Various load-balance cleanups/optimizations

@@ -5018,7 +5018,7 @@ static struct rq *find_busiest_queue(str
 				     struct sched_group *group)
 {
 	struct rq *busiest = NULL, *rq;
-	unsigned long max_load = 0;
+	unsigned long busiest_load = 0, busiest_power = SCHED_POWER_SCALE;
 	int i;
 
 	for_each_cpu(i, sched_group_cpus(group)) {

@@ -5049,10 +5049,9 @@ static struct rq *find_busiest_queue(str
 		 * the load can be moved away from the cpu that is potentially
 		 * running at a lower capacity.
 		 */
-		wl = (wl * SCHED_POWER_SCALE) / power;
-
-		if (wl > max_load) {
-			max_load = wl;
+		if (wl * busiest_power > busiest_load * power) {
+			busiest_load = wl;
+			busiest_power = power;
 			busiest = rq;
 		}
 	}

@@ -4510,22 +4510,17 @@ fix_small_capacity(struct sched_domain *
  * @group: sched_group whose statistics are to be updated.
  * @load_idx: Load index of sched_domain of this_cpu for load calc.
  * @local_group: Does group contain this_cpu.
- * @balance: Should we balance.
  * @sgs: variable to hold the statistics for this group.
  */
 static inline void update_sg_lb_stats(struct lb_env *env,
 			struct sched_group *group, int load_idx,
-			int local_group, int *balance, struct sg_lb_stats *sgs)
+			int local_group, struct sg_lb_stats *sgs)
 {
 	unsigned long nr_running, max_nr_running, min_nr_running;
 	unsigned long load, max_cpu_load, min_cpu_load;
-	unsigned int balance_cpu = -1, first_idle_cpu = 0;
 	unsigned long avg_load_per_task = 0;
 	int i;
 
-	if (local_group)
-		balance_cpu = group_balance_cpu(group);
-
 	/* Tally up the load of all CPUs in the group */
 	max_cpu_load = 0;
 	min_cpu_load = ~0UL;

@@ -4538,15 +4533,9 @@ static inline void update_sg_lb_stats(st
 		nr_running = rq->nr_running;
 
 		/* Bias balancing toward cpus of our domain */
-		if (local_group) {
-			if (idle_cpu(i) && !first_idle_cpu &&
-					cpumask_test_cpu(i, sched_group_mask(group))) {
-				first_idle_cpu = 1;
-				balance_cpu = i;
-			}
-
+		if (local_group)
 			load = target_load(i, load_idx);
-		} else {
+		else {
 			load = source_load(i, load_idx);
 			if (load > max_cpu_load)
 				max_cpu_load = load;

@@ -4566,22 +4555,9 @@ static inline void update_sg_lb_stats(st
 			sgs->idle_cpus++;
 	}
 
-	/*
-	 * First idle cpu or the first cpu(busiest) in this sched group
-	 * is eligible for doing load balancing at this and above
-	 * domains. In the newly idle case, we will allow all the cpu's
-	 * to do the newly idle load balance.
-	 */
-	if (local_group) {
-		if (env->idle != CPU_NEWLY_IDLE) {
-			if (balance_cpu != env->dst_cpu) {
-				*balance = 0;
-				return;
-			}
-			update_group_power(env->sd, env->dst_cpu);
-		} else if (time_after_eq(jiffies, group->sgp->next_update))
-			update_group_power(env->sd, env->dst_cpu);
-	}
+	if (local_group && (env->idle != CPU_NEWLY_IDLE ||
+			time_after_eq(jiffies, group->sgp->next_update)))
+		update_group_power(env->sd, env->dst_cpu);
 
 	/* Adjust by relative CPU power of the group */
 	sgs->avg_load = (sgs->group_load*SCHED_POWER_SCALE) / group->sgp->power;

@@ -4663,7 +4639,7 @@ static bool update_sd_pick_busiest(struc
  * @sds: variable to hold the statistics for this sched_domain.
  */
 static inline void update_sd_lb_stats(struct lb_env *env,
-					int *balance, struct sd_lb_stats *sds)
+					struct sd_lb_stats *sds)
 {
 	struct sched_domain *child = env->sd->child;
 	struct sched_group *sg = env->sd->groups;

@@ -4680,10 +4656,7 @@ static inline void update_sd_lb_stats(st
 
 		local_group = cpumask_test_cpu(env->dst_cpu, sched_group_cpus(sg));
 		memset(&sgs, 0, sizeof(sgs));
-		update_sg_lb_stats(env, sg, load_idx, local_group, balance, &sgs);
-
-		if (local_group && !(*balance))
-			return;
+		update_sg_lb_stats(env, sg, load_idx, local_group, &sgs);
 
 		sds->total_load += sgs.group_load;
 		sds->total_pwr += sg->sgp->power;

@@ -4916,8 +4889,6 @@ static inline void calculate_imbalance(s
  * to restore balance.
  *
  * @env: The load balancing environment.
- * @balance: Pointer to a variable indicating if this_cpu
- *	is the appropriate cpu to perform load balancing at this_level.
  *
  * Return:	- The busiest group if imbalance exists.
  *		- If no imbalance and user has opted for power-savings balance,

@@ -4925,7 +4896,7 @@ static inline void calculate_imbalance(s
  *		   put to idle by rebalancing its tasks onto our group.
  */
 static struct sched_group *
-find_busiest_group(struct lb_env *env, int *balance)
+find_busiest_group(struct lb_env *env)
 {
 	struct sd_lb_stats sds;
 

