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

Quentin Perret
2018-08-09T15:55:53Z

Hi Patrick,

On Thursday 09 Aug 2018 at 16:41:56 (+0100), Patrick Bellasi wrote:
> > IIUC, not far below this you should still have something like:
> >
> > if (rt_rq_is_runnable(&rq->rt))
> > return max;
>
> Do you mean that when RT tasks are RUNNABLE we still want to got to
> MAX? Not sure to understand... since this patch is actually to clamp
> the RT class...

Argh, reading my message again it wasn't very clear indeed. Sorry about
that ...

What I'm try to say is that your patch does _not_ remove the snippet of code
above from sugov_get_util(). So I think that when a RT task is runnable,
you will not reach the end of the function where the clamping is done.
And this is not what you want AFAICT.

Does that make any sense ?

>
> > So you won't reach the actual clamping code at the end of the function
> > when a RT task is runnable no?
>
> ... mmm... no... this patch is to clamp RT tasks... Am I missing
> something?
>
> > Also, I always had the feeling that the default for RT should be
> > util_min == 1024, and then users could decide to lower the bar if they
> > want to.
>
> Mmm... good point! This would keep the policy unaltered for RT tasks.
>
> I want to keep sched class specific code in uclamp at minimum but
> likely this should be achievable by just properly initializing the
> task-specific util_min for RT tasks, if the original task has
> UCLAM_NOT_VALID.

+1, it'd be nice to keep the cross-class mess to a minimum IMO. But
hopefully this RT thing isn't too ugly to implement ...

>
> > For the specific case of RT, that feels more natural than
> > applying a max util clamp IMO. What do you think ?
>
> I'll look better into this for the next posting!

Sounds good :-)

Thanks,
Quentin