Commit 94beeae8 authored Jul 02, 2015 by Morten Rasmussen Committed by Dmitry Shmidt Jan 31, 2017

ANDROID: sched: Prevent unnecessary active balance of single task in sched group



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 a
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@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Signed-off-by: Andres Oportus <andresoportus@google.com>

parent 90f309fb

kernel/sched/fair.c

+5 −1

Original line number	Diff line number	Diff line
		@@ -6311,6 +6311,7 @@ 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;

		@@ -7281,6 +7282,8 @@ 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)
		@@ -7903,6 +7906,7 @@ static int load_balance(int this_cpu, struct rq *this_rq,
		* excessive cache_hot migrations and active balances.
		*/
		if (idle != CPU_NEWLY_IDLE)
		if (env.src_grp_nr_running > 1)
		sd->nr_balance_failed++;

		if (need_active_balance(&env)) {