[PATCH v3] sched/fair: Fix unfairness caused by stalled tg_load_avg_contrib when the last task migrates out.

[PATCH v3] sched/fair: Fix unfairness caused by stalled tg_load_avg_contrib when the last task migrates out.

[xupengbo]

When a task is migrated out, there is a probability that the tg->load_avg
value will become abnormal. The reason is as follows.

1. Due to the 1ms update period limitation in update_tg_load_avg(), there
is a possibility that the reduced load_avg is not updated to tg->load_avg
when a task migrates out.
2. Even though __update_blocked_fair() traverses the leaf_cfs_rq_list and
calls update_tg_load_avg() for cfs_rqs that are not fully decayed, the key
function cfs_rq_is_decayed() does not check whether
cfs->tg_load_avg_contrib is null. Consequently, in some cases,
__update_blocked_fair() removes cfs_rqs whose avg.load_avg has not been
updated to tg->load_avg.

I added a check of cfs_rq->tg_load_avg_contrib in cfs_rq_is_decayed(),
which blocks the case (2.) mentioned above.
After some preliminary discussion and analysis, I think it is feasible to
directly check if cfs_rq->tg_load_avg_contrib is 0 in cfs_rq_is_decay().
So patch v3 was submitted.

Fixes: 1528c661c24b ("sched/fair: Ratelimit update to tg->load_avg")
Signed-off-by: xupengbo <xupengbo@oppo.com>
---
Changes:
v1 -> v2: 
- Another option to fix the bug. Check cfs_rq->tg_load_avg_contrib in 
cfs_rq_is_decayed() to avoid early removal from the leaf_cfs_rq_list.
- Link to v1 : https://lore.kernel.org/cgroups/20250804130326.57523-1-xupengbo@oppo.com/
v2 -> v3:
- Check if cfs_rq->tg_load_avg_contrib is 0 derectly.
- Link to v2 : https://lore.kernel.org/cgroups/20250805144121.14871-1-xupengbo@oppo.com/

Please send emails to a different email address <xupengbo1029@163.com>
after September 3, 2025, after that date <xupengbo@oppo.com> will expire
for personal reasons.

Thanks,
Xu Pengbo

 kernel/sched/fair.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b173a059315c..df348cb6a5f3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4062,6 +4062,9 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
 	if (child_cfs_rq_on_list(cfs_rq))
 		return false;
 
+	if (cfs_rq->tg_laod_avg_contrib)
+		return false;
+
 	return true;
 }
 

base-commit: fab1beda7597fac1cecc01707d55eadb6bbe773c
-- 
2.43.0

Re: [PATCH v2] sched/fair: Fix unfairness caused by stalled tg_load_avg_contrib when the last task migrates out.

[xupengbo]

>Hi xupengbo,
>
>On Wed, Aug 06, 2025 at 04:38:10PM +0800, xupengbo wrote:
>... ... 
>> 
>> It actually becomes:
>>     if (cfs_rq->tg_load_avg_contrib > 0)
>> if cfs_rq->tg_load_avg_contrib == 0 , it will be false. As it is an unsigned
>> long, this condition is equivalent to :
>>     if (cfs_rq->tg_load_avg_contrib)
>
>I suppose we have reached a conclusion that the right fix is to add a
>check of cfs_rq->tg_load_avg_contrib in cfs_rq_is_decayed()? Something
>like below:
>
>diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>index af33d107d8034..3ebcb683063f0 100644
>--- a/kernel/sched/fair.c
>+++ b/kernel/sched/fair.c
>@@ -4056,6 +4056,9 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
> 	if (child_cfs_rq_on_list(cfs_rq))
> 		return false;
> 
>+	if (cfs_rq->tg_load_avg_contrib)
>+		return false;
>+
> 	return true;
> }
> 
>If you also agree, can you send an updated patch to fix this problem?
>Thanks.

I already sent an updated patch v3.
Link: https://lore.kernel.org/cgroups/20250826075743.19106-1-xupengbo@oppo.com/

Thanks,
xupengbo

Re: [PATCH v3] sched/fair: Fix unfairness caused by stalled tg_load_avg_contrib when the last task migrates out.

[Aaron Lu]

On Tue, Aug 26, 2025 at 03:57:42PM +0800, xupengbo wrote:
> When a task is migrated out, there is a probability that the tg->load_avg
> value will become abnormal. The reason is as follows.
> 
> 1. Due to the 1ms update period limitation in update_tg_load_avg(), there
> is a possibility that the reduced load_avg is not updated to tg->load_avg
> when a task migrates out.
> 2. Even though __update_blocked_fair() traverses the leaf_cfs_rq_list and
> calls update_tg_load_avg() for cfs_rqs that are not fully decayed, the key
> function cfs_rq_is_decayed() does not check whether
> cfs->tg_load_avg_contrib is null. Consequently, in some cases,
> __update_blocked_fair() removes cfs_rqs whose avg.load_avg has not been
> updated to tg->load_avg.
> 
> I added a check of cfs_rq->tg_load_avg_contrib in cfs_rq_is_decayed(),
> which blocks the case (2.) mentioned above.
> After some preliminary discussion and analysis, I think it is feasible to
> directly check if cfs_rq->tg_load_avg_contrib is 0 in cfs_rq_is_decay().
> So patch v3 was submitted.
> 
> Fixes: 1528c661c24b ("sched/fair: Ratelimit update to tg->load_avg")
> Signed-off-by: xupengbo <xupengbo@oppo.com>

With the below typo fixed:

Tested-by: Aaron Lu <ziqianlu@bytedance.com>
Reviewed-by: Aaron Lu <ziqianlu@bytedance.com>

> ---
> Changes:
> v1 -> v2: 
> - Another option to fix the bug. Check cfs_rq->tg_load_avg_contrib in 
> cfs_rq_is_decayed() to avoid early removal from the leaf_cfs_rq_list.
> - Link to v1 : https://lore.kernel.org/cgroups/20250804130326.57523-1-xupengbo@oppo.com/
> v2 -> v3:
> - Check if cfs_rq->tg_load_avg_contrib is 0 derectly.
> - Link to v2 : https://lore.kernel.org/cgroups/20250805144121.14871-1-xupengbo@oppo.com/
> 
> Please send emails to a different email address <xupengbo1029@163.com>
> after September 3, 2025, after that date <xupengbo@oppo.com> will expire
> for personal reasons.
> 
> Thanks,
> Xu Pengbo
> 
>  kernel/sched/fair.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b173a059315c..df348cb6a5f3 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4062,6 +4062,9 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
>  	if (child_cfs_rq_on_list(cfs_rq))
>  		return false;
>  
> +	if (cfs_rq->tg_laod_avg_contrib)
                       ~~~~
typo: tg_load_avg_contrib

> +		return false;
> +
>  	return true;
>  }
>  
> 
> base-commit: fab1beda7597fac1cecc01707d55eadb6bbe773c
> -- 
> 2.43.0
>