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

[Aaron Lu <ziqianlu@bytedance.com>]

On Wed, Aug 06, 2025 at 02:31:58PM +0800, xupengbo wrote:
> > >On Tue, 5 Aug 2025 at 16:42, xupengbo <xupengbo@oppo.com> 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. I follow the condition in
> > > update_tg_load_avg() instead of directly checking if
> > > cfs_rq->tg_load_avg_contrib is null. I think it's necessary to keep the
> > > condition consistent in both places, otherwise unexpected problems may
> > > occur.
> > >
> > > Thanks for your comments,
> > > Xu Pengbo
> > >
> > > 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/T/#u
> > >
> > >  kernel/sched/fair.c | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index b173a059315c..a35083a2d006 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -4062,6 +4062,11 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
> > >         if (child_cfs_rq_on_list(cfs_rq))
> > >                 return false;
> > >
> > > +       long delta = cfs_rq->avg.load_avg - cfs_rq->tg_load_avg_contrib;
> > > +
> > > +       if (abs(delta) > cfs_rq->tg_load_avg_contrib / 64)
> > 
> >I don't understand why you use the above condition instead of if
> >(!cfs_rq->tg_load_avg_contrib). Can you elaborate ?
> > 
> >strictly speaking we want to keep the cfs_rq in the list if
> >(cfs_rq->tg_load_avg_contrib != cfs_rq->avg.load_avg) and
> >cfs_rq->avg.load_avg == 0 when we test this condition
> 
> 
> I use this condition primarily based on the function update_tg_load_avg().
> I want to absolutely avoid a situation where cfs_rq_is_decay() returns 
> false but update_tg_load_avg() cannot update its value due to the delta 
> check, which may cause the cfs_rq to remain on the list permanently. 
> Honestly, I am not sure if this will happen, so I took this conservative 
> approach.

Hmm...it doesn't seem we need worry about this situation.

Because when cfs_rq->load_avg is 0, abs(delta) will be
cfs_rq->tg_load_avg_contrib and the following condition:

	if (abs(delta) > cfs_rq->tg_load_avg_contrib / 64)
becomes:
	if (cfs_rq->tg_load_avg_contrib > cfs_rq->tg_load_avg_contrib / 64)

which should always be true, right?

Thanks,
Aaron

> 
> In fact, in the second if-condition of cfs_rq_is_decay(), the comment in 
> the load_avg_is_decayed() function states:"_avg must be null when _sum is 
> null because _avg = _sum / divider". Therefore, when we check this newly 
> added condition, cfs_rq->avg.load_avg should already be 0, right?
> 
> After reading your comments, I carefully considered the differences 
> between these two approaches. Here, my condition is similar
> to cfs_rq->tg_load_avg_contrib != cfs_rq->avg.load_avg but weaker. In 
> fact, when cfs_rq->avg.load_avg is already 0, 
> abs(delta) > cfs_rq->tg_load_avg_contrib / 64 is equivalent to 
> cfs_rq->tg_load_avg_contrib > cfs_rq->tg_load_avg_contrib / 64,
> Further reasoning leads to the condition cfs_rq->tg_load_avg_contrib > 0.
> However if cfs_rq->avg.load_avg is not necessarily 0 at this point, then
> the condition you propose is obviously more accurate, simpler than the
> delta check, and requires fewer calculations.
> 
> I think our perspectives differ. From the perspective of 
> update_tg_load_avg(), the semantics of this condition are as follows: if
> there is no 1ms update limit, and update_tg_load_avg() can continue 
> updating after checking the delta, then in cfs_rq_is_decayed() we should
> return false to keep the cfs_rq in the list for subsequent updates. As 
> mentioned in the first paragraph, this avoids that tricky situation. From
> the perspective of cfs_rq_is_decayed(), the semantics of the condition you
> proposed are that if cfs_rq->avg.load_avg is already 0, then it cannot be
> removed from the list before all load_avg are updated to tg. That makes 
> sense to me, but I still feel like there's a little bit of a risk. Am I 
> being paranoid?
> 
> How do you view these two lines of thinking?
> 
> It's a pleasure to discuss this with you, 
> xupengbo.
> 
> > > +               return false;
> > > +
> > >         return true;
> > >  }
> > >
> > > --
> > > 2.43.0
> > >