Re: [PATCH v5] sched/fair: Fix unfairness caused by stalled tg_load_avg_contrib when the last task migrates out.

["Aaron Lu" <ziqianlu@bytedance.com>]

Hello,

On Wed, Aug 27, 2025 at 10:22:07AM +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.
> 
> Add a check of cfs_rq->tg_load_avg_contrib in cfs_rq_is_decayed(),
> which fixes the case (2.) mentioned above.
> 
> Fixes: 1528c661c24b ("sched/fair: Ratelimit update to tg->load_avg")
> Tested-by: Aaron Lu <ziqianlu@bytedance.com>
> Reviewed-by: Aaron Lu <ziqianlu@bytedance.com>
> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
> Signed-off-by: xupengbo <xupengbo@oppo.com>

I wonder if there are any more concerns about this patch? If no, I hope
this fix can be merged. It's a rare case but it does happen for some
specific setup.

Sorry if this is a bad timing, but I just hit an oncall where this exact
problem occurred so I suppose it's worth a ping :)

Best regards,
Aaron