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

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