Re: [PATCH v3 5/6] sched/fair: Consider SMT in ASYM_PACKING load balance

[Ricardo Neri]

On Fri, May 14, 2021 at 11:47:45AM +0200, Peter Zijlstra wrote:
> On Thu, May 13, 2021 at 08:49:08AM -0700, Ricardo Neri wrote:
> >  include/linux/sched/topology.h |   1 +
> >  kernel/sched/fair.c            | 101 +++++++++++++++++++++++++++++++++
> >  2 files changed, 102 insertions(+)
> > 
> > diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> > index 8f0f778b7c91..43bdb8b1e1df 100644
> > --- a/include/linux/sched/topology.h
> > +++ b/include/linux/sched/topology.h
> > @@ -57,6 +57,7 @@ static inline int cpu_numa_flags(void)
> >  #endif
> >  
> >  extern int arch_asym_cpu_priority(int cpu);
> > +extern bool arch_asym_check_smt_siblings(void);
> >  
> >  struct sched_domain_attr {
> >  	int relax_domain_level;
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index c8b66a5d593e..3d6cc027e6e6 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -106,6 +106,15 @@ int __weak arch_asym_cpu_priority(int cpu)
> >  	return -cpu;
> >  }
> >  
> > +/*
> > + * For asym packing, first check the state of SMT siblings before deciding to
> > + * pull tasks.
> > + */
> > +bool __weak arch_asym_check_smt_siblings(void)
> > +{
> > +	return false;
> > +}
> > +
> >  /*
> >   * The margin used when comparing utilization with CPU capacity.
> >   *
> 
> > @@ -8458,6 +8550,9 @@ sched_asym(struct lb_env *env, struct sd_lb_stats *sds,  struct sg_lb_stats *sgs
> >  	if (group == sds->local)
> >  		return false;
> >  
> > +	if (arch_asym_check_smt_siblings())
> > +		return asym_can_pull_tasks(env->dst_cpu, sds, sgs, group);
> > +
> >  	return sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu);
> >  }
> 
> So I'm thinking that this is a property of having ASYM_PACKING at a core
> level, rather than some arch special. Wouldn't something like this be
> more appropriate?
> 
> ---
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -57,7 +57,6 @@ static inline int cpu_numa_flags(void)
>  #endif
>  
>  extern int arch_asym_cpu_priority(int cpu);
> -extern bool arch_asym_check_smt_siblings(void);
>  
>  struct sched_domain_attr {
>  	int relax_domain_level;
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -107,15 +107,6 @@ int __weak arch_asym_cpu_priority(int cp
>  }
>  
>  /*
> - * For asym packing, first check the state of SMT siblings before deciding to
> - * pull tasks.
> - */
> -bool __weak arch_asym_check_smt_siblings(void)
> -{
> -	return false;
> -}
> -
> -/*
>   * The margin used when comparing utilization with CPU capacity.
>   *
>   * (default: ~20%)
> @@ -8550,7 +8541,8 @@ sched_asym(struct lb_env *env, struct sd
>  	if (group == sds->local)
>  		return false;
>  
> -	if (arch_asym_check_smt_siblings())
> +	if ((sds->local->flags & SD_SHARE_CPUCAPACITY) ||
> +	    (group->flags & SD_SHARE_CPUCAPACITY))
>  		return asym_can_pull_tasks(env->dst_cpu, sds, sgs, group);

Thanks Peter for the quick review! This makes sense to me. The only
reason we proposed arch_asym_check_smt_siblings() is because we were
about breaking powerpc (I need to study how they set priorities for SMT,
if applicable). If you think this is not an issue I can post a
v4 with this update.

Thanks and BR,
Ricardo