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Subject[RFC,v2 1/3] sched: set loop_max after rq lock is taken
Date
From: Uladzislau 2 Rezki <uladzislau2.rezki@sonymobile.com>

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@sonymobile.com>
---
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 6559d19..4be7193 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8073,12 +8073,17 @@ 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
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