@@ -4935,14 +4906,7 @@ find_busiest_group(struct lb_env *env, i
 	 * Compute the various statistics relavent for load balancing at
 	 * this level.
 	 */
-	update_sd_lb_stats(env, balance, &sds);
-
-	/*
-	 * this_cpu is not the appropriate cpu to perform load balancing at
-	 * this level.
-	 */
-	if (!(*balance))
-		goto ret;
+	update_sd_lb_stats(env, &sds);
 
 	if ((env->idle == CPU_IDLE || env->idle == CPU_NEWLY_IDLE) &&
 	    check_asym_packing(env, &sds))

@@ -5006,7 +4970,6 @@ find_busiest_group(struct lb_env *env, i
 	return sds.busiest;
 
 out_balanced:
-ret:
 	env->imbalance = 0;
 	return NULL;
 }

@@ -5088,13 +5051,47 @@ static int need_active_balance(struct lb
 
 static int active_load_balance_cpu_stop(void *data);
 
+static int should_we_balance(struct lb_env *env)
+{
+	struct sched_group *sg = env->sd->groups;
+	struct cpumask *sg_cpus, *sg_mask;
+	int cpu, balance_cpu = -1;
+
+	/*
+	 * In the newly idle case, we will allow all the cpu's
+	 * to do the newly idle load balance.
+	 */
+	if (env->idle == CPU_NEWLY_IDLE)
+		return 1;
+
+	sg_cpus = sched_group_cpus(sg);
+	sg_mask = sched_group_mask(sg);
+	/* Try to find first idle cpu */
+	for_each_cpu_and(cpu, sg_cpus, env->cpus) {
+		if (!cpumask_test_cpu(cpu, sg_mask) || !idle_cpu(cpu))
+			continue;
+
+		balance_cpu = cpu;
+		break;
+	}
+
+	if (balance_cpu == -1)
+		balance_cpu = group_balance_cpu(sg);
+
+	/*
+	 * First idle cpu or the first cpu(busiest) in this sched group
+	 * is eligible for doing load balancing at this and above domains.
+	 */
+	return balance_cpu != env->dst_cpu;
+}
+
 /*
  * Check this_cpu to ensure it is balanced within domain. Attempt to move
  * tasks if there is an imbalance.
  */
 static int load_balance(int this_cpu, struct rq *this_rq,
 			struct sched_domain *sd, enum cpu_idle_type idle,
-			int *balance)
+			int *should_balance)
 {
 	int ld_moved, cur_ld_moved, active_balance = 0;
 	struct sched_group *group;

@@ -5123,12 +5120,11 @@ static int load_balance(int this_cpu, st
 
 	schedstat_inc(sd, lb_count[idle]);
 
-redo:
-	group = find_busiest_group(&env, balance);
-
-	if (*balance == 0)
+	if (!(*should_balance = should_we_balance(&env)))
 		goto out_balanced;
 
+redo:
+	group = find_busiest_group(&env);
 	if (!group) {
 		schedstat_inc(sd, lb_nobusyg[idle]);
 		goto out_balanced;

@@ -5340,7 +5336,7 @@ void idle_balance(int this_cpu, struct r
 	rcu_read_lock();
 	for_each_domain(this_cpu, sd) {
 		unsigned long interval;
-		int balance = 1;
+		int should_balance;
 
 		if (!(sd->flags & SD_LOAD_BALANCE))
 			continue;

@@ -5348,7 +5344,8 @@ void idle_balance(int this_cpu, struct r
 		if (sd->flags & SD_BALANCE_NEWIDLE) {
 			/* If we've pulled tasks over stop searching: */
 			pulled_task = load_balance(this_cpu, this_rq,
-						   sd, CPU_NEWLY_IDLE, &balance);
+						   sd, CPU_NEWLY_IDLE,
+						   &should_balance);
 		}
 
 		interval = msecs_to_jiffies(sd->balance_interval);

@@ -5586,7 +5583,7 @@ void update_max_interval(void)
  */
 static void rebalance_domains(int cpu, enum cpu_idle_type idle)
 {
-	int balance = 1;
+	int should_balance = 1;
 	struct rq *rq = cpu_rq(cpu);
 	unsigned long interval;
 	struct sched_domain *sd;

@@ -5618,7 +5615,7 @@ static void rebalance_domains(int cpu, e
 		}
 
 		if (time_after_eq(jiffies, sd->last_balance + interval)) {
-			if (load_balance(cpu, rq, sd, idle, &balance)) {
+			if (load_balance(cpu, rq, sd, idle, &should_balance)) {
 				/*
 				 * The LBF_SOME_PINNED logic could have changed
 				 * env->dst_cpu, so we can't know our idle

@@ -5641,7 +5638,7 @@ static void rebalance_domains(int cpu, e
 		 * CPU in our sched group which is doing load balancing more
 		 * actively.
 		 */
-		if (!balance)
+		if (!should_balance)
 			break;
 	}
 	rcu_read_unlock();

