sched/fair: Rework load_balance()

The load_balance() algorithm contains some heuristics which have become meaningless since the rework of the scheduler's metrics like the introduction of PELT. Furthermore, load is an ill-suited metric for solving certain task placement imbalance scenarios. For instance, in the presence of idle CPUs, we should simply try to get at least one task per CPU, whereas the current load-based algorithm can actually leave idle CPUs alone simply because the load is somewhat balanced. The current algorithm ends up creating virtual and meaningless values like the avg_load_per_task or tweaks the state of a group to make it overloaded whereas it's not, in order to try to migrate tasks. load_balance() should better qualify the imbalance of the group and clearly define what has to be moved to fix this imbalance. The type of sched_group has been extended to better reflect the type of imbalance. We now have: group_has_spare group_fully_busy group_misfit_task group_asym_packing group_imbalanced group_overloaded Based on the type of sched_group, load_balance now sets what it wants to move in order to fix the imbalance. It can be some load as before but also some utilization, a number of task or a type of task: migrate_task migrate_util migrate_load migrate_misfit This new load_balance() algorithm fixes several pending wrong tasks placement: - the 1 task per CPU case with asymmetric system - the case of cfs task preempted by other class - the case of tasks not evenly spread on groups with spare capacity Also the load balance decisions have been consolidated in the 3 functions below after removing the few bypasses and hacks of the current code: - update_sd_pick_busiest() select the busiest sched_group. - find_busiest_group() checks if there is an imbalance between local and busiest group. - calculate_imbalance() decides what have to be moved. Finally, the now unused field total_running of struct sd_lb_stats has been removed. Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> Cc: Ben Segall <bsegall@google.com> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com> Cc: Juri Lelli <juri.lelli@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mel Gorman <mgorman@suse.de> Cc: Mike Galbraith <efault@gmx.de> Cc: Morten.Rasmussen@arm.com Cc: Peter Zijlstra <peterz@infradead.org> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: hdanton@sina.com Cc: parth@linux.ibm.com Cc: pauld@redhat.com Cc: quentin.perret@arm.com Cc: riel@surriel.com Cc: srikar@linux.vnet.ibm.com Cc: valentin.schneider@arm.com Link: https://lkml.kernel.org/r/1571405198-27570-5-git-send-email-vincent.guittot@linaro.org [ Small readability and spelling updates. ] Signed-off-by: Ingo Molnar <mingo@kernel.org>
2019-10-18 15:26:31 +02:00 · 2019-10-18 15:26:31 +02:00 · 0b0695f2b3
parent fcf0553db6
commit 0b0695f2b3
1 changed files with 407 additions and 214 deletions
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@ -7058,11 +7058,26 @@ static unsigned long __read_mostly max_load_balance_interval = HZ/10;

 enum fbq_type { regular, remote, all };

+/*
+ * group_type describes the group of CPUs at the moment of the load balance.
+ * The enum is ordered by pulling priority, with the group with lowest priority
+ * first so the groupe_type can be simply compared when selecting the busiest
+ * group. see update_sd_pick_busiest().
+ */
 enum group_type {
-	group_other = 0,
+	group_has_spare = 0,
+	group_fully_busy,
 	group_misfit_task,
+	group_asym_packing,
 	group_imbalanced,
-	group_overloaded,
+	group_overloaded
+};
+
+enum migration_type {
+	migrate_load = 0,
+	migrate_util,
+	migrate_task,
+	migrate_misfit
 };

 #define LBF_ALL_PINNED	0x01
@ -7095,7 +7110,7 @@ struct lb_env {
 	unsigned int		loop_max;

 	enum fbq_type		fbq_type;
-	enum group_type		src_grp_type;
+	enum migration_type	migration_type;
 	struct list_head	tasks;
 };

@ -7318,7 +7333,7 @@ static struct task_struct *detach_one_task(struct lb_env *env)
 static const unsigned int sched_nr_migrate_break = 32;

 /*
- * detach_tasks() -- tries to detach up to imbalance runnable load from
+ * detach_tasks() -- tries to detach up to imbalance load/util/tasks from
 * busiest_rq, as part of a balancing operation within domain "sd".
 *
 * Returns number of detached tasks if successful and 0 otherwise.
@ -7326,8 +7341,8 @@ static const unsigned int sched_nr_migrate_break = 32;
 static int detach_tasks(struct lb_env *env)
 {
 	struct list_head *tasks = &env->src_rq->cfs_tasks;
+	unsigned long util, load;
 	struct task_struct *p;
-	unsigned long load;
 	int detached = 0;

 	lockdep_assert_held(&env->src_rq->lock);
@ -7360,19 +7375,51 @@ static int detach_tasks(struct lb_env *env)
 		if (!can_migrate_task(p, env))
 			goto next;

