Re: [PATCH 06/10] sched/fair: Clear the target CPU from the cpumask of CPUs searched

[Vincent Guittot]

On Fri, 4 Dec 2020 at 12:30, Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, Dec 04, 2020 at 11:56:36AM +0100, Vincent Guittot wrote:
> > > The intent was that the sibling might still be an idle candidate. In
> > > the current draft of the series, I do not even clear this so that the
> > > SMT sibling is considered as an idle candidate. The reasoning is that if
> > > there are no idle cores then an SMT sibling of the target is as good an
> > > idle CPU to select as any.
> >
> > Isn't the purpose of select_idle_smt ?
> >
>
> Only in part.
>
> > select_idle_core() looks for an idle core and opportunistically saves
> > an idle CPU candidate to skip select_idle_cpu. In this case this is
> > useless loops for select_idle_core() because we are sure that the core
> > is not idle
> >
>
> If select_idle_core() finds an idle candidate other than the sibling,
> it'll use it if there is no idle core -- it picks a busy sibling based
> on a linear walk of the cpumask. Similarly, select_idle_cpu() is not

My point is that it's a waste of time to loop the sibling cpus of
target in select_idle_core because it will not help to find an idle
core. The sibling  cpus will then be check either by select_idle_cpu
of select_idle_smt

> guaranteed to scan the sibling first (ordering) or even reach the sibling
> (throttling). select_idle_smt() is a last-ditch effort.
>
> --
> Mel Gorman
> SUSE Labs