@@ -4277,36 +4277,6 @@ static unsigned long task_h_load(struct
 
 /********** Helpers for find_busiest_group ************************/
 /*
- * sd_lb_stats - Structure to store the statistics of a sched_domain
- * 		during load balancing.
- */
-struct sd_lb_stats {
-	struct sched_group *busiest; /* Busiest group in this sd */
-	struct sched_group *this;  /* Local group in this sd */
-	unsigned long total_load;  /* Total load of all groups in sd */
-	unsigned long total_pwr;   /*	Total power of all groups in sd */
-	unsigned long avg_load;	   /* Average load across all groups in sd */
-
-	/** Statistics of this group */
-	unsigned long this_load;
-	unsigned long this_load_per_task;
-	unsigned long this_nr_running;
-	unsigned long this_has_capacity;
-	unsigned int  this_idle_cpus;
-
-	/* Statistics of the busiest group */
-	unsigned int  busiest_idle_cpus;
-	unsigned long max_load;
-	unsigned long busiest_load_per_task;
-	unsigned long busiest_nr_running;
-	unsigned long busiest_group_capacity;
-	unsigned long busiest_has_capacity;
-	unsigned int  busiest_group_weight;
-
-	int group_imb; /* Is there imbalance in this sd */
-};
-
-/*
  * sg_lb_stats - stats of a sched_group required for load_balancing
  */
 struct sg_lb_stats {

@@ -4314,6 +4284,7 @@ struct sg_lb_stats {
 	unsigned long group_load; /* Total load over the CPUs of the group */
 	unsigned long sum_nr_running; /* Nr tasks running in the group */
 	unsigned long sum_weighted_load; /* Weighted load of group's tasks */
+	unsigned long load_per_task;
 	unsigned long group_capacity;
 	unsigned long idle_cpus;
 	unsigned long group_weight;

@@ -4321,6 +4292,21 @@ struct sg_lb_stats {
 	int group_has_capacity; /* Is there extra capacity in the group? */
 };
 
+/*
+ * sd_lb_stats - Structure to store the statistics of a sched_domain
+ *		 during load balancing.
+ */
+struct sd_lb_stats {
+	struct sched_group *busiest;	/* Busiest group in this sd */
+	struct sched_group *this;	/* Local group in this sd */
+	unsigned long total_load;	/* Total load of all groups in sd */
+	unsigned long total_pwr;	/* Total power of all groups in sd */
+	unsigned long avg_load;	/* Average load across all groups in sd */
+
+	struct sg_lb_stats this_stat;	/* Statistics of this group */
+	struct sg_lb_stats busiest_stat;/* Statistics of the busiest group */
+};
+
 /**
  * get_sd_load_idx - Obtain the load index for a given sched domain.
  * @sd: The sched_domain whose load_idx is to be obtained.

@@ -4533,10 +4519,11 @@ static inline void update_sg_lb_stats(st
 		nr_running = rq->nr_running;
 
 		/* Bias balancing toward cpus of our domain */
-		if (local_group)
+		if (local_group) {
 			load = target_load(i, load_idx);
-		else {
+		} else {
 			load = source_load(i, load_idx);
+
 			if (load > max_cpu_load)
 				max_cpu_load = load;
 			if (min_cpu_load > load)

@@ -4578,10 +4565,12 @@ static inline void update_sg_lb_stats(st
 	    (max_nr_running - min_nr_running) > 1)
 		sgs->group_imb = 1;
 
-	sgs->group_capacity = DIV_ROUND_CLOSEST(group->sgp->power,
-						SCHED_POWER_SCALE);
+	sgs->group_capacity =
+		DIV_ROUND_CLOSEST(group->sgp->power, SCHED_POWER_SCALE);
+
 	if (!sgs->group_capacity)
 		sgs->group_capacity = fix_small_capacity(env->sd, group);
+
 	sgs->group_weight = group->group_weight;
 
 	if (sgs->group_capacity > sgs->sum_nr_running)

@@ -4606,7 +4595,7 @@ static bool update_sd_pick_busiest(struc
 				   struct sched_group *sg,
 				   struct sg_lb_stats *sgs)
 {
-	if (sgs->avg_load <= sds->max_load)
+	if (sgs->avg_load <= sds->busiest_stat.avg_load)
 		return false;
 
 	if (sgs->sum_nr_running > sgs->group_capacity)

@@ -4643,7 +4632,7 @@ static inline void update_sd_lb_stats(st
 {
 	struct sched_domain *child = env->sd->child;
 	struct sched_group *sg = env->sd->groups;
-	struct sg_lb_stats sgs;
+	struct sg_lb_stats tmp_sgs;
 	int load_idx, prefer_sibling = 0;
 
 	if (child && child->flags & SD_PREFER_SIBLING)

@@ -4652,14 +4641,17 @@ static inline void update_sd_lb_stats(st
 	load_idx = get_sd_load_idx(env->sd, env->idle);
 
 	do {
+		struct sg_lb_stats *sgs = &tmp_sgs;
 		int local_group;
 
 		local_group = cpumask_test_cpu(env->dst_cpu, sched_group_cpus(sg));
-		memset(&sgs, 0, sizeof(sgs));
-		update_sg_lb_stats(env, sg, load_idx, local_group, &sgs);
+		if (local_group) {
+			sds->this = sg;
+			sgs = &sds->this_stat;
+		}
 
-		sds->total_load += sgs.group_load;
-		sds->total_pwr += sg->sgp->power;
+		memset(sgs, 0, sizeof(*sgs));
+		update_sg_lb_stats(env, sg, load_idx, local_group, sgs);
 
 		/*
 		 * In case the child domain prefers tasks go to siblings

@@ -4671,26 +4663,17 @@ static inline void update_sd_lb_stats(st
 		 * heaviest group when it is already under-utilized (possible
 		 * with a large weight task outweighs the tasks on the system).
 		 */
-		if (prefer_sibling && !local_group && sds->this_has_capacity)
-			sgs.group_capacity = min(sgs.group_capacity, 1UL);
+		if (prefer_sibling && !local_group &&
+				sds->this && sds->this_stat.group_has_capacity)
+			sgs->group_capacity = min(sgs->group_capacity, 1UL);
 