-		load = task_h_load(p);
+		switch (env->migration_type) {
+		case migrate_load:
+			load = task_h_load(p);

-		if (sched_feat(LB_MIN) && load < 16 && !env->sd->nr_balance_failed)
-			goto next;
+			if (sched_feat(LB_MIN) &&
+			    load < 16 && !env->sd->nr_balance_failed)
+				goto next;

-		if ((load / 2) > env->imbalance)
-			goto next;
+			if (load/2 > env->imbalance)
+				goto next;
+
+			env->imbalance -= load;
+			break;
+
+		case migrate_util:
+			util = task_util_est(p);
+
+			if (util > env->imbalance)
+				goto next;
+
+			env->imbalance -= util;
+			break;
+
+		case migrate_task:
+			env->imbalance--;
+			break;
+
+		case migrate_misfit:
+			load = task_h_load(p);
+
+			/*
+			 * Load of misfit task might decrease a bit since it has
+			 * been recorded. Be conservative in the condition.
+			 */
+			if (load/2 < env->imbalance)
+				goto next;
+
+			env->imbalance = 0;
+			break;
+		}

 		detach_task(p, env);
 		list_add(&p->se.group_node, &env->tasks);

 		detached++;
-		env->imbalance -= load;

 #ifdef CONFIG_PREEMPTION
 		/*
@ -7386,7 +7433,7 @@ static int detach_tasks(struct lb_env *env)

 		/*
 		 * We only want to steal up to the prescribed amount of
-		 * runnable load.
+		 * load/util/tasks.
 		 */
 		if (env->imbalance <= 0)
 			break;
@ -7651,7 +7698,6 @@ struct sg_lb_stats {
 	unsigned int idle_cpus;
 	unsigned int group_weight;
 	enum group_type group_type;
-	int group_no_capacity;
 	unsigned int group_asym_packing; /* Tasks should be moved to preferred CPU */
 	unsigned long group_misfit_task_load; /* A CPU has a task too big for its capacity */
 #ifdef CONFIG_NUMA_BALANCING
@ -7667,10 +7713,10 @@ struct sg_lb_stats {
 struct sd_lb_stats {
 	struct sched_group *busiest;	/* Busiest group in this sd */
 	struct sched_group *local;	/* Local group in this sd */
-	unsigned long total_running;
 	unsigned long total_load;	/* Total load of all groups in sd */
 	unsigned long total_capacity;	/* Total capacity of all groups in sd */
 	unsigned long avg_load;	/* Average load across all groups in sd */
+	unsigned int prefer_sibling; /* tasks should go to sibling first */

 	struct sg_lb_stats busiest_stat;/* Statistics of the busiest group */
 	struct sg_lb_stats local_stat;	/* Statistics of the local group */
@ -7681,19 +7727,18 @@ static inline void init_sd_lb_stats(struct sd_lb_stats *sds)
 	/*
 	 * Skimp on the clearing to avoid duplicate work. We can avoid clearing
 	 * local_stat because update_sg_lb_stats() does a full clear/assignment.
-	 * We must however clear busiest_stat::avg_load because
-	 * update_sd_pick_busiest() reads this before assignment.
+	 * We must however set busiest_stat::group_type and
+	 * busiest_stat::idle_cpus to the worst busiest group because
+	 * update_sd_pick_busiest() reads these before assignment.
 	 */
 	*sds = (struct sd_lb_stats){
 		.busiest = NULL,
 		.local = NULL,
-		.total_running = 0UL,
 		.total_load = 0UL,
 		.total_capacity = 0UL,
 		.busiest_stat = {
-			.avg_load = 0UL,
-			.sum_h_nr_running = 0,
-			.group_type = group_other,
+			.idle_cpus = UINT_MAX,
+			.group_type = group_has_spare,
 		},
 	};
 }
@ -7935,19 +7980,26 @@ group_smaller_max_cpu_capacity(struct sched_group *sg, struct sched_group *ref)
 }

 static inline enum
-group_type group_classify(struct sched_group *group,
+group_type group_classify(struct lb_env *env,
+			  struct sched_group *group,
 			  struct sg_lb_stats *sgs)
 {
-	if (sgs->group_no_capacity)
+	if (group_is_overloaded(env, sgs))
 		return group_overloaded;

 	if (sg_imbalanced(group))
 		return group_imbalanced;

