sched/fair: Make sure to try to detach at least one movable task

During load balance, we try at most env->loop_max time to move a task.
But it can happen that the loop_max LRU tasks (ie tail of
the cfs_tasks list) can't be moved to dst_cpu because of affinity.
In this case, loop in the list until we found at least one.

The maximum of detached tasks remained the same as before.
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220825122726.20819-2-vincent.guittot@linaro.org

kernel/sched/fair.c

@@ -8049,8 +8049,12 @@ static int detach_tasks(struct lb_env *env)
 		p = list_last_entry(tasks, struct task_struct, se.group_node);
 
 		env->loop++;
-		/* We've more or less seen every task there is, call it quits */
-		if (env->loop > env->loop_max)
+		/*
+		 * We've more or less seen every task there is, call it quits
+		 * unless we haven't found any movable task yet.
+		 */
+		if (env->loop > env->loop_max &&
+		    !(env->flags & LBF_ALL_PINNED))
 			break;
 
 		/* take a breather every nr_migrate tasks */
@@ -10179,7 +10183,9 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 
 		if (env.flags & LBF_NEED_BREAK) {
 			env.flags &= ~LBF_NEED_BREAK;
-			goto more_balance;
+			/* Stop if we tried all running tasks */
+			if (env.loop < busiest->nr_running)
+				goto more_balance;
 		}
 
 		/*