[RFC 1/3] sched: set loop_max after rq lock is taken

[Uladzislau Rezki]

From: Uladzislau 2 Rezki <uladzislau2.rezki@xxxxxxxxxxxxxx>

While doing a load balance there is a race in setting
loop_max variable since nr_running can be changed causing
incorect iteration loops.

As a result we may skip some candidates or check the same
tasks again.

Signed-off-by: Uladzislau 2 Rezki <uladzislau2.rezki@xxxxxxxxxxxxxx>
---
 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 c242944..c5d9351 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7744,12 +7744,17 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 		 * correctly treated as an imbalance.
 		 */
 		env.flags |= LBF_ALL_PINNED;
-		env.loop_max  = min(sysctl_sched_nr_migrate, busiest->nr_running);
 
 more_balance:
 		raw_spin_lock_irqsave(&busiest->lock, flags);
 
 		/*
+		 * Set loop_max when rq's lock is taken to prevent a race.
+		 */
+		env.loop_max = min(sysctl_sched_nr_migrate,
+						busiest->nr_running);
+
+		/*
 		 * cur_ld_moved - load moved in current iteration
 		 * ld_moved     - cumulative load moved across iterations
 		 */
-- 
2.1.4