+	if (sgs->group_asym_packing)
+		return group_asym_packing;
+
 	if (sgs->group_misfit_task_load)
 		return group_misfit_task;

-	return group_other;
+	if (!group_has_capacity(env, sgs))
+		return group_fully_busy;
+
+	return group_has_spare;
 }

 static bool update_nohz_stats(struct rq *rq, bool force)
@ -7984,10 +8036,12 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 				      struct sg_lb_stats *sgs,
 				      int *sg_status)
 {
-	int i, nr_running;
+	int i, nr_running, local_group;

 	memset(sgs, 0, sizeof(*sgs));

+	local_group = cpumask_test_cpu(env->dst_cpu, sched_group_span(group));
+
 	for_each_cpu_and(i, sched_group_span(group), env->cpus) {
 		struct rq *rq = cpu_rq(i);

@ -8012,9 +8066,16 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 		/*
 		 * No need to call idle_cpu() if nr_running is not 0
 		 */
-		if (!nr_running && idle_cpu(i))
+		if (!nr_running && idle_cpu(i)) {
 			sgs->idle_cpus++;
+			/* Idle cpu can't have misfit task */
+			continue;
+		}

+		if (local_group)
+			continue;
+
+		/* Check for a misfit task on the cpu */
 		if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
 		    sgs->group_misfit_task_load < rq->misfit_task_load) {
 			sgs->group_misfit_task_load = rq->misfit_task_load;
@ -8022,14 +8083,24 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 		}
 	}

-	/* Adjust by relative CPU capacity of the group */
+	/* Check if dst CPU is idle and preferred to this group */
+	if (env->sd->flags & SD_ASYM_PACKING &&
+	    env->idle != CPU_NOT_IDLE &&
+	    sgs->sum_h_nr_running &&
+	    sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu)) {
+		sgs->group_asym_packing = 1;
+	}
+
 	sgs->group_capacity = group->sgc->capacity;
-	sgs->avg_load = (sgs->group_load*SCHED_CAPACITY_SCALE) / sgs->group_capacity;

 	sgs->group_weight = group->group_weight;

-	sgs->group_no_capacity = group_is_overloaded(env, sgs);
-	sgs->group_type = group_classify(group, sgs);
+	sgs->group_type = group_classify(env, group, sgs);
+
+	/* Computing avg_load makes sense only when group is overloaded */
+	if (sgs->group_type == group_overloaded)
+		sgs->avg_load = (sgs->group_load * SCHED_CAPACITY_SCALE) /
+				sgs->group_capacity;
 }

 /**
@ -8052,6 +8123,10 @@ static bool update_sd_pick_busiest(struct lb_env *env,
 {
 	struct sg_lb_stats *busiest = &sds->busiest_stat;

+	/* Make sure that there is at least one task to pull */
+	if (!sgs->sum_h_nr_running)
+		return false;
+
 	/*
 	 * Don't try to pull misfit tasks we can't help.
 	 * We can use max_capacity here as reduction in capacity on some
@ -8060,7 +8135,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
 	 */
 	if (sgs->group_type == group_misfit_task &&
 	    (!group_smaller_max_cpu_capacity(sg, sds->local) ||
-	     !group_has_capacity(env, &sds->local_stat)))
+	     sds->local_stat.group_type != group_has_spare))
 		return false;

 	if (sgs->group_type > busiest->group_type)
@ -8069,62 +8144,80 @@ static bool update_sd_pick_busiest(struct lb_env *env,
 	if (sgs->group_type < busiest->group_type)
 		return false;

