linux-kernel - Re: [PATCH] sched/fair: Clear ->h_load

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Message-ID: <CAKfTPtD-RJoMEHSQToF_578KZ=WszR+xStxNghiWpv4asnHBoA@mail.gmail.com>
Date: Wed, 15 Oct 2025 15:14:37 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Peng Wang <peng_wang@...ux.alibaba.com>, mingo@...hat.com, juri.lelli@...hat.com, 
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com, 
	mgorman@...e.de, vschneid@...hat.com, vdavydov.dev@...il.com, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Clear ->h_load_next after hierarchical load

On Wed, 15 Oct 2025 at 14:44, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Oct 15, 2025 at 08:19:50PM +0800, Peng Wang wrote:
>
> > We found that the task_group corresponding to the problematic se
> > is not in the parent task_group’s children list, indicating that
> > h_load_next points to an invalid address. Consider the following
> > cgroup and task hierarchy:
> >
> >          A
> >         / \
> >        /   \
> >       B     E
> >      / \    |
> >     /   \   t2
> >    C     D
> >    |     |
> >    t0    t1
> >
> > Here follows a timing sequence that may be responsible for triggering
> > the problem:
> >
> > CPU X                   CPU Y                   CPU Z
> > wakeup t0
> > set list A->B->C
> > traverse A->B->C
> > t0 exits
> > destroy C
> >                         wakeup t2
> >                         set list A->E           wakeup t1
> >                                                 set list A->B->D
> >                         traverse A->B->C
> >                         panic
> >
> > CPU Z sets ->h_load_next list to A->B->D, but due to arm64 weaker memory
> > ordering, Y may observe A->B before it sees B->D, then in this time window,
> > it can traverse A->B->C and reach an invalid se.
>
> Hmm, I rather think we should ensure update_cfs_rq_h_load() is
> serialized against unregister_fair_sched_group().

The bug has been reported for v5.10 which probably don't have fixed
done "recently"
commit b027789e5e50 ("sched/fair: Prevent dead task groups from
regaining cfs_rq's")

>
> And I'm thinking that really shouldn't be hard; note how
> sched_unregister_group() already has an RCU grace period. So all we need
> to ensure is that task_h_load() is called in a context that stops RCU
> grace periods (rcu_read_lock(), preempt_disable(), local_irq_disable(),
> local_bh_disable()).
>
> A very quick scan makes me think at the very least the usage in
>
>   task_numa_migrate()
>     task_numa_find_cpu()
>       task_h_load()
>
> fails here; probably more.