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

[Peter Zijlstra]

On Fri, Nov 28, 2025 at 07:54:45PM +0800, Aaron Lu wrote:
> 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@xxxxxxxxxxxxx>
> > Reviewed-by: Aaron Lu <ziqianlu@xxxxxxxxxxxxx>
> > Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > Signed-off-by: xupengbo <xupengbo@xxxxxxxx>
> 
> 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 :)

Totally missed it. Seems okay, let me go queue the thing.