-	if (sgs->avg_load <= busiest->avg_load)
+	/*
+	 * The candidate and the current busiest group are the same type of
+	 * group. Let check which one is the busiest according to the type.
+	 */
+
+	switch (sgs->group_type) {
+	case group_overloaded:
+		/* Select the overloaded group with highest avg_load. */
+		if (sgs->avg_load <= busiest->avg_load)
+			return false;
+		break;
+
+	case group_imbalanced:
+		/*
+		 * Select the 1st imbalanced group as we don't have any way to
+		 * choose one more than another.
+		 */
 		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_h_nr_running <= sgs->group_weight &&
-	    group_smaller_min_cpu_capacity(sds->local, sg))
-		return false;
-
-	/*
-	 * If we have more than one misfit sg go with the biggest misfit.
-	 */
-	if (sgs->group_type == group_misfit_task &&
-	    sgs->group_misfit_task_load < busiest->group_misfit_task_load)
-		return false;
-
-asym_packing:
-	/* This is the busiest node in its class. */
-	if (!(env->sd->flags & SD_ASYM_PACKING))
-		return true;
-
-	/* No ASYM_PACKING if target CPU is already busy */
-	if (env->idle == CPU_NOT_IDLE)
-		return true;
-	/*
-	 * ASYM_PACKING needs to move all the work to the highest
-	 * prority CPUs in the group, therefore mark all groups
-	 * of lower priority than ourself as busy.
-	 *
-	 * This is primarily intended to used at the sibling level.  Some
-	 * cores like POWER7 prefer to use lower numbered SMT threads.  In the
-	 * case of POWER7, it can move to lower SMT modes only when higher
-	 * threads are idle.  When in lower SMT modes, the threads will
-	 * perform better since they share less core resources.  Hence when we
-	 * have idle threads, we want them to be the higher ones.
-	 */
-	if (sgs->sum_h_nr_running &&
-	    sched_asym_prefer(env->dst_cpu, sg->asym_prefer_cpu)) {
-		sgs->group_asym_packing = 1;
-		if (!sds->busiest)
-			return true;
-
+	case group_asym_packing:
 		/* Prefer to move from lowest priority CPU's work */
-		if (sched_asym_prefer(sds->busiest->asym_prefer_cpu,
-				      sg->asym_prefer_cpu))
-			return true;
+		if (sched_asym_prefer(sg->asym_prefer_cpu, sds->busiest->asym_prefer_cpu))
+			return false;
+		break;
+
+	case group_misfit_task:
+		/*
+		 * If we have more than one misfit sg go with the biggest
+		 * misfit.
+		 */
+		if (sgs->group_misfit_task_load < busiest->group_misfit_task_load)
+			return false;
+		break;
+
+	case group_fully_busy:
+		/*
+		 * Select the fully busy group with highest avg_load. In
+		 * theory, there is no need to pull task from such kind of
+		 * group because tasks have all compute capacity that they need
+		 * but we can still improve the overall throughput by reducing
+		 * contention when accessing shared HW resources.
+		 *
+		 * XXX for now avg_load is not computed and always 0 so we
+		 * select the 1st one.
+		 */
+		if (sgs->avg_load <= busiest->avg_load)
+			return false;
+		break;
+
+	case group_has_spare:
+		/*
+		 * Select not overloaded group with lowest number of
+		 * idle cpus. We could also compare the spare capacity
+		 * which is more stable but it can end up that the
+		 * group has less spare capacity but finally more idle
+		 * CPUs which means less opportunity to pull tasks.
+		 */
+		if (sgs->idle_cpus >= busiest->idle_cpus)
+			return false;
+		break;
 	}

-	return false;
+	/*
+	 * 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 ((env->sd->flags & SD_ASYM_CPUCAPACITY) &&
+	    (sgs->group_type <= group_fully_busy) &&
+	    (group_smaller_min_cpu_capacity(sds->local, sg)))
+		return false;
+
+	return true;
 }

 #ifdef CONFIG_NUMA_BALANCING
@ -8162,13 +8255,13 @@ static inline enum fbq_type fbq_classify_rq(struct rq *rq)
 * @env: The load balancing environment.
 * @sds: variable to hold the statistics for this sched_domain.
 */
+
 static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sds)
 {
 	struct sched_domain *child = env->sd->child;
 	struct sched_group *sg = env->sd->groups;
 	struct sg_lb_stats *local = &sds->local_stat;
 	struct sg_lb_stats tmp_sgs;
-	bool prefer_sibling = child && child->flags & SD_PREFER_SIBLING;
 	int sg_status = 0;

 #ifdef CONFIG_NO_HZ_COMMON
@ -8195,22 +8288,6 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
 		if (local_group)
 			goto next_group;

