[PATCH 5.16 0266/1017] sched/fair: Improve consistency of allowed NUMA balance calculations

From: Greg Kroah-Hartman
Date: Tue Apr 05 2022 - 07:46:09 EST

From: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

[ 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@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
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
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;
- * 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);