Re: [PATCH v5 6/6] sched/fair: Consider SMT in ASYM_PACKING load balance

[Vincent Guittot]

On Fri, 17 Sept 2021 at 20:47, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Fri, Sep 17, 2021 at 05:25:02PM +0200, Vincent Guittot wrote:
>
> > With the removal of the condition !sds->local_stat.sum_nr_running
> > which seems useless because dst_cpu is idle and not SMT, this patch
> > looks good to me
>
> I've made it look like this. Thanks!

Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

Thanks

>
> ---
> Subject: sched/fair: Consider SMT in ASYM_PACKING load balance
> From: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> Date: Fri, 10 Sep 2021 18:18:19 -0700
>
> From: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
>
> When deciding to pull tasks in ASYM_PACKING, it is necessary not only to
> check for the idle state of the destination CPU, dst_cpu, but also of
> its SMT siblings.
>
> If dst_cpu is idle but its SMT siblings are busy, performance suffers
> if it pulls tasks from a medium priority CPU that does not have SMT
> siblings.
>
> Implement asym_smt_can_pull_tasks() to inspect the state of the SMT
> siblings of both dst_cpu and the CPUs in the candidate busiest group.
>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Reviewed-by: Len Brown <len.brown@xxxxxxxxx>
> Link: https://lkml.kernel.org/r/20210911011819.12184-7-ricardo.neri-calderon@xxxxxxxxxxxxxxx
> ---
>  kernel/sched/fair.c |   92 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 92 insertions(+)
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8538,10 +8538,96 @@ group_type group_classify(unsigned int i
>         return group_has_spare;
>  }
>
> +/**
> + * asym_smt_can_pull_tasks - Check whether the load balancing CPU can pull tasks
> + * @dst_cpu:   Destination CPU of the load balancing
> + * @sds:       Load-balancing data with statistics of the local group
> + * @sgs:       Load-balancing statistics of the candidate busiest group
> + * @sg:                The candidate busiest group
> + *
> + * Check the state of the SMT siblings of both @sds::local and @sg and decide
> + * if @dst_cpu can pull tasks.
> + *
> + * If @dst_cpu does not have SMT siblings, it can pull tasks if two or more of
> + * the SMT siblings of @sg are busy. If only one CPU in @sg is busy, pull tasks
> + * only if @dst_cpu has higher priority.
> + *
> + * If both @dst_cpu and @sg have SMT siblings, and @sg has exactly one more
> + * busy CPU than @sds::local, let @dst_cpu pull tasks if it has higher priority.
> + * Bigger imbalances in the number of busy CPUs will be dealt with in
> + * update_sd_pick_busiest().
> + *
> + * If @sg does not have SMT siblings, only pull tasks if all of the SMT siblings
> + * of @dst_cpu are idle and @sg has lower priority.
> + */
> +static bool asym_smt_can_pull_tasks(int dst_cpu, struct sd_lb_stats *sds,
> +                                   struct sg_lb_stats *sgs,
> +                                   struct sched_group *sg)
> +{
> +#ifdef CONFIG_SCHED_SMT
> +       bool local_is_smt, sg_is_smt;
> +       int sg_busy_cpus;
> +
> +       local_is_smt = sds->local->flags & SD_SHARE_CPUCAPACITY;
> +       sg_is_smt = sg->flags & SD_SHARE_CPUCAPACITY;
> +
> +       sg_busy_cpus = sgs->group_weight - sgs->idle_cpus;
> +
> +       if (!local_is_smt) {
> +               /*
> +                * If we are here, @dst_cpu is idle and does not have SMT
> +                * siblings. Pull tasks if candidate group has two or more
> +                * busy CPUs.
> +                */
> +               if (sg_busy_cpus >= 2) /* implies sg_is_smt */
> +                       return true;
> +
> +               /*
> +                * @dst_cpu does not have SMT siblings. @sg may have SMT
> +                * siblings and only one is busy. In such case, @dst_cpu
> +                * can help if it has higher priority and is idle (i.e.,
> +                * it has no running tasks).
> +                */
> +               return sched_asym_prefer(dst_cpu, sg->asym_prefer_cpu);
> +       }
> +
> +       /* @dst_cpu has SMT siblings. */
> +
> +       if (sg_is_smt) {
> +               int local_busy_cpus = sds->local->group_weight -
> +                                     sds->local_stat.idle_cpus;
> +               int busy_cpus_delta = sg_busy_cpus - local_busy_cpus;
> +
> +               if (busy_cpus_delta == 1)
> +                       return sched_asym_prefer(dst_cpu, sg->asym_prefer_cpu);
> +
> +               return false;
> +       }
> +
> +       /*
> +        * @sg does not have SMT siblings. Ensure that @sds::local does not end
> +        * up with more than one busy SMT sibling and only pull tasks if there
> +        * are not busy CPUs (i.e., no CPU has running tasks).
> +        */
> +       if (!sds->local_stat.sum_nr_running)
> +               return sched_asym_prefer(dst_cpu, sg->asym_prefer_cpu);
> +
> +       return false;
> +#else
> +       /* Always return false so that callers deal with non-SMT cases. */
> +       return false;
> +#endif
> +}
> +
>  static inline bool
>  sched_asym(struct lb_env *env, struct sd_lb_stats *sds,  struct sg_lb_stats *sgs,
>            struct sched_group *group)
>  {
> +       /* Only do SMT checks if either local or candidate have SMT siblings */
> +       if ((sds->local->flags & SD_SHARE_CPUCAPACITY) ||
> +           (group->flags & SD_SHARE_CPUCAPACITY))
> +               return asym_smt_can_pull_tasks(env->dst_cpu, sds, sgs, group);
> +
>         return sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu);
>  }
>
> @@ -9547,6 +9633,12 @@ static struct rq *find_busiest_queue(str
>                     nr_running == 1)
>                         continue;
>
> +               /* Make sure we only pull tasks from a CPU of lower priority */
> +               if ((env->sd->flags & SD_ASYM_PACKING) &&
> +                   sched_asym_prefer(i, env->dst_cpu) &&
> +                   nr_running == 1)
> +                       continue;
> +
>                 switch (env->migration_type) {
>                 case migrate_load:
>                         /*