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

[Waiman Long <longman@redhat.com>]

On 3/18/26 2:43 PM, Paul E. McKenney wrote:
> On Wed, Mar 18, 2026 at 11:02:16AM -0400, Waiman Long wrote:
>> 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?
> It does, thank you!
>
> Tested-by: Paul E. McKenney <paulmck@kernel.org>
>
Thanks for the confirmation.

Cheers,
Longman