Re: [PATCH 3/4] sched/fair: Consider SMT in ASYM_PACKING load balance

[Ricardo Neri]

On Thu, Apr 08, 2021 at 01:10:39PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 06, 2021 at 04:17:10PM -0700, Ricardo Neri wrote:
> > On Tue, Apr 06, 2021 at 01:17:28PM +0200, Peter Zijlstra wrote:
> > > On Mon, Apr 05, 2021 at 09:11:07PM -0700, Ricardo Neri wrote:
> > > > @@ -8507,6 +8619,10 @@ static bool update_sd_pick_busiest(struct lb_env *env,
> > > >  	if (!sgs->sum_h_nr_running)
> > > >  		return false;
> > > >  
> > > > +	if (sgs->group_type == group_asym_packing &&
> > > > +	    !asym_can_pull_tasks(env->dst_cpu, sds, sgs, sg))
> > > > +		return false;
> > > 
> > > All of this makes my head hurt; but afaict this isn't right.
> > > 
> > > Your update_sg_lb_stats() change makes that we unconditionally set
> > > sgs->group_asym_packing, and then this is to undo that. But it's not
> > > clear this covers all cases right.
> > 
> > We could not make a decision to set sgs->group_asym_packing in
> > update_sg_lb_stats() because we don't have information about the dst_cpu
> > and its SMT siblings if any. That is the reason I proposed to delay the
> > decision to update_sd_pick_busiest(), where we can compare local and
> > sgs.
> 
> Yeah, I sorta got that.
> 
> > > Even if !sched_asym_prefer(), we could end up selecting this sg as
> > > busiest, but you're just bailing out here.
> > 
> > Even if sgs->group_asym_packing is unconditionally set, sgs can still
> > be classified as group_overloaded and group_imbalanced. In such cases
> > we wouldn't bailout. sgs could not be classified as group_fully_busy
> > or group_has_spare and we would bailout, though. Is your concern about
> > these? I can fixup these two cases.
> 
> Yes. Either explain (in a comment) why those cases are not relevant, or
> handle them properly.
> 
> Because when reading this, it wasn't at all obvious that this is correct
> or as intended.

Sure Peter, I will post a v2 handling the remaining cases properly.

Thanks and BR,
Ricardo