[PATCH] sched/fair: Do not scan non-movable tasks several times

[Konstantin Khorenko]

If busiest rq is small, nr_running < SCHED_NR_MIGRATE_BREAK and all
tasks are not movable, detach_tasks() should not iterate more than tasks
available in the busiest rq.

Previously the (env->loop > env->loop_max) condition prevented us from
scanning non-movable tasks more than rq size times, but after we start
checking the LBF_ALL_PINNED flag, the "all tasks are not movable" case
is under threat.

Signed-off-by: Konstantin Khorenko <khorenko@xxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d7a3c63a2171..faa2a765e899 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11219,7 +11219,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 		.dst_rq		= this_rq,
 		.dst_grpmask    = group_balance_mask(sd->groups),
 		.idle		= idle,
-		.loop_break	= SCHED_NR_MIGRATE_BREAK,
 		.cpus		= cpus,
 		.fbq_type	= all,
 		.tasks		= LIST_HEAD_INIT(env.tasks),
@@ -11265,6 +11264,12 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 		 * correctly treated as an imbalance.
 		 */
 		env.loop_max  = min(sysctl_sched_nr_migrate, busiest->nr_running);
+		/*
+		 * If busiest rq is small, nr_running < SCHED_NR_MIGRATE_BREAK
+		 * and all tasks are not movable, detach_tasks() should not
+		 * iterate more than tasks available in rq.
+		 */
+		env.loop_break = min(SCHED_NR_MIGRATE_BREAK, busiest->nr_running);
 
 more_balance:
 		rq_lock_irqsave(busiest, &rf);
-- 
2.39.3