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    Subject[PATCH 5.16 0266/1017] sched/fair: Improve consistency of allowed NUMA balance calculations
    Date
    From: Mel Gorman <mgorman@techsingularity.net>

    [ Upstream commit 2cfb7a1b031b0e816af7a6ee0c6ab83b0acdf05a ]

    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@techsingularity.net>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
    Link: https://lore.kernel.org/r/20220208094334.16379-2-mgorman@techsingularity.net
    Signed-off-by: Sasha Levin <sashal@kernel.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 069e01772d92..9637766e220d 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -9062,9 +9062,10 @@ 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));
    }

    /*
    @@ -9198,12 +9199,13 @@ 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;
    }

    @@ -9409,7 +9411,7 @@ 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;
    --
    2.34.1


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