linux-kernel - Re: [PATCH 1/2] sched/fair: Improve consistency of allowed NUMA balance calculations

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Date:   Mon, 14 Feb 2022 10:48:35 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Valentin Schneider <Valentin.Schneider@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Mike Galbraith <efault@....de>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Gautham Shenoy <gautham.shenoy@....com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] sched/fair: Improve consistency of allowed NUMA
 balance calculations

On Tue, 8 Feb 2022 at 10:43, Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> There are inconsistencies when determining if a NUMA imbalance is allowed
> that should be corrected.
>
> o allow_numa_imbalance changes types and is not always examining
>   the destination group so both the type should be corrected as
>   well as the naming.
> o find_idlest_group uses the sched_domain's weight instead of the
>   group weight which is different to find_busiest_group
> o find_busiest_group uses the source group instead of the destination
>   which is different to task_numa_find_cpu
> o Both find_idlest_group and find_busiest_group should account
>   for the number of running tasks if a move was allowed to be
>   consistent with task_numa_find_cpu
>
> Fixes: 7d2b5dd0bcc4 ("sched/numa: Allow a floating imbalance between NUMA nodes")
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

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

> ---
>  kernel/sched/fair.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 095b0aa378df..4592ccf82c34 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9003,9 +9003,10 @@ static bool update_pick_idlest(struct sched_group *idlest,
>   * This is an approximation as the number of running tasks may not be
>   * related to the number of busy CPUs due to sched_setaffinity.
>   */
> -static inline bool allow_numa_imbalance(int dst_running, int dst_weight)
> +static inline bool
> +allow_numa_imbalance(unsigned int running, unsigned int weight)
>  {
> -       return (dst_running < (dst_weight >> 2));
> +       return (running < (weight >> 2));
>  }
>
>  /*
> @@ -9139,12 +9140,13 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
>                                 return idlest;
>  #endif
>                         /*
> -                        * Otherwise, keep the task on this node to stay close
> -                        * its wakeup source and improve locality. If there is
> -                        * a real need of migration, periodic load balance will
> -                        * take care of it.
> +                        * Otherwise, keep the task close to the wakeup source
> +                        * and improve locality if the number of running tasks
> +                        * would remain below threshold where an imbalance is
> +                        * allowed. If there is a real need of migration,
> +                        * periodic load balance will take care of it.
>                          */
> -                       if (allow_numa_imbalance(local_sgs.sum_nr_running, sd->span_weight))
> +                       if (allow_numa_imbalance(local_sgs.sum_nr_running + 1, local_sgs.group_weight))
>                                 return NULL;
>                 }
>
> @@ -9350,7 +9352,7 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>                 /* Consider allowing a small imbalance between NUMA groups */
>                 if (env->sd->flags & SD_NUMA) {
>                         env->imbalance = adjust_numa_imbalance(env->imbalance,
> -                               busiest->sum_nr_running, busiest->group_weight);
> +                               local->sum_nr_running + 1, local->group_weight);
>                 }
>
>                 return;
> --
> 2.31.1
>