sched/fair: Add per-CPU min capacity to sched_group_capacity

struct sched_group_capacity currently represents the compute capacity
sum of all CPUs in the sched_group.

Unless it is divided by the group_weight to get the average capacity
per CPU, it hides differences in CPU capacity for mixed capacity systems
(e.g. high RT/IRQ utilization or ARM big.LITTLE).

But even the average may not be sufficient if the group covers CPUs of
different capacities.

Instead, by extending struct sched_group_capacity to indicate min per-CPU
capacity in the group a suitable group for a given task utilization can
more easily be found such that CPUs with reduced capacity can be avoided
for tasks with high utilization (not implemented by this patch).

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/1476452472-24740-4-git-send-email-morten.rasmussen@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -5708,7 +5708,7 @@ static int sched_domain_debug_one(struct sched_domain *sd, int cpu, int level,
 		printk(KERN_CONT " %*pbl",
 		       cpumask_pr_args(sched_group_cpus(group)));
 		if (group->sgc->capacity != SCHED_CAPACITY_SCALE) {
-			printk(KERN_CONT " (cpu_capacity = %d)",
+			printk(KERN_CONT " (cpu_capacity = %lu)",
 				group->sgc->capacity);
 		}
 
@@ -6185,6 +6185,7 @@ build_overlap_sched_groups(struct sched_domain *sd, int cpu)
 		 * die on a /0 trap.
 		 */
 		sg->sgc->capacity = SCHED_CAPACITY_SCALE * cpumask_weight(sg_span);
+		sg->sgc->min_capacity = SCHED_CAPACITY_SCALE;
 
 		/*
 		 * Make sure the first group of this domain contains the

kernel/sched/fair.c

@@ -6909,13 +6909,14 @@ static void update_cpu_capacity(struct sched_domain *sd, int cpu)
 
 	cpu_rq(cpu)->cpu_capacity = capacity;
 	sdg->sgc->capacity = capacity;
+	sdg->sgc->min_capacity = capacity;
 }
 
 void update_group_capacity(struct sched_domain *sd, int cpu)
 {
 	struct sched_domain *child = sd->child;
 	struct sched_group *group, *sdg = sd->groups;
-	unsigned long capacity;
+	unsigned long capacity, min_capacity;
 	unsigned long interval;
 
 	interval = msecs_to_jiffies(sd->balance_interval);
@@ -6928,6 +6929,7 @@ void update_group_capacity(struct sched_domain *sd, int cpu)
 	}
 
 	capacity = 0;
+	min_capacity = ULONG_MAX;
 
 	if (child->flags & SD_OVERLAP) {
 		/*
@@ -6952,11 +6954,12 @@ void update_group_capacity(struct sched_domain *sd, int cpu)
 			 */
 			if (unlikely(!rq->sd)) {
 				capacity += capacity_of(cpu);
-				continue;
+			} else {
+				sgc = rq->sd->groups->sgc;
+				capacity += sgc->capacity;
 			}
 
-			sgc = rq->sd->groups->sgc;
-			capacity += sgc->capacity;
+			min_capacity = min(capacity, min_capacity);
 		}
 	} else  {
 		/*
@@ -6966,12 +6969,16 @@ void update_group_capacity(struct sched_domain *sd, int cpu)
 
 		group = child->groups;
 		do {
-			capacity += group->sgc->capacity;
+			struct sched_group_capacity *sgc = group->sgc;
+
+			capacity += sgc->capacity;
+			min_capacity = min(sgc->min_capacity, min_capacity);
 			group = group->next;
 		} while (group != child->groups);
 	}
 
 	sdg->sgc->capacity = capacity;
+	sdg->sgc->min_capacity = min_capacity;
 }
 
 /*

kernel/sched/sched.h

@@ -892,7 +892,8 @@ struct sched_group_capacity {
 	 * CPU capacity of this group, SCHED_CAPACITY_SCALE being max capacity
 	 * for a single CPU.
 	 */
-	unsigned int capacity;
+	unsigned long capacity;
+	unsigned long min_capacity; /* Min per-CPU capacity in group */
 	unsigned long next_update;
 	int imbalance; /* XXX unrelated to capacity but shared group state */