Re: [PATCH v3 07/24] sched/fair: Compute IPC class scores for load balancing

[Vincent Guittot]

On Tue, 7 Feb 2023 at 06:01, Ricardo Neri
<ricardo.neri-calderon@xxxxxxxxxxxxxxx> wrote:
>
> Compute the joint total (both current and prospective) IPC class score of
> a scheduling group and the local scheduling group.
>
> These IPCC statistics are used during idle load balancing. The candidate
> scheduling group will have one fewer busy CPU after load balancing. This
> observation is important for cores with SMT support.
>
> The IPCC score of scheduling groups composed of SMT siblings needs to
> consider that the siblings share CPU resources. When computing the total
> IPCC score of the scheduling group, divide score of each sibling by the
> number of busy siblings.
>
> Collect IPCC statistics for asym_packing and fully_busy scheduling groups.

IPCC statistics collect scores of current tasks, so they are
meaningful only when trying to migrate one of those running tasks.
Using such score when pulling other tasks is just meaningless. And I
don't see how you ensure such correct use of ipcc score

> When picking a busiest group, they are used to break ties between otherwise
> identical groups.
>
> Cc: Ben Segall <bsegall@xxxxxxxxxx>
> Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Cc: Ionela Voinescu <ionela.voinescu@xxxxxxx>
> Cc: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Cc: Len Brown <len.brown@xxxxxxxxx>
> Cc: Lukasz Luba <lukasz.luba@xxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> Cc: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Tim C. Chen <tim.c.chen@xxxxxxxxx>
> Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: linux-pm@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> ---
> Changes since v2:
>  * Also collect IPCC stats for fully_busy sched groups.

Why have you added fully_busy case ? it's worth explaining the
rational because there is a lot of change to use the ipcc score of the
wrong task to take the decision

>  * Restrict use of IPCC stats to SD_ASYM_PACKING. (Ionela)
>  * Handle errors of arch_get_ipcc_score(). (Ionela)
>
> Changes since v1:
>  * Implemented cleanups and reworks from PeterZ. I took all his
>    suggestions, except the computation of the  IPC score before and after
>    load balancing. We are computing not the average score, but the *total*.
>  * Check for the SD_SHARE_CPUCAPACITY to compute the throughput of the SMT
>    siblings of a physical core.
>  * Used the new interface names.
>  * Reworded commit message for clarity.
> ---
>  kernel/sched/fair.c | 68 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 68 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d773380a95b3..b6165aa8a376 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8901,6 +8901,8 @@ struct sg_lb_stats {
>         unsigned long min_score; /* Min(score(rq->curr->ipcc)) */
>         unsigned short min_ipcc; /* Class of the task with the minimum IPCC score in the rq */
>         unsigned long sum_score; /* Sum(score(rq->curr->ipcc)) */
> +       long ipcc_score_after; /* Prospective IPCC score after load balancing */
> +       unsigned long ipcc_score_before; /* IPCC score before load balancing */
>  #endif
>  };
>
> @@ -9287,6 +9289,62 @@ static void update_sg_lb_ipcc_stats(int dst_cpu, struct sg_lb_stats *sgs,
>         }
>  }
>
> +static void update_sg_lb_stats_scores(struct sg_lb_stats *sgs,
> +                                     struct sched_group *sg,
> +                                     struct lb_env *env)
> +{
> +       unsigned long score_on_dst_cpu, before;
> +       int busy_cpus;
> +       long after;
> +
> +       if (!sched_ipcc_enabled())
> +               return;
> +
> +       /*
> +        * IPCC scores are only useful during idle load balancing. For now,
> +        * only asym_packing uses IPCC scores.
> +        */
> +       if (!(env->sd->flags & SD_ASYM_PACKING) ||
> +           env->idle == CPU_NOT_IDLE)
> +               return;
> +
> +       /*
> +        * IPCC scores are used to break ties only between these types of
> +        * groups.
> +        */
> +       if (sgs->group_type != group_fully_busy &&
> +           sgs->group_type != group_asym_packing)
> +               return;
> +
> +       busy_cpus = sgs->group_weight - sgs->idle_cpus;
> +
> +       /* No busy CPUs in the group. No tasks to move. */
> +       if (!busy_cpus)
> +               return;
> +
> +       score_on_dst_cpu = arch_get_ipcc_score(sgs->min_ipcc, env->dst_cpu);
> +
> +       /*
> +        * Do not use IPC scores. sgs::ipcc_score_{after, before} will be zero
> +        * and not used.
> +        */
> +       if (IS_ERR_VALUE(score_on_dst_cpu))
> +               return;
> +
> +       before = sgs->sum_score;
> +       after = before - sgs->min_score;

IIUC, you assume that you will select the cpu with the min score.
How do you ensure this ? otherwise all your comparisong are useless

> +
> +       /* SMT siblings share throughput. */
> +       if (busy_cpus > 1 && sg->flags & SD_SHARE_CPUCAPACITY) {
> +               before /= busy_cpus;
> +               /* One sibling will become idle after load balance. */
> +               after /= busy_cpus - 1;
> +       }
> +
> +       sgs->ipcc_score_after = after + score_on_dst_cpu;
> +       sgs->ipcc_score_before = before;

I'm not sure to understand why you are computing the sum_score,
ipcc_score_before and ipcc_score_after ?
Why is it not sufficient to check if score_on_dst_cpu will be higher
than current min_score ?

> +}
> +
>  #else /* CONFIG_IPC_CLASSES */
>  static void update_sg_lb_ipcc_stats(int dst_cpu, struct sg_lb_stats *sgs,
>                                     struct rq *rq)
> @@ -9296,6 +9354,13 @@ static void update_sg_lb_ipcc_stats(int dst_cpu, struct sg_lb_stats *sgs,
>  static void init_rq_ipcc_stats(struct sg_lb_stats *sgs)
>  {
>  }
> +
> +static void update_sg_lb_stats_scores(struct sg_lb_stats *sgs,
> +                                     struct sched_group *sg,
> +                                     struct lb_env *env)
> +{
> +}
> +
>  #endif /* CONFIG_IPC_CLASSES */
>
>  /**
> @@ -9457,6 +9522,9 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>
>         sgs->group_type = group_classify(env->sd->imbalance_pct, group, sgs);
>
> +       if (!local_group)
> +               update_sg_lb_stats_scores(sgs, group, env);
> +
>         /* Computing avg_load makes sense only when group is overloaded */
>         if (sgs->group_type == group_overloaded)
>                 sgs->avg_load = (sgs->group_load * SCHED_CAPACITY_SCALE) /
> --
> 2.25.1
>