-		/*
-		 * In case the child domain prefers tasks go to siblings
-		 * first, lower the sg capacity so that we'll try
-		 * and move all the excess tasks away. We lower the capacity
-		 * of a group only if the local group has the capacity to fit
-		 * these excess tasks. The extra check prevents the case where
-		 * you always pull from the heaviest group when it is already
-		 * under-utilized (possible with a large weight task outweighs
-		 * the tasks on the system).
-		 */
-		if (prefer_sibling && sds->local &&
-		    group_has_capacity(env, local) &&
-		    (sgs->sum_h_nr_running > local->sum_h_nr_running + 1)) {
-			sgs->group_no_capacity = 1;
-			sgs->group_type = group_classify(sg, sgs);
-		}

 		if (update_sd_pick_busiest(env, sds, sg, sgs)) {
 			sds->busiest = sg;
@ -8219,13 +8296,15 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd

 next_group:
 		/* Now, start updating sd_lb_stats */
-		sds->total_running += sgs->sum_h_nr_running;
 		sds->total_load += sgs->group_load;
 		sds->total_capacity += sgs->group_capacity;

 		sg = sg->next;
 	} while (sg != env->sd->groups);

+	/* Tag domain that child domain prefers tasks go to siblings first */
+	sds->prefer_sibling = child && child->flags & SD_PREFER_SIBLING;
+
 #ifdef CONFIG_NO_HZ_COMMON
 	if ((env->flags & LBF_NOHZ_AGAIN) &&
 	    cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd))) {
@ -8263,69 +8342,149 @@ next_group:
 */
 static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *sds)
 {
-	unsigned long max_pull, load_above_capacity = ~0UL;
 	struct sg_lb_stats *local, *busiest;

 	local = &sds->local_stat;
 	busiest = &sds->busiest_stat;

-	if (busiest->group_asym_packing) {
-		env->imbalance = busiest->group_load;
+	if (busiest->group_type == group_misfit_task) {
+		/* Set imbalance to allow misfit tasks to be balanced. */
+		env->migration_type = migrate_misfit;
+		env->imbalance = busiest->group_misfit_task_load;
+		return;
+	}
+
+	if (busiest->group_type == group_asym_packing) {
+		/*
+		 * In case of asym capacity, we will try to migrate all load to
+		 * the preferred CPU.
+		 */
+		env->migration_type = migrate_task;
+		env->imbalance = busiest->sum_h_nr_running;
+		return;
+	}
+
+	if (busiest->group_type == group_imbalanced) {
+		/*
+		 * In the group_imb case we cannot rely on group-wide averages
+		 * to ensure CPU-load equilibrium, try to move any task to fix
+		 * the imbalance. The next load balance will take care of
+		 * balancing back the system.
+		 */
+		env->migration_type = migrate_task;
+		env->imbalance = 1;
 		return;
 	}

 	/*
-	 * Avg load of busiest sg can be less and avg load of local sg can
-	 * be greater than avg load across all sgs of sd because avg load
-	 * factors in sg capacity and sgs with smaller group_type are
-	 * skipped when updating the busiest sg:
+	 * Try to use spare capacity of local group without overloading it or
+	 * emptying busiest
 	 */
-	if (busiest->group_type != group_misfit_task &&
-	    (busiest->avg_load <= sds->avg_load ||
-	     local->avg_load >= sds->avg_load)) {
-		env->imbalance = 0;
+	if (local->group_type == group_has_spare) {
+		if (busiest->group_type > group_fully_busy) {
+			/*
+			 * If busiest is overloaded, try to fill spare
+			 * capacity. This might end up creating spare capacity
+			 * in busiest or busiest still being overloaded but
+			 * there is no simple way to directly compute the
+			 * amount of load to migrate in order to balance the
+			 * system.
+			 */
+			env->migration_type = migrate_util;
+			env->imbalance = max(local->group_capacity, local->group_util) -
+					 local->group_util;
+
+			/*
+			 * In some cases, the group's utilization is max or even
+			 * higher than capacity because of migrations but the
+			 * local CPU is (newly) idle. There is at least one
+			 * waiting task in this overloaded busiest group. Let's
+			 * try to pull it.
+			 */
+			if (env->idle != CPU_NOT_IDLE && env->imbalance == 0) {
+				env->migration_type = migrate_task;
+				env->imbalance = 1;
+			}
+
+			return;
+		}
+
+		if (busiest->group_weight == 1 || sds->prefer_sibling) {
+			unsigned int nr_diff = busiest->sum_h_nr_running;
+			/*
+			 * When prefer sibling, evenly spread running tasks on
+			 * groups.
+			 */
+			env->migration_type = migrate_task;
+			lsub_positive(&nr_diff, local->sum_h_nr_running);
+			env->imbalance = nr_diff >> 1;
+			return;
+		}
+
+		/*
+		 * If there is no overload, we just want to even the number of
+		 * idle cpus.
+		 */
+		env->migration_type = migrate_task;
+		env->imbalance = max_t(long, 0, (local->idle_cpus -
+						 busiest->idle_cpus) >> 1);
 		return;
 	}

