Re: [PATCH] sched/fair: Fix the logic about active_balance in load_balance()

From: Qi Zheng
Date: Mon Aug 03 2020 - 08:34:20 EST

Hi Dietmar,

I understand, thank you for your review and very detailed explanation.

Qi Zheng

On 2020/8/3 下午3:36, Dietmar Eggemann wrote:
On 02/08/2020 06:51, Qi Zheng wrote:
I think the unbalance scenario here should be that we need to
do active balance but it is not actually done. So fix it.

Signed-off-by: Qi Zheng <arch0.zheng@xxxxxxxxx>
kernel/sched/fair.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2ba8f230feb9..6d8c53718b67 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9710,7 +9710,7 @@ static int load_balance(int this_cpu, struct rq *this_rq,
} else
sd->nr_balance_failed = 0;
- if (likely(!active_balance) || voluntary_active_balance(&env)) {
+ if (likely(!active_balance) && voluntary_active_balance(&env)) {
/* We were unbalanced, so reset the balancing interval */
sd->balance_interval = sd->min_interval;
} else {

Active balance is potentially already been done when we reach this code.

See 'if (need_active_balance(&env))' and 'if (!busiest->active_balance)'
further up.

Here we only reset sd->balance_interval in case:
(A) the last load balance wasn't an active one
(B) the reason for the active load balance was:
(1) asym packing
(2) capacity of src_cpu is reduced compared to the one of dst_cpu
(3) misfit handling

(B) is done to not unnecessarily increase of balance interval, see
commit 46a745d90585 ("sched/fair: Fix unnecessary increase of balance