[RFCv5 PATCH 36/46] sched: Prevent unnecessary active balance of single task in sched group

[Morten Rasmussen]

Scenarios with the busiest group having just one task and the local
being idle on topologies with sched groups with different numbers of
cpus manage to dodge all load-balance bailout conditions resulting the
nr_balance_failed counter to be incremented. This eventually causes an
pointless active migration of the task. This patch prevents this by not
incrementing the counter when the busiest group only has one task.
ASYM_PACKING migrations and migrations due to reduced capacity should
still take place as these are explicitly captured by
need_active_balance().

A better solution would be to not attempt the load-balance in the first
place, but that requires significant changes to the order of bailout
conditions and statistics gathering.

cc: Ingo Molnar <mingo@xxxxxxxxxx>
cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

Signed-off-by: Morten Rasmussen <morten.rasmussen@xxxxxxx>
---
 kernel/sched/fair.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8e0cbd4..f7cb6c9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6127,6 +6127,7 @@ struct lb_env {
 	int			new_dst_cpu;
 	enum cpu_idle_type	idle;
 	long			imbalance;
+	unsigned int		src_grp_nr_running;
 	/* The set of CPUs under consideration for load-balancing */
 	struct cpumask		*cpus;
 
@@ -7150,6 +7151,8 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
 	if (env->sd->flags & SD_NUMA)
 		env->fbq_type = fbq_classify_group(&sds->busiest_stat);
 
+	env->src_grp_nr_running = sds->busiest_stat.sum_nr_running;
+
 	if (!env->sd->parent) {
 		/* update overload indicator if we are at root domain */
 		if (env->dst_rq->rd->overload != overload)
@@ -7788,7 +7791,8 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 		 * excessive cache_hot migrations and active balances.
 		 */
 		if (idle != CPU_NEWLY_IDLE)
-			sd->nr_balance_failed++;
+			if (env.src_grp_nr_running > 1)
+				sd->nr_balance_failed++;
 
 		if (need_active_balance(&env)) {
 			raw_spin_lock_irqsave(&busiest->lock, flags);
-- 
1.9.1

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