sched/fair: Use utilization to select misfit task

Utilization is used to detect a misfit task but the load is then used to
select the task on the CPU which can lead to select a small task with
high weight instead of the task that triggered the misfit migration.

Check that task can't fit the CPU's capacity when selecting the misfit
task instead of using the load.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Acked-by: Valentin Schneider <valentin.schneider@arm.com>
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
Link: https://lkml.kernel.org/r/1571405198-27570-9-git-send-email-vincent.guittot@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -7408,13 +7408,8 @@ static int detach_tasks(struct lb_env *env)
 			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)
+			/* This is not a misfit task */
+			if (task_fits_capacity(p, capacity_of(env->src_cpu)))
 				goto next;
 
 			env->imbalance = 0;
@@ -8358,7 +8353,7 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
 	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;
+		env->imbalance = 1;
 		return;
 	}