 	/*
-	 * If there aren't any idle CPUs, avoid creating some.
+	 * Local is fully busy but has to take more load to relieve the
+	 * busiest group
 	 */
-	if (busiest->group_type == group_overloaded &&
-	    local->group_type   == group_overloaded) {
-		load_above_capacity = busiest->sum_h_nr_running * SCHED_CAPACITY_SCALE;
-		if (load_above_capacity > busiest->group_capacity) {
-			load_above_capacity -= busiest->group_capacity;
-			load_above_capacity *= scale_load_down(NICE_0_LOAD);
-			load_above_capacity /= busiest->group_capacity;
-		} else
-			load_above_capacity = ~0UL;
+	if (local->group_type < group_overloaded) {
+		/*
+		 * Local will become overloaded so the avg_load metrics are
+		 * finally needed.
+		 */
+
+		local->avg_load = (local->group_load * SCHED_CAPACITY_SCALE) /
+				  local->group_capacity;
+
+		sds->avg_load = (sds->total_load * SCHED_CAPACITY_SCALE) /
+				sds->total_capacity;
 	}

 	/*
-	 * We're trying to get all the CPUs to the average_load, so we don't
-	 * want to push ourselves above the average load, nor do we wish to
-	 * reduce the max loaded CPU below the average load. At the same time,
-	 * we also don't want to reduce the group load below the group
-	 * capacity. Thus we look for the minimum possible imbalance.
+	 * Both group are or will become overloaded and we're trying to get all
+	 * the CPUs to the average_load, so we don't want to push ourselves
+	 * above the average load, nor do we wish to reduce the max loaded CPU
+	 * below the average load. At the same time, we also don't want to
+	 * reduce the group load below the group capacity. Thus we look for
+	 * the minimum possible imbalance.
 	 */
-	max_pull = min(busiest->avg_load - sds->avg_load, load_above_capacity);
-
-	/* How much load to actually move to equalise the imbalance */
+	env->migration_type = migrate_load;
 	env->imbalance = min(
-		max_pull * busiest->group_capacity,
+		(busiest->avg_load - sds->avg_load) * busiest->group_capacity,
 		(sds->avg_load - local->avg_load) * local->group_capacity
 	) / SCHED_CAPACITY_SCALE;
-
-	/* Boost imbalance to allow misfit task to be balanced. */
-	if (busiest->group_type == group_misfit_task) {
-		env->imbalance = max_t(long, env->imbalance,
-				       busiest->group_misfit_task_load);
-	}
-
 }

 /******* find_busiest_group() helpers end here *********************/

+/*
+ * Decision matrix according to the local and busiest group type:
+ *
+ * busiest \ local has_spare fully_busy misfit asym imbalanced overloaded
+ * has_spare        nr_idle   balanced   N/A    N/A  balanced   balanced
+ * fully_busy       nr_idle   nr_idle    N/A    N/A  balanced   balanced
+ * misfit_task      force     N/A        N/A    N/A  force      force
+ * asym_packing     force     force      N/A    N/A  force      force
+ * imbalanced       force     force      N/A    N/A  force      force
+ * overloaded       force     force      N/A    N/A  force      avg_load
+ *
+ * N/A :      Not Applicable because already filtered while updating
+ *            statistics.
+ * balanced : The system is balanced for these 2 groups.
+ * force :    Calculate the imbalance as load migration is probably needed.
+ * avg_load : Only if imbalance is significant enough.
+ * nr_idle :  dst_cpu is not busy and the number of idle CPUs is quite
+ *            different in groups.
+ */
+
 /**
 * find_busiest_group - Returns the busiest group within the sched_domain
 * if there is an imbalance.
@ -8360,17 +8519,17 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
 	local = &sds.local_stat;
 	busiest = &sds.busiest_stat;

-	/* ASYM feature bypasses nice load balance check */
-	if (busiest->group_asym_packing)
-		goto force_balance;
-
 	/* There is no busy sibling group to pull tasks from */
-	if (!sds.busiest || busiest->sum_h_nr_running == 0)
+	if (!sds.busiest)
 		goto out_balanced;

