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

[Waiman Long <longman@redhat.com>]

On 3/24/26 5:41 AM, Paul E. McKenney wrote:
> On Wed, Mar 18, 2026 at 11:43:37AM -0700, 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>
> This did fix the problem, except for PREEMPT_RT kernels (which I have
> not yet bisected).  If there is another patch for that configuration,
> could you please let me know?

Thank for the notice. I haven't much testing with respect to PREEMPT_RT 
kernel. I will try to run some tests on PREEMPT_RT kernel and see if 
there is any problem. Please let me know if you found out the new cpuset 
code is at fault after bisection.

Cheers,
Longman