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

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Message-ID: <CAKfTPtDJW4yU2=_4stdS1bggHwAA8K2On_ruV63=_H9=YEgdkw@mail.gmail.com>
Date: Wed, 22 Oct 2025 14:52:18 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Peng Wang <peng_wang@...ux.alibaba.com>
Cc: Peter Zijlstra <peterz@...radead.org>, 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, 22 Oct 2025 at 11:00, Peng Wang <peng_wang@...ux.alibaba.com> wrote:
>
> On Wed, Oct 15, 2025 at 03:14:37PM +0200, Vincent Guittot wrote:
> > 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")
>
> Hi, Vincent and Peter,
>
> We have already integrated this commit, but the bug persists.
>
> Do you think we should explicitly clear the h_load_next list?
>
> Even though update_cfs_rq_h_load runs under an RCU lock, ARM's
> weak memory ordering could still allow readers to observe stale
> values in the list.

I'm worried about the increase of contention on the cache with this write.

Could we check cfs_rq->h_load_next  and clear it if needed in
unregister_fair_sched_group() instead ?

>
> >
> > >
> > > 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.