-	/* XXX broken for overlapping NUMA groups */
-	sds.avg_load = (SCHED_CAPACITY_SCALE * sds.total_load)
-						/ sds.total_capacity;
+	/* Misfit tasks should be dealt with regardless of the avg load */
+	if (busiest->group_type == group_misfit_task)
+		goto force_balance;
+
+	/* ASYM feature bypasses nice load balance check */
+	if (busiest->group_type == group_asym_packing)
+		goto force_balance;

 	/*
 	 * If the busiest group is imbalanced the below checks don't
@ -8380,56 +8539,65 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
 	if (busiest->group_type == group_imbalanced)
 		goto force_balance;

-	/*
-	 * When dst_cpu is idle, prevent SMP nice and/or asymmetric group
-	 * capacities from resulting in underutilization due to avg_load.
-	 */
-	if (env->idle != CPU_NOT_IDLE && group_has_capacity(env, local) &&
-	    busiest->group_no_capacity)
-		goto force_balance;
-
-	/* Misfit tasks should be dealt with regardless of the avg load */
-	if (busiest->group_type == group_misfit_task)
-		goto force_balance;
-
 	/*
 	 * If the local group is busier than the selected busiest group
 	 * don't try and pull any tasks.
 	 */
-	if (local->avg_load >= busiest->avg_load)
+	if (local->group_type > busiest->group_type)
 		goto out_balanced;

 	/*
-	 * Don't pull any tasks if this group is already above the domain
-	 * average load.
+	 * When groups are overloaded, use the avg_load to ensure fairness
+	 * between tasks.
 	 */
-	if (local->avg_load >= sds.avg_load)
-		goto out_balanced;
-
-	if (env->idle == CPU_IDLE) {
+	if (local->group_type == group_overloaded) {
 		/*
-		 * This CPU is idle. If the busiest group is not overloaded
-		 * and there is no imbalance between this and busiest group
-		 * wrt idle CPUs, it is balanced. The imbalance becomes
-		 * significant if the diff is greater than 1 otherwise we
-		 * might end up to just move the imbalance on another group
+		 * If the local group is more loaded than the selected
+		 * busiest group don't try to pull any tasks.
 		 */
-		if ((busiest->group_type != group_overloaded) &&
-				(local->idle_cpus <= (busiest->idle_cpus + 1)))
+		if (local->avg_load >= busiest->avg_load)
 			goto out_balanced;
-	} else {
+
+		/* XXX broken for overlapping NUMA groups */
+		sds.avg_load = (sds.total_load * SCHED_CAPACITY_SCALE) /
+				sds.total_capacity;
+
 		/*
-		 * In the CPU_NEWLY_IDLE, CPU_NOT_IDLE cases, use
-		 * imbalance_pct to be conservative.
+		 * Don't pull any tasks if this group is already above the
+		 * domain average load.
+		 */
+		if (local->avg_load >= sds.avg_load)
+			goto out_balanced;
+
+		/*
+		 * If the busiest group is more loaded, use imbalance_pct to be
+		 * conservative.
 		 */
 		if (100 * busiest->avg_load <=
 				env->sd->imbalance_pct * local->avg_load)
 			goto out_balanced;
 	}

+	/* Try to move all excess tasks to child's sibling domain */
+	if (sds.prefer_sibling && local->group_type == group_has_spare &&
+	    busiest->sum_h_nr_running > local->sum_h_nr_running + 1)
+		goto force_balance;
+
+	if (busiest->group_type != group_overloaded &&
+	     (env->idle == CPU_NOT_IDLE ||
+	      local->idle_cpus <= (busiest->idle_cpus + 1)))
+		/*
+		 * If the busiest group is not overloaded
+		 * and there is no imbalance between this and busiest group
+		 * wrt. idle CPUs, it is balanced. The imbalance
+		 * becomes significant if the diff is greater than 1 otherwise
+		 * we might end up just moving the imbalance to another
+		 * group.
+		 */
+		goto out_balanced;
+
 force_balance:
 	/* Looks like there is an imbalance. Compute it */
-	env->src_grp_type = busiest->group_type;
 	calculate_imbalance(env, &sds);
 	return env->imbalance ? sds.busiest : NULL;