-		if (local_group) {
-			sds->this_load = sgs.avg_load;
-			sds->this = sg;
-			sds->this_nr_running = sgs.sum_nr_running;
-			sds->this_load_per_task = sgs.sum_weighted_load;
-			sds->this_has_capacity = sgs.group_has_capacity;
-			sds->this_idle_cpus = sgs.idle_cpus;
-		} else if (update_sd_pick_busiest(env, sds, sg, &sgs)) {
-			sds->max_load = sgs.avg_load;
+		/* Now, start updating sd_lb_stats */
+		sds->total_load += sgs->group_load;
+		sds->total_pwr += sg->sgp->power;
+
+		if (!local_group && update_sd_pick_busiest(env, sds, sg, sgs)) {
 			sds->busiest = sg;
-			sds->busiest_nr_running = sgs.sum_nr_running;
-			sds->busiest_idle_cpus = sgs.idle_cpus;
-			sds->busiest_group_capacity = sgs.group_capacity;
-			sds->busiest_load_per_task = sgs.sum_weighted_load;
-			sds->busiest_has_capacity = sgs.group_has_capacity;
-			sds->busiest_group_weight = sgs.group_weight;
-			sds->group_imb = sgs.group_imb;
+			sds->busiest_stat = *sgs;
 		}
 
 		sg = sg->next;

@@ -4734,8 +4717,8 @@ static int check_asym_packing(struct lb_
 	if (env->dst_cpu > busiest_cpu)
 		return 0;
 
-	env->imbalance = DIV_ROUND_CLOSEST(
-		sds->max_load * sds->busiest->sgp->power, SCHED_POWER_SCALE);
+	env->imbalance = DIV_ROUND_CLOSEST(sds->busiest_stat.avg_load *
+				sds->busiest->sgp->power, SCHED_POWER_SCALE);
 
 	return 1;
 }

@@ -4753,24 +4736,23 @@ void fix_small_imbalance(struct lb_env *
 	unsigned long tmp, pwr_now = 0, pwr_move = 0;
 	unsigned int imbn = 2;
 	unsigned long scaled_busy_load_per_task;
+	struct sg_lb_stats *this, *busiest;
 
-	if (sds->this_nr_running) {
-		sds->this_load_per_task /= sds->this_nr_running;
-		if (sds->busiest_load_per_task >
-				sds->this_load_per_task)
-			imbn = 1;
-	} else {
-		sds->this_load_per_task =
-			cpu_avg_load_per_task(env->dst_cpu);
-	}
+	this = &sds->this_stat;
+	busiest = &sds->busiest_stat;
+
+	if (!this->sum_nr_running)
+		this->load_per_task = cpu_avg_load_per_task(env->dst_cpu);
+	else if (busiest->load_per_task > this->load_per_task)
+		imbn = 1;
 
-	scaled_busy_load_per_task = sds->busiest_load_per_task
-					 * SCHED_POWER_SCALE;
-	scaled_busy_load_per_task /= sds->busiest->sgp->power;
+	scaled_busy_load_per_task =
+		(busiest->load_per_task * SCHED_POWER_SCALE) /
+		sds->busiest->sgp->power;
 
-	if (sds->max_load - sds->this_load + scaled_busy_load_per_task >=
-			(scaled_busy_load_per_task * imbn)) {
-		env->imbalance = sds->busiest_load_per_task;
+	if (busiest->avg_load - this->avg_load + scaled_busy_load_per_task >=
+	    (scaled_busy_load_per_task * imbn)) {
+		env->imbalance = busiest->load_per_task;
 		return;
 	}
 

@@ -4781,33 +4763,36 @@ void fix_small_imbalance(struct lb_env *
 	 */
 
 	pwr_now += sds->busiest->sgp->power *
-			min(sds->busiest_load_per_task, sds->max_load);
+			min(busiest->load_per_task, busiest->avg_load);
 	pwr_now += sds->this->sgp->power *
-			min(sds->this_load_per_task, sds->this_load);
+			min(this->load_per_task, this->avg_load);
 	pwr_now /= SCHED_POWER_SCALE;
 
 	/* Amount of load we'd subtract */
-	tmp = (sds->busiest_load_per_task * SCHED_POWER_SCALE) /
+	tmp = (busiest->load_per_task * SCHED_POWER_SCALE) /
 		sds->busiest->sgp->power;
-	if (sds->max_load > tmp)
+	if (busiest->avg_load > tmp) {
 		pwr_move += sds->busiest->sgp->power *
-			min(sds->busiest_load_per_task, sds->max_load - tmp);
+			    min(busiest->load_per_task,
+				busiest->avg_load - tmp);
+	}
 
 	/* Amount of load we'd add */
-	if (sds->max_load * sds->busiest->sgp->power <
-		sds->busiest_load_per_task * SCHED_POWER_SCALE)
-		tmp = (sds->max_load * sds->busiest->sgp->power) /
+	if (busiest->avg_load * sds->busiest->sgp->power <
+	    busiest->load_per_task * SCHED_POWER_SCALE) {
+		tmp = (busiest->avg_load * sds->busiest->sgp->power) /
 			sds->this->sgp->power;
-	else
-		tmp = (sds->busiest_load_per_task * SCHED_POWER_SCALE) /
+	} else {
+		tmp = (busiest->load_per_task * SCHED_POWER_SCALE) /
 			sds->this->sgp->power;
+	}
 	pwr_move += sds->this->sgp->power *
-			min(sds->this_load_per_task, sds->this_load + tmp);
+			min(this->load_per_task, this->avg_load + tmp);
 	pwr_move /= SCHED_POWER_SCALE;
 
