Re: [RFC PATCH 09/23] sched/fair: Use task-class performance score to pick the busiest group

Ricardo Neri
2022-10-06T19:03:07Z

On Thu, Oct 06, 2022 at 10:37:52AM +0200, Peter Zijlstra wrote:
> On Wed, Oct 05, 2022 at 04:38:41PM -0700, Ricardo Neri wrote:
> > On Tue, Sep 27, 2022 at 01:01:32PM +0200, Peter Zijlstra wrote:
> > > On Fri, Sep 09, 2022 at 04:11:51PM -0700, Ricardo Neri wrote:
>
> > > > @@ -9049,6 +9111,12 @@ static bool update_sd_pick_busiest(struct lb_env *env,
> > > > /* Prefer to move from lowest priority CPU's work */
> > > > if (sched_asym_prefer(sg->asym_prefer_cpu, sds->busiest->asym_prefer_cpu))
> > > > return false;
> > > > +
> > > > + /* @sg and @sds::busiest have the same priority. */
> > > > + if (sched_asym_class_pick(sds->busiest, sg, &sds->busiest_stat, sgs))
> > > > + return false;
> > > > +
> > > > + /* @sg has lower priority than @sds::busiest. */
> > > > break;
> > > >
> > > > case group_misfit_task:
> > >
> > > So why does only this one instance of asym_prefer() require tie
> > > breaking?
> >
> > This is the only place in which two sched groups with running tasks and of
> > equal priority are compared.
> >
> > In all other places sched_asym_prefer() is used to compare the destination
> > CPU with others. Since asym_packing is done only when the destination CPU is
> > idle, there is no need to break this tie.

>
> That would make for a fine comment, no? Because as presented one is left
> wondering, why if asym_prefer() needs tie breaking, only this one site
> needs it.

Sure. I will add this comment.

>
> > > And while looking through this, I must ask about the comment that goes
> > > with sched_set_itmt_core_prio() vs the sg->asym_prefer_cpu assignment in
> > > init_sched_groups_capacity(), what-up ?!
> >
> > Are you referring to this comment?
> >
> > "No need to rebuild sched domain after updating
> > the CPU priorities. The sched domains have no
> > dependency on CPU priorities"
> >
> > If yes, then it looks wrong to me. Sched domains are rebuilt after updating
> > priorities.

I can included in the series a patch removing this comment.

Thanks and BR,
Ricardo