[PATCH v5] sched/fair: do not scan twice in detach_tasks()

[Huang Shijie <shijie@os.amperecomputing.com>]

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>
---
v4 --> v5:
    Set the env.loop_max once the rq lock is taken.
    v4:https://lore.kernel.org/all/20250721023939.19703-1-shijie@os.amperecomputing.com/

v3 --> v4:
    Changed the commit message suggested by Valentin Schneider.
    v3: https://lore.kernel.org/all/20250718063523.9232-1-shijie@os.amperecomputing.com/

v2 --> v3:
    Fix a typo in the commit message.
    v2: https://lore.kernel.org/all/20250718054709.8781-1-shijie@os.amperecomputing.com/

v1 --> v2:
    Add more comment from Valentin Schneider
    v1: https://lore.kernel.org/all/20250707083636.38380-1-shijie@os.amperecomputing.com/
---
 kernel/sched/fair.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7cc9d50e3e11..9c1f21d59b5c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11708,12 +11708,15 @@ 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
-- 
2.40.1