 	/* Move if we gain throughput */
 	if (pwr_move > pwr_now)
-		env->imbalance = sds->busiest_load_per_task;
+		env->imbalance = busiest->load_per_task;
 }
 
 /**

@@ -4819,11 +4804,22 @@ void fix_small_imbalance(struct lb_env *
 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 *this, *busiest;
 
-	sds->busiest_load_per_task /= sds->busiest_nr_running;
-	if (sds->group_imb) {
-		sds->busiest_load_per_task =
-			min(sds->busiest_load_per_task, sds->avg_load);
+	this = &sds->this_stat;
+	if (this->sum_nr_running) {
+		this->load_per_task =
+			this->sum_weighted_load / this->sum_nr_running;
+	}
+
+	busiest = &sds->busiest_stat;
+	/* busiest must have some tasks */
+	busiest->load_per_task =
+		busiest->sum_weighted_load / busiest->sum_nr_running;
+
+	if (busiest->group_imb) {
+		busiest->load_per_task =
+			min(busiest->load_per_task, sds->avg_load);
 	}
 
 	/*

@@ -4831,20 +4827,19 @@ static inline void calculate_imbalance(s
 	 * max load less than avg load(as we skip the groups at or below
 	 * its cpu_power, while calculating max_load..)
 	 */
-	if (sds->max_load < sds->avg_load) {
+	if (busiest->avg_load < sds->avg_load) {
 		env->imbalance = 0;
 		return fix_small_imbalance(env, sds);
 	}
 
-	if (!sds->group_imb) {
+	if (!busiest->group_imb) {
 		/*
 		 * Don't want to pull so many tasks that a group would go idle.
 		 */
-		load_above_capacity = (sds->busiest_nr_running -
-						sds->busiest_group_capacity);
+		load_above_capacity =
+			(busiest->sum_nr_running - busiest->group_capacity);
 
 		load_above_capacity *= (SCHED_LOAD_SCALE * SCHED_POWER_SCALE);
-
 		load_above_capacity /= sds->busiest->sgp->power;
 	}
 

@@ -4858,12 +4853,14 @@ static inline void calculate_imbalance(s
 	 * Be careful of negative numbers as they'll appear as very large values
 	 * with unsigned longs.
 	 */
-	max_pull = min(sds->max_load - sds->avg_load, load_above_capacity);
+	max_pull = min(busiest->avg_load - sds->avg_load,
+		       load_above_capacity);
 
 	/* How much load to actually move to equalise the imbalance */
-	env->imbalance = min(max_pull * sds->busiest->sgp->power,
-		(sds->avg_load - sds->this_load) * sds->this->sgp->power)
-			/ SCHED_POWER_SCALE;
+	env->imbalance = min(
+		max_pull * sds->busiest->sgp->power,
+		(sds->avg_load - this->avg_load) * sds->this->sgp->power
+	) / SCHED_POWER_SCALE;
 
 	/*
 	 * if *imbalance is less than the average load per runnable task

@@ -4871,9 +4868,8 @@ static inline void calculate_imbalance(s
 	 * a think about bumping its value to force at least one task to be
 	 * moved
 	 */
-	if (env->imbalance < sds->busiest_load_per_task)
+	if (env->imbalance < busiest->load_per_task)
 		return fix_small_imbalance(env, sds);
-
 }
 
 /******* find_busiest_group() helpers end here *********************/

@@ -4895,54 +4891,56 @@ static inline void calculate_imbalance(s
  *		   return the least loaded group whose CPUs can be
  *		   put to idle by rebalancing its tasks onto our group.
  */
-static struct sched_group *
-find_busiest_group(struct lb_env *env)
+static struct sched_group *find_busiest_group(struct lb_env *env)
 {
+	struct sg_lb_stats *this, *busiest;
 	struct sd_lb_stats sds;
 
-	memset(&sds, 0, sizeof(sds));
+	memset(&sds, 0, sizeof(sds) - 2*sizeof(struct sg_lb_stats));
 
 	/*
 	 * Compute the various statistics relavent for load balancing at
 	 * this level.
 	 */
 	update_sd_lb_stats(env, &sds);
+	this = &sds.this_stat;
+	busiest = &sds.busiest_stat;
 
 	if ((env->idle == CPU_IDLE || env->idle == CPU_NEWLY_IDLE) &&
 	    check_asym_packing(env, &sds))
 		return sds.busiest;
 
 	/* There is no busy sibling group to pull tasks from */
-	if (!sds.busiest || sds.busiest_nr_running == 0)
+	if (!sds.busiest || busiest->sum_nr_running == 0)
 		goto out_balanced;
 
-	sds.avg_load = (SCHED_POWER_SCALE * sds.total_load) / sds.total_pwr;
+	sds.avg_load = (SCHED_POWER_SCALE * sds.total_load) / sds.total_pwr;
 
 	/*
 	 * If the busiest group is imbalanced the below checks don't
 	 * work because they assumes all things are equal, which typically
 	 * isn't true due to cpus_allowed constraints and the like.
 	 */
-	if (sds.group_imb)
+	if (busiest->group_imb)
 		goto force_balance;
 
