Re: [PATCH 4/6] sched/fair: Skip prefer sibling move between SMT group and non-SMT group

[Tim Chen]

On Fri, 2023-05-05 at 15:22 +0200, Peter Zijlstra wrote:
> On Thu, May 04, 2023 at 09:09:54AM -0700, Tim Chen wrote:
> > From: Tim C Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > 
> > Do not try to move tasks between non SMT sched group and SMT sched
> > group for "prefer sibling" load balance.
> > Let asym_active_balance_busiest() handle that case properly.
> > Otherwise we could get task bouncing back and forth between
> > the SMT sched group and non SMT sched group.
> > 
> > Reviewed-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > ---
> >  kernel/sched/fair.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 8a325db34b02..58ef7d529731 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -10411,8 +10411,12 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
> >  	/*
> >  	 * Try to move all excess tasks to a sibling domain of the busiest
> >  	 * group's child domain.
> > +	 *
> > +	 * Do not try to move between non smt sched group and smt sched
> > +	 * group. Let asym active balance properly handle that case.
> >  	 */
> >  	if (sds.prefer_sibling && local->group_type == group_has_spare &&
> > +	    !asymmetric_groups(sds.busiest, sds.local) &&
> >  	    busiest->sum_nr_running > local->sum_nr_running + 1)
> >  		goto force_balance;
> 
> This seems to have the hidden assumption that a !SMT core is somehow
> 'less' that an SMT code. Should this not also look at
> sched_asym_prefer() to establush this is so?
> 
> I mean, imagine I have a regular system and just offline one smt sibling
> for giggles.

I don't quite follow your point as asymmetric_groups() returns false even
one smt sibling is offlined.

Even say sds.busiest has 1 SMT and sds.local has 2 SMT, both sched groups still
have SD_SHARE_CPUCAPACITY flag turned on.  So asymmetric_groups() return
false and the load balancing logic is not changed for regular non-hybrid system.

I may be missing something.

Tim