Re: [PATCH v6 7/8] cgroup/cpuset: Defer housekeeping_update() calls from CPU hotplug to workqueue

[Waiman Long <longman@redhat.com>]

On 3/3/26 5:48 PM, Frederic Weisbecker wrote:
> Le Tue, Mar 03, 2026 at 11:00:54AM -0500, Waiman Long a écrit :
>> On 2/26/26 11:06 AM, Frederic Weisbecker wrote:
>> If you look at the work function, it will make a copy of HK_TYPE_DOMAIN
>> cpumask while holding rcu_read_lock().
> Where?
>
>> So the current hotplug operation must
>> have finished at that point.
> I'm confused. This is called from sched_cpu_deactivate(), right?
> So the work is scheduled at that point. But the work does cpuset_full_lock()
> which includes cpu hotplug read lock, so the sched domain rebuild can only
> happen at the end of cpu_down().
>
> This means that between CPUHP_TEARDOWN_CPU and CPUHP_OFFLINE, the offline
> CPU still appears in the scheduler topology because the scheduler domains
> haven't been rebuilt.
>
> And even if the work wouldn't cpu hotplug read lock, what guarantees that
> it executes before reaching CPUHP_TEARDOWN_CPU?
>
>> Of course, if there is another hot-add/remove
>> operation right after the rcu_read_lock is released, the cpumask passed down
>> to housekeeping_update() may not be the latest one. In this case, another
>> work will be scheduled to call housekeeping_update() with the new cpumask
>> again.
> I'm not so much worried about housekeeping_update() (yet). I'm worried about
> topology rebuild to happen before CPUHP_TEARDOWN_CPU. Offline CPUs shouldn't
> exist in the topology.

Yes, I am aware that this could be a problem. I am working on a fix 
patch that will always do a rebuild_sched_domains_cpuslocked() call 
directly in the hotplug path if needed as shown in the patch that I sent 
to Jon. I want to get a confirmation first before I send it out. There 
will be other minor code/comment adjustments as well.

Cheers,
Longman