sched/fair: Avoid pulling tasks from non-overloaded higher capacity groups

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.

Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dietmar.eggemann@arm.com
Cc: freedom.tan@mediatek.com
Cc: keita.kobayashi.ym@renesas.com
Cc: mgalbraith@suse.de
Cc: sgurrappadi@nvidia.com
Cc: vincent.guittot@linaro.org
Cc: yuyang.du@intel.com
Link: http://lkml.kernel.org/r/1476452472-24740-5-git-send-email-morten.rasmussen@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 25 insertions, 0 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index faf8f18616e6..ee39bfda5ae5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7073,6 +7073,17 @@ 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)
@@ -7176,6 +7187,20 @@ 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;