Re: [PATCH v2 -next] cgroup: remove offline draining in root destruction to avoid hung_tasks

[Hillf Danton <hdanton@sina.com>]

On Fri, Jul 25, 2025 at 09:42:05AM +0800, Chen Ridong <chenridong@huaweicloud.com> wrote:
> > On Tue, Jul 22, 2025 at 11:27:33AM +0000, Chen Ridong <chenridong@huaweicloud.com> wrote:
> >> CPU0                            CPU1
> >> mount perf_event                umount net_prio
> >> cgroup1_get_tree                cgroup_kill_sb
> >> rebind_subsystems               // root destruction enqueues
> >> 				// cgroup_destroy_wq
> >> // kill all perf_event css
> >>                                 // one perf_event css A is dying
> >>                                 // css A offline enqueues cgroup_destroy_wq
> >>                                 // root destruction will be executed first
> >>                                 css_free_rwork_fn
> >>                                 cgroup_destroy_root
> >>                                 cgroup_lock_and_drain_offline
> >>                                 // some perf descendants are dying
> >>                                 // cgroup_destroy_wq max_active = 1
> >>                                 // waiting for css A to die
> >>
> >> Problem scenario:
> >> 1. CPU0 mounts perf_event (rebind_subsystems)
> >> 2. CPU1 unmounts net_prio (cgroup_kill_sb), queuing root destruction work
> >> 3. A dying perf_event CSS gets queued for offline after root destruction
> >> 4. Root destruction waits for offline completion, but offline work is
> >>    blocked behind root destruction in cgroup_destroy_wq (max_active=1)
> > 
> > What's concerning me is why umount of net_prio hierarhy waits for
> > draining of the default hierachy? (Where you then run into conflict with
> > perf_event that's implicit_on_dfl.)
> > 
/*
 * cgroup destruction makes heavy use of work items and there can be a lot
 * of concurrent destructions.  Use a separate workqueue so that cgroup
 * destruction work items don't end up filling up max_active of system_wq
 * which may lead to deadlock.
 */

If task hung could be reliably reproduced, it is right time to cut
max_active off for cgroup_destroy_wq according to its comment.