For asymmetric cpu capacity systems it is counter-productive for
throughput if low capacity cpus are pulling tasks from non-overloaded
cpus with higher capacity. The assumption is that higher cpu capacity is
preferred over running alone in a group with lower cpu capacity.

This patch rejects higher cpu capacity groups with one or less task per
cpu as potential busiest group which could otherwise lead to a series of
failing load-balancing attempts leading to a force-migration.

cc: Ingo Molnar <mi...@redhat.com>
cc: Peter Zijlstra <pet...@infradead.org>

Signed-off-by: Morten Rasmussen <morten.rasmus...@arm.com>
---
 kernel/sched/fair.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 28e42cb41d7b..a5efafda23ef 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7069,6 +7069,17 @@ group_is_overloaded(struct lb_env *env, struct 
sg_lb_stats *sgs)
        return false;
 }
 
+/*
+ * group_smaller_cpu_capacity: Returns true if sched_group sg has smaller
+ * per-cpu capacity than sched_group ref.
+ */
+static inline bool
+group_smaller_cpu_capacity(struct sched_group *sg, struct sched_group *ref)
+{
+       return sg->sgc->min_capacity * capacity_margin <
+                                               ref->sgc->min_capacity * 1024;
+}
+
 static inline enum
 group_type group_classify(struct sched_group *group,
                          struct sg_lb_stats *sgs)
@@ -7172,6 +7183,20 @@ static bool update_sd_pick_busiest(struct lb_env *env,
        if (sgs->avg_load <= busiest->avg_load)
                return false;
 
+       if (!(env->sd->flags & SD_ASYM_CPUCAPACITY))
+               goto asym_packing;
+
+       /*
+        * Candidate sg has no more than one task per cpu and
+        * has higher per-cpu capacity. Migrating tasks to less
+        * capable cpus may harm throughput. Maximize throughput,
+        * power/energy consequences are not considered.
+        */
+       if (sgs->sum_nr_running <= sgs->group_weight &&
+           group_smaller_cpu_capacity(sds->local, sg))
+               return false;
+
+asym_packing:
        /* This is the busiest node in its class. */
        if (!(env->sd->flags & SD_ASYM_PACKING))
                return true;
-- 
2.7.4

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