 9 Aug 2018 at 17:34, Patrick Bellasi <patrick.bellasi@arm.com> wrote:
>
> On 07-Aug 15:26, Juri Lelli wrote:
> > Hi,
> >
> > On 06/08/18 17:39, Patrick Bellasi wrote:
> >
> > [...]
> >
> > > @@ -223,13 +224,25 @@ static unsigned long sugov_get_util(struct sugov_cpu *sg_cpu)
> > > * utilization (PELT windows are synchronized) we can directly add them
> > > * to obtain the CPU's actual utilization.
> > > *
> > > - * CFS utilization can be boosted or capped, depending on utilization
> > > - * clamp constraints configured for currently RUNNABLE tasks.
> > > + * CFS and RT utilizations can be boosted or capped, depending on
> > > + * utilization constraints enforce by currently RUNNABLE tasks.
> > > + * They are individually clamped to ensure fairness across classes,
> > > + * meaning that CFS always gets (if possible) the (minimum) required
> > > + * bandwidth on top of that required by higher priority classes.
> >
> > Is this a stale comment written before UCLAMP_SCHED_CLASS was
> > introduced? It seems to apply to the below if branch only.
>
> Yes, you right... I'll update the comment.
>
> > > */
> > > - util = cpu_util_cfs(rq);
> > > - if (util)
> > > - util = uclamp_util(cpu_of(rq), util);
> > > - util += cpu_util_rt(rq);
> > > + 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()

>
> We had an internal discussion and we found pro/cons for both... but
> I'm not sure keeping the sched_feat is a good solution on the long
> run, i.e. mainline merge ;)
>
> --
> #include <best/regards.h>
>
> Patrick Bellasi