sched/fair: do not scan twice in detach_tasks()

detach_tasks() uses struct lb_env.loop_max as an env.src_rq->cfs_tasks
iteration count limit. It is however set without the source RQ lock held.

This means that env.loop_max and the actual length of env.src_rq->cfs_tasks
as observed within detach_tasks() can differ. This can cause some tasks to
be unnecessarily iterated over more than once, for instance:

  env.loop_max := 4
  detach_tasks()
    // Here env.src->cfs_tasks only contains two tasks which can't be
    // migrated anywhere, so they're put back in the list each time.
    env.src->cfs_tasks := [p1, p0]
    // The iteration goes:
    p0; cfs_tasks := [p0, p1]
    p1; cfs_tasks := [p1, p0]
    p0; cfs_tasks := [p0, p1]
    p1; cfs_tasks := [p1, p0]

    // IOW we iterate over each task twice

In the Specjbb test, the similar issues can be caught many times.
(Over 330,000 times in a 30-minites Specjbb test)

This patch sets env.loop_max only once RQ lock is taken,
and uses busiest->cfs.h_nr_queued for setting the env.loop_max.

After this patch, I cannot catch any above issue in the Specjbb test.

Signed-off-by: Huang Shijie <shijie@os.amperecomputing.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>

Author: HuangShijie2024

kernel/sched/fair.c

@@ -11932,12 +11932,15 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
 		 * still unbalanced. ld_moved simply stays zero, so it is
 		 * correctly treated as an imbalance.
 		 */
-		env.loop_max  = min(sysctl_sched_nr_migrate, busiest->nr_running);
-
 more_balance:
 		rq_lock_irqsave(busiest, &rf);
 		update_rq_clock(busiest);
 
+		if (!env.loop_max)
+			env.loop_max  = min(sysctl_sched_nr_migrate, busiest->cfs.h_nr_queued);
+		else
+			env.loop_max  = min(env.loop_max, busiest->cfs.h_nr_queued);
+
 		/*
 		 * cur_ld_moved - load moved in current iteration
 		 * ld_moved     - cumulative load moved across iterations