sched/fair: Fix unfairness caused by stalled tg_load_avg_contrib when the last task...

sched/fair: Fix unfairness caused by stalled tg_load_avg_contrib when the last task migrates out

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")
Signed-off-by: xupengbo <xupengbo@oppo.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Aaron Lu <ziqianlu@bytedance.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Tested-by: Aaron Lu <ziqianlu@bytedance.com>
Link: https://patch.msgid.link/20250827022208.14487-1-xupengbo@oppo.com