[tip: sched/core] sched/fair: Improve consistency of allowed NUMA balance calculations

[tip-bot2 for Mel Gorman]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     2cfb7a1b031b0e816af7a6ee0c6ab83b0acdf05a
Gitweb:        https://git.kernel.org/tip/2cfb7a1b031b0e816af7a6ee0c6ab83b0acdf05a
Author:        Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
AuthorDate:    Tue, 08 Feb 2022 09:43:33 
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 11 Feb 2022 23:30:08 +01:00

sched/fair: Improve consistency of allowed NUMA balance calculations

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@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Gautham R. Shenoy <gautham.shenoy@xxxxxxx>
Link: https://lore.kernel.org/r/20220208094334.16379-2-mgorman@xxxxxxxxxxxxxxxxxxx
---
 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 5dca13f..ea71016 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9005,9 +9005,10 @@ 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));
 }
 
 /*
@@ -9141,12 +9142,13 @@ 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;
 		}
 
@@ -9352,7 +9354,7 @@ 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;