 	/* SD_BALANCE_NEWIDLE trumps SMP nice when underutilized */
-	if (env->idle == CPU_NEWLY_IDLE && sds.this_has_capacity &&
-			!sds.busiest_has_capacity)
+	if (env->idle == CPU_NEWLY_IDLE && this->group_has_capacity &&
+	    !busiest->group_has_capacity)
 		goto force_balance;
 
 	/*
 	 * If the local group is more busy than the selected busiest group
 	 * don't try and pull any tasks.
 	 */
-	if (sds.this_load >= sds.max_load)
+	if (this->avg_load >= busiest->avg_load)
 		goto out_balanced;
 
 	/*
 	 * Don't pull any tasks if this group is already above the domain
 	 * average load.
 	 */
-	if (sds.this_load >= sds.avg_load)
+	if (this->avg_load >= sds.avg_load)
 		goto out_balanced;
 
 	if (env->idle == CPU_IDLE) {

@@ -4952,15 +4950,16 @@ find_busiest_group(struct lb_env *env)
 		 * there is no imbalance between this and busiest group
 		 * wrt to idle cpu's, it is balanced.
 		 */
-		if ((sds.this_idle_cpus <= sds.busiest_idle_cpus + 1) &&
-		    sds.busiest_nr_running <= sds.busiest_group_weight)
+		if ((this->idle_cpus <= busiest->idle_cpus + 1) &&
+		    busiest->sum_nr_running <= busiest->group_weight)
 			goto out_balanced;
 	} else {
 		/*
 		 * In the CPU_NEWLY_IDLE, CPU_NOT_IDLE cases, use
 		 * imbalance_pct to be conservative.
 		 */
-		if (100 * sds.max_load <= env->sd->imbalance_pct * sds.this_load)
+		if (100 * busiest->avg_load <=
+				env->sd->imbalance_pct * this->avg_load)
 			goto out_balanced;
 	}
 

@@ -4504,7 +4504,6 @@ static inline void update_sg_lb_stats(st
 {
 	unsigned long nr_running, max_nr_running, min_nr_running;
 	unsigned long load, max_cpu_load, min_cpu_load;
-	unsigned long avg_load_per_task = 0;
 	int i;
 
 	/* Tally up the load of all CPUs in the group */

@@ -4559,9 +4558,9 @@ static inline void update_sg_lb_stats(st
 	 *      the hierarchy?
 	 */
 	if (sgs->sum_nr_running)
-		avg_load_per_task = sgs->sum_weighted_load / sgs->sum_nr_running;
+		sgs->load_per_task = sgs->sum_weighted_load / sgs->sum_nr_running;
 
-	if ((max_cpu_load - min_cpu_load) >= avg_load_per_task &&
+	if ((max_cpu_load - min_cpu_load) >= sgs->load_per_task &&
 	    (max_nr_running - min_nr_running) > 1)
 		sgs->group_imb = 1;
 

@@ -4807,15 +4806,7 @@ static inline void calculate_imbalance(s
 	struct sg_lb_stats *this, *busiest;
 
 	this = &sds->this_stat;
-	if (this->sum_nr_running) {
-		this->load_per_task =
-			this->sum_weighted_load / this->sum_nr_running;
-	}
-
 	busiest = &sds->busiest_stat;
-	/* busiest must have some tasks */
-	busiest->load_per_task =
-		busiest->sum_weighted_load / busiest->sum_nr_running;
 
 	if (busiest->group_imb) {
 		busiest->load_per_task =

@@ -4288,6 +4288,7 @@ struct sg_lb_stats {
 	unsigned long group_capacity;
 	unsigned long idle_cpus;
 	unsigned long group_weight;
+	unsigned long group_power;
 	int group_imb; /* Is there an imbalance in the group ? */
 	int group_has_capacity; /* Is there extra capacity in the group? */
 };

@@ -4546,7 +4547,8 @@ static inline void update_sg_lb_stats(st
 		update_group_power(env->sd, env->dst_cpu);
 
 	/* Adjust by relative CPU power of the group */
-	sgs->avg_load = (sgs->group_load*SCHED_POWER_SCALE) / group->sgp->power;
+	sgs->group_power = group->sgp->power;
+	sgs->avg_load = (sgs->group_load*SCHED_POWER_SCALE) / sgs->group_power;
 
 	/*
 	 * Consider the group unbalanced when the imbalance is larger

@@ -4565,7 +4567,7 @@ static inline void update_sg_lb_stats(st
 		sgs->group_imb = 1;
 
 	sgs->group_capacity =
-		DIV_ROUND_CLOSEST(group->sgp->power, SCHED_POWER_SCALE);
+		DIV_ROUND_CLOSEST(sgs->group_power, SCHED_POWER_SCALE);
 
 	if (!sgs->group_capacity)
 		sgs->group_capacity = fix_small_capacity(env->sd, group);

@@ -4668,7 +4670,7 @@ static inline void update_sd_lb_stats(st
 
 		/* Now, start updating sd_lb_stats */
 		sds->total_load += sgs->group_load;
-		sds->total_pwr += sg->sgp->power;
+		sds->total_pwr += sgs->group_power;
 
 		if (!local_group && update_sd_pick_busiest(env, sds, sg, sgs)) {
 			sds->busiest = sg;

@@ -4716,8 +4718,9 @@ static int check_asym_packing(struct lb_
 	if (env->dst_cpu > busiest_cpu)
 		return 0;
 
