Re: [BUG] cgroups/cpusets: Spurious CPU-hotplug failures

[Waiman Long <longman@redhat.com>]

On 3/18/26 8:53 AM, Paul E. McKenney wrote:
> Hello!
>
> Running rcutorture on v7.0-rc3 results in spurious CPU-hotplug failures,
> most frequently on the TREE03 scenario, which suffers about ten such
> failures per hundred hours of test time.  Repeat-by is as follows:
>
> tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 80 --duration 100h --configs "100*TREE03" --trust-make
>
> Though a faster repeat-by instead uses kvm-remote.sh and lots of systems.
>
> Bisection converges here:
>
> 6df415aa46ec ("cgroup/cpuset: Defer housekeeping_update() calls from CPU hotplug to workqueue")
>
> Reverting this commit gets rid of the spurious CPU-hotplug failures.
> Of course, this also gets rid of some ability to do dynamic nohz_full
> processing.
>
> Now, the problem might be that the workqueue handler might still be
> in flight by the time that rcutorture fired up the next CPU-hotplug
> operation, especially given that the TREE03 scenario only waits 200
> milliseconds between these operations.  This suggests waiting for this
> handler before ending each CPU-hotplug operation.  And the crude patch
> below does make the problem go away.
>
> This alleged fix is quite heavy-handed, and also fragile in that if
> hk_sd_workfn() uses a different workqueue, this breaks.  It might be
> better to call into the cgroups/cpusets code and to use flush_work()
> to wait only on hk_sd_workfn() and nothing else.  But it seemed best to
> keep things trivial to start with.
>
> Either way, please consider the patch below to be part of this bug report
> rather than a proper fix.
>
> Thoughts?
>
> 							Thanx, Paul
There is a fix commit ca174c705db5 ("cgroup/cpuset: Call
rebuild_sched_domains() directly in hotplug") in rc4 that may help. Could
you try out the rc4 kernel to see if that can resolve the problem that 
you have?

Thanks,
Longman