[RFC PATCH 08/23] sched/fair: Compute task-class performance scores for load balancing

[Ricardo Neri]

Compute both the current and the prospective the task-class performance
of a scheduling group. As task-class statistics are used during asym_
packing load balancing, the scheduling group will become idle.

For a scheduling group with only one CPU, the prospective performance is
the performance of its current task if placed on the destination CPU.

In a scheduling group composed of SMT siblings the current tasks of all
CPUs share the resources of the core. Divide the task-class performance of
scheduling group by the number of busy CPUs.

After load balancing, the throughput of the siblings that remain busy
increases. Plus, the destination CPU now contributes to the overall
throughput.

Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Len Brown <len.brown@xxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
Cc: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Tim C. Chen <tim.c.chen@xxxxxxxxx>
Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 53 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 58a435a04c1c..97731f81b570 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8405,6 +8405,8 @@ struct sg_lb_stats {
 	enum group_type group_type;
 	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 */
+	long task_class_score_after; /* Prospective task-class score after load balancing */
+	long task_class_score_before; /* Task-class score before load balancing */
 #ifdef CONFIG_NUMA_BALANCING
 	unsigned int nr_numa_running;
 	unsigned int nr_preferred_running;
@@ -8732,6 +8734,49 @@ static void update_rq_task_classes_stats(struct sg_lb_task_class_stats *class_sg
 	class_sgs->min_score = score;
 	class_sgs->p_min_score = rq->curr;
 }
+
+static void compute_ilb_sg_task_class_scores(struct sg_lb_task_class_stats *class_sgs,
+					     struct sg_lb_stats *sgs,
+					     int dst_cpu)
+{
+	int group_score, group_score_without, score_on_dst_cpu;
+	int busy_cpus = sgs->group_weight - sgs->idle_cpus;
+
+	if (!sched_task_classes_enabled())
+		return;
+
+	/* No busy CPUs in the group. No tasks to move. */
+	if (!busy_cpus)
+		return;
+
+	score_on_dst_cpu = arch_get_task_class_score(class_sgs->p_min_score->class,
+						     dst_cpu);
+
+	/*
+	 * The simpest case. The single busy CPU in the current group will
+	 * become idle after pulling its current task. The destination CPU is
+	 * idle.
+	 */
+	if (busy_cpus == 1) {
+		sgs->task_class_score_before = class_sgs->sum_score;
+		sgs->task_class_score_after = score_on_dst_cpu;
+		return;
+	}
+
+	/*
+	 * Now compute the group score with and without the task with the
+	 * lowest score. We assume that the tasks that remain in the group share
+	 * the CPU resources equally.
+	 */
+	group_score = class_sgs->sum_score / busy_cpus;
+
+	group_score_without =  (class_sgs->sum_score - class_sgs->min_score) /
+			       (busy_cpus - 1);
+
+	sgs->task_class_score_after = group_score_without + score_on_dst_cpu;
+	sgs->task_class_score_before = group_score;
+}
+
 #else /* CONFIG_SCHED_TASK_CLASSES */
 static void update_rq_task_classes_stats(struct sg_lb_task_class_stats *class_sgs,
 					 struct rq *rq)
@@ -8741,6 +8786,13 @@ static void update_rq_task_classes_stats(struct sg_lb_task_class_stats *class_sg
 static void init_rq_task_classes_stats(struct sg_lb_task_class_stats *class_sgs)
 {
 }
+
+static void compute_ilb_sg_task_class_scores(struct sg_lb_task_class_stats *class_sgs,
+					     struct sg_lb_stats *sgs,
+					     int dst_cpu)
+{
+}
+
 #endif /* CONFIG_SCHED_TASK_CLASSES */
 
 /**
@@ -8920,6 +8972,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 	if (!local_group && env->sd->flags & SD_ASYM_PACKING &&
 	    env->idle != CPU_NOT_IDLE && sgs->sum_h_nr_running &&
 	    sched_asym(env, sds, sgs, group)) {
+		compute_ilb_sg_task_class_scores(&class_stats, sgs, env->dst_cpu);
 		sgs->group_asym_packing = 1;
 	}
 
-- 
2.25.1