-	env->imbalance = DIV_ROUND_CLOSEST(sds->busiest_stat.avg_load *
-				sds->busiest->sgp->power, SCHED_POWER_SCALE);
+	env->imbalance = DIV_ROUND_CLOSEST(
+		sds->busiest_stat.avg_load * sds->busiest_stat.group_power,
+		SCHED_POWER_SCALE);
 
 	return 1;
 }

@@ -4747,7 +4750,7 @@ void fix_small_imbalance(struct lb_env *
 
 	scaled_busy_load_per_task =
 		(busiest->load_per_task * SCHED_POWER_SCALE) /
-		sds->busiest->sgp->power;
+		busiest->group_power;
 
 	if (busiest->avg_load - this->avg_load + scaled_busy_load_per_task >=
 	    (scaled_busy_load_per_task * imbn)) {

@@ -4761,32 +4764,32 @@ void fix_small_imbalance(struct lb_env *
 	 * moving them.
 	 */
 
-	pwr_now += sds->busiest->sgp->power *
+	pwr_now += busiest->group_power *
 			min(busiest->load_per_task, busiest->avg_load);
-	pwr_now += sds->this->sgp->power *
+	pwr_now += this->group_power *
 			min(this->load_per_task, this->avg_load);
 	pwr_now /= SCHED_POWER_SCALE;
 
 	/* Amount of load we'd subtract */
 	tmp = (busiest->load_per_task * SCHED_POWER_SCALE) /
-		sds->busiest->sgp->power;
+		busiest->group_power;
 	if (busiest->avg_load > tmp) {
-		pwr_move += sds->busiest->sgp->power *
+		pwr_move += busiest->group_power *
 			    min(busiest->load_per_task,
 				busiest->avg_load - tmp);
 	}
 
 	/* Amount of load we'd add */
-	if (busiest->avg_load * sds->busiest->sgp->power <
+	if (busiest->avg_load * busiest->group_power <
 	    busiest->load_per_task * SCHED_POWER_SCALE) {
-		tmp = (busiest->avg_load * sds->busiest->sgp->power) /
-			sds->this->sgp->power;
+		tmp = (busiest->avg_load * busiest->group_power) /
+		      this->group_power;
 	} else {
 		tmp = (busiest->load_per_task * SCHED_POWER_SCALE) /
-			sds->this->sgp->power;
+		      this->group_power;
 	}
-	pwr_move += sds->this->sgp->power *
-			min(this->load_per_task, this->avg_load + tmp);
+	pwr_move += this->group_power *
+		    min(this->load_per_task, this->avg_load + tmp);
 	pwr_move /= SCHED_POWER_SCALE;
 
 	/* Move if we gain throughput */

@@ -4831,7 +4834,7 @@ static inline void calculate_imbalance(s
 			(busiest->sum_nr_running - busiest->group_capacity);
 
 		load_above_capacity *= (SCHED_LOAD_SCALE * SCHED_POWER_SCALE);
-		load_above_capacity /= sds->busiest->sgp->power;
+		load_above_capacity /= busiest->group_power;
 	}
 
 	/*

@@ -4849,8 +4852,8 @@ static inline void calculate_imbalance(s
 
 	/* How much load to actually move to equalise the imbalance */
 	env->imbalance = min(
-		max_pull * sds->busiest->sgp->power,
-		(sds->avg_load - this->avg_load) * sds->this->sgp->power
+		max_pull * busiest->group_power,
+		(sds->avg_load - this->avg_load) * this->group_power
 	) / SCHED_POWER_SCALE;
 
 	/*

@@ -4491,6 +4491,81 @@ fix_small_capacity(struct sched_domain *
 	return 0;
 }
 
+/*
+ * Group imbalance indicates (and tries to solve) the problem where balancing
+ * groups is inadequate due to tsk_cpus_allowed() constraints.
+ *
+ * Imagine a situation of two groups of 4 cpus each and 4 tasks each with a
+ * cpumask covering 1 cpu of the first group and 3 cpus of the second group.
+ * Something like:
+ *
+ * 	{ 0 1 2 3 } { 4 5 6 7 }
+ * 	        *     * * *
+ *
+ * If we were to balance group-wise we'd place two tasks in the first group and
+ * two tasks in the second group. Clearly this is undesired as it will overload
+ * cpu 3 and leave one of the cpus in the second group unused.
+ *
+ * The current solution to this issue is detecting the skew in the first group
+ * by noticing it has a cpu that is overloaded while the remaining cpus are
+ * idle -- or rather, there's a distinct imbalance in the cpus; see
+ * sg_imbalanced().
+ *
+ * When this is so detected; this group becomes a candidate for busiest; see
+ * update_sd_pick_busiest(). And calculcate_imbalance() and
+ * find_busiest_group() avoid some of the usual balance conditional to allow it
+ * to create an effective group imbalance.
+ *
+ * This is a somewhat tricky proposition since the next run might not find the
+ * group imbalance and decide the groups need to be balanced again. A most
+ * subtle and fragile situation.
+ */
+
+struct sg_imb_stats {
+	unsigned long max_nr_running, min_nr_running;
+	unsigned long max_cpu_load, min_cpu_load;
+};
+
+static inline void init_sg_imb_stats(struct sg_imb_stats *sgi)
+{
+	sgi->max_cpu_load = sgi->max_nr_running = 0UL;
+	sgi->min_cpu_load = sgi->min_nr_running = ~0UL;
+}
+
+static inline void
+update_sg_imb_stats(struct sg_imb_stats *sgi,
+		    unsigned long load, unsigned long nr_running)
+{
+	if (load > sgi->max_cpu_load)
+		sgi->max_cpu_load = load;
+	if (sgi->min_cpu_load > load)
+		sgi->min_cpu_load = load;
+
+	if (nr_running > sgi->max_nr_running)
+		sgi->max_nr_running = nr_running;
+	if (sgi->min_nr_running > nr_running)
+		sgi->min_nr_running = nr_running;
+}
+
+static inline int
+sg_imbalanced(struct sg_lb_stats *sgs, struct sg_imb_stats *sgi)
+{
+	/*
+	 * Consider the group unbalanced when the imbalance is larger
+	 * than the average weight of a task.
+	 *
+	 * APZ: with cgroup the avg task weight can vary wildly and
+	 *      might not be a suitable number - should we keep a
+	 *      normalized nr_running number somewhere that negates
+	 *      the hierarchy?
+	 */
+	if ((sgi->max_cpu_load - sgi->min_cpu_load) >= sgs->load_per_task &&
+	    (sgi->max_nr_running - sgi->min_nr_running) > 1)
+		return 1;
+
+	return 0;
+}
+
 /**
  * update_sg_lb_stats - Update sched_group's statistics for load balancing.
  * @env: The load balancing environment.