@ -8445,11 +8613,13 @@ static struct rq *find_busiest_queue(struct lb_env *env,
 				     struct sched_group *group)
 {
 	struct rq *busiest = NULL, *rq;
-	unsigned long busiest_load = 0, busiest_capacity = 1;
+	unsigned long busiest_util = 0, busiest_load = 0, busiest_capacity = 1;
+	unsigned int busiest_nr = 0;
 	int i;

 	for_each_cpu_and(i, sched_group_span(group), env->cpus) {
-		unsigned long capacity, load;
+		unsigned long capacity, load, util;
+		unsigned int nr_running;
 		enum fbq_type rt;

 		rq = cpu_rq(i);
@ -8477,20 +8647,8 @@ static struct rq *find_busiest_queue(struct lb_env *env,
 		if (rt > env->fbq_type)
 			continue;

-		/*
-		 * For ASYM_CPUCAPACITY domains with misfit tasks we simply
-		 * seek the "biggest" misfit task.
-		 */
-		if (env->src_grp_type == group_misfit_task) {
-			if (rq->misfit_task_load > busiest_load) {
-				busiest_load = rq->misfit_task_load;
-				busiest = rq;
-			}
-
-			continue;
-		}
-
 		capacity = capacity_of(i);
+		nr_running = rq->cfs.h_nr_running;

 		/*
 		 * For ASYM_CPUCAPACITY domains, don't pick a CPU that could
@ -8500,35 +8658,70 @@ static struct rq *find_busiest_queue(struct lb_env *env,
 		 */
 		if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
 		    capacity_of(env->dst_cpu) < capacity &&
-		    rq->nr_running == 1)
+		    nr_running == 1)
 			continue;

-		load = cpu_runnable_load(rq);
+		switch (env->migration_type) {
+		case migrate_load:
+			/*
+			 * When comparing with load imbalance, use
+			 * cpu_runnable_load() which is not scaled with the CPU
+			 * capacity.
+			 */
+			load = cpu_runnable_load(rq);

-		/*
-		 * When comparing with imbalance, use cpu_runnable_load()
-		 * which is not scaled with the CPU capacity.
-		 */
+			if (nr_running == 1 && load > env->imbalance &&
+			    !check_cpu_capacity(rq, env->sd))
+				break;

-		if (rq->nr_running == 1 && load > env->imbalance &&
-		    !check_cpu_capacity(rq, env->sd))
-			continue;
+			/*
+			 * For the load comparisons with the other CPUs,
+			 * consider the cpu_runnable_load() scaled with the CPU
+			 * capacity, so that the load can be moved away from
+			 * the CPU that is potentially running at a lower
+			 * capacity.
+			 *
+			 * Thus we're looking for max(load_i / capacity_i),
+			 * crosswise multiplication to rid ourselves of the
+			 * division works out to:
+			 * load_i * capacity_j > load_j * capacity_i;
+			 * where j is our previous maximum.
+			 */
+			if (load * busiest_capacity > busiest_load * capacity) {
+				busiest_load = load;
+				busiest_capacity = capacity;
+				busiest = rq;
+			}
+			break;
+
+		case migrate_util:
+			util = cpu_util(cpu_of(rq));
+
+			if (busiest_util < util) {
+				busiest_util = util;
+				busiest = rq;
+			}
+			break;
+
+		case migrate_task:
+			if (busiest_nr < nr_running) {
+				busiest_nr = nr_running;
+				busiest = rq;
+			}
+			break;
+
+		case migrate_misfit:
+			/*
+			 * For ASYM_CPUCAPACITY domains with misfit tasks we
+			 * simply seek the "biggest" misfit task.
+			 */
+			if (rq->misfit_task_load > busiest_load) {
+				busiest_load = rq->misfit_task_load;
+				busiest = rq;
+			}
+
+			break;

-		/*
-		 * For the load comparisons with the other CPU's, consider
-		 * the cpu_runnable_load() scaled with the CPU capacity, so
-		 * that the load can be moved away from the CPU that is
-		 * potentially running at a lower capacity.
-		 *
-		 * Thus we're looking for max(load_i / capacity_i), crosswise
-		 * multiplication to rid ourselves of the division works out
-		 * to: load_i * capacity_j > load_j * capacity_i;  where j is
-		 * our previous maximum.
-		 */
-		if (load * busiest_capacity > busiest_load * capacity) {
-			busiest_load = load;
-			busiest_capacity = capacity;
-			busiest = rq;
 		}
 	}

@ -8574,7 +8767,7 @@ voluntary_active_balance(struct lb_env *env)
 			return 1;
 	}

-	if (env->src_grp_type == group_misfit_task)
+	if (env->migration_type == migrate_misfit)
 		return 1;

 	return 0;