2020-08-02 04:55:32

by Qi Zheng

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

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 <[email protected]>
---
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 {
--
2.25.1

2020-08-03 07:39:50

by Dietmar Eggemann

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

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 <[email protected]>
> ---
> 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
interval").

2020-08-03 12:37:18

by Qi Zheng

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

Hi Dietmar,

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

Yours,
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 <[email protected]>
>> ---
>> 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
> interval").
>