@@ -4503,15 +4578,12 @@ static inline void update_sg_lb_stats(st
 			struct sched_group *group, int load_idx,
 			int local_group, struct sg_lb_stats *sgs)
 {
-	unsigned long nr_running, max_nr_running, min_nr_running;
-	unsigned long load, max_cpu_load, min_cpu_load;
+	struct sg_imb_stats sgi;
+	unsigned long nr_running;
+	unsigned long load;
 	int i;
 
-	/* Tally up the load of all CPUs in the group */
-	max_cpu_load = 0;
-	min_cpu_load = ~0UL;
-	max_nr_running = 0;
-	min_nr_running = ~0UL;
+	init_sg_imb_stats(&sgi);
 
 	for_each_cpu_and(i, sched_group_cpus(group), env->cpus) {
 		struct rq *rq = cpu_rq(i);

@@ -4523,16 +4595,7 @@ static inline void update_sg_lb_stats(st
 			load = target_load(i, load_idx);
 		} else {
 			load = source_load(i, load_idx);
-
-			if (load > max_cpu_load)
-				max_cpu_load = load;
-			if (min_cpu_load > load)
-				min_cpu_load = load;
-
-			if (nr_running > max_nr_running)
-				max_nr_running = nr_running;
-			if (min_nr_running > nr_running)
-				min_nr_running = nr_running;
+			update_sg_imb_stats(&sgi, load, nr_running);
 		}
 
 		sgs->group_load += load;

@@ -4550,21 +4613,10 @@ static inline void update_sg_lb_stats(st
 	sgs->group_power = group->sgp->power;
 	sgs->avg_load = (sgs->group_load*SCHED_POWER_SCALE) / sgs->group_power;
 
-	/*
-	 * Consider the group unbalanced when the imbalance is larger
-	 * than the average weight of a task.
-	 *
-	 * APZ: with cgroup the avg task weight can vary wildly and
-	 *      might not be a suitable number - should we keep a
-	 *      normalized nr_running number somewhere that negates
-	 *      the hierarchy?
-	 */
 	if (sgs->sum_nr_running)
 		sgs->load_per_task = sgs->sum_weighted_load / sgs->sum_nr_running;
 
-	if ((max_cpu_load - min_cpu_load) >= sgs->load_per_task &&
-	    (max_nr_running - min_nr_running) > 1)
-		sgs->group_imb = 1;
+	sgs->group_imb = sg_imbalanced(sgs, &sgi);
 
 	sgs->group_capacity =
 		DIV_ROUND_CLOSEST(sgs->group_power, SCHED_POWER_SCALE);

@@ -4812,6 +4864,10 @@ static inline void calculate_imbalance(s
 	busiest = &sds->busiest_stat;
 
 	if (busiest->group_imb) {
+		/*
+		 * In the group_imb case we cannot rely on group-wide averages
+		 * to ensure cpu-load equilibrium, look at wider averages. XXX
+		 */
 		busiest->load_per_task =
 			min(busiest->load_per_task, sds->avg_load);
 	}

@@ -4829,6 +4885,8 @@ static inline void calculate_imbalance(s
 	if (!busiest->group_imb) {
 		/*
 		 * Don't want to pull so many tasks that a group would go idle.
+		 * Except of course for the group_imb case, since then we might
+		 * have to drop below capacity to reach cpu-load equilibrium.
 		 */
 		load_above_capacity =
 			(busiest->sum_nr_running - busiest->group_capacity);

@@ -4844,11 +4902,8 @@ static inline void calculate_imbalance(s
 	 * we also don't want to reduce the group load below the group capacity
 	 * (so that we can implement power-savings policies etc). Thus we look
 	 * for the minimum possible imbalance.
-	 * Be careful of negative numbers as they'll appear as very large values
-	 * with unsigned longs.
 	 */
-	max_pull = min(busiest->avg_load - sds->avg_load,
-		       load_above_capacity);
+	max_pull = min(busiest->avg_load - sds->avg_load, load_above_capacity);
 
 	/* How much load to actually move to equalise the imbalance */
 	env->imbalance = min(

@@ -4912,7 +4967,7 @@ static struct sched_group *find_busiest_
 
 	/*
 	 * If the busiest group is imbalanced the below checks don't
-	 * work because they assumes all things are equal, which typically
+	 * work because they assume all things are equal, which typically
 	 * isn't true due to cpus_allowed constraints and the like.
 	 */
 	